CA1096464A

CA1096464A - Fuel ignition system having interlock protection and electronic valve leak detection

Info

Publication number: CA1096464A
Application number: CA289,994A
Authority: CA
Inventors: Russell B. Matthews
Original assignee: Johnson Controls Inc
Current assignee: Johnson Controls Inc
Priority date: 1977-03-07
Filing date: 1977-11-01
Publication date: 1981-02-24
Also published as: JPS53110126A; DE2809843C2; NL7713579A; DE2809843A1

Abstract

FUEL IGNITION SYSTEM HAVING INTERLOCK PROTECTION AND ELECTRONIC VALVE LEAK DETECTION Abstract of the Disclosure: A fuel ignition system of the pilot ignition type including a control circuit for operating pilot and main valves of a redundant valve assembly, the control circuit having control relays which provide an interlock on start-up to prevent the energization of the valves under certain failure conditions, the control circuit including a flame sensing circuit which is operable to prevent start-up of the system when a flame is provided as the result of a leak con-dition for the pilot valve, and to terminate an ignition cycle causing complete shut off of fuel whenever a flame is provided as the result of a leak condition for the main valve. ;

Description

BAC~GRO~ND ~F T~F I ~NTION

1. Field of the Invention. This inven-tion relates to fuel ignitior~ systems of the pilot igni-tion -type and more particularly, to a control circuit Eor use in such systems for prov:iding an interlock on start-up under certain failure conditions.

2. Description of the Prior Art. In known fuel ignition systems of the pilot ignition type, a pilot valve is operated in response to the closure of thermostatically con-trolled contacts to supply fuel to a pilot outlet for ignition by a suitable i~niter to establish a pilot flame. A pilot ;~
flame sensing clrcuit detects the pilot flame an~ effects the energization of a main valve which supplies fuel to a main burner apparatus for ignition by the pilot flame. ~-Typically, the operation of the main value is con- ' trolled by a relay of the flame sensing circuit which has ~;
normally open contacts connected in the energizing path for the main valve -to maintain the main valve deenergized until a pilot flame is established. When a pilot flame is established, the flame sensing circuit energizes the relay which closes its contacts to connect the main valve to an energizing circuit to permit the main valve to operateO
After the heating demand has been met, the thermo statically controlled contacts open to effect deenergization of ~
the fuel valves and cause the flame to be extinguished. The ~-flame sensing circuit responsively causes the relay to be de-energized, opening its contarts to disconnect the main valve from the energizing circuit in preparation for the next -, . ' . ' ' , :

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heating cycle. However, should the relay con-tacts which control the energization oE the main valve become welded together Eollowing a successful ignition cycle, then, when the relay is deenergized, the main valve remains connected to the energizing circuit and will be energized when khe thermos-tatically con-trolled contacts close in response to the call Eor heat, even though a pilot flame is not established. Similarly, the main valve will also be connected to the energizing circuit for a circuit failure which permits the relay of the flame sensing circuit to be energized in the absence of a pilot flame. For such failure conditions, both the ~ilot valve and the main valve will be energized when the thermostatically controlled contacts close, permitting fuel to emanate from the pilot out-let and the main burner, an undesireable condition.
Various interlock arrangements have been proposed in the prior art as exemplified by the U.S. Patents 3,449,055 to L. C. Blackett, 3,644,074 to P. J. Cade and 3j709,783 to J. S.
Warren, in which the energization of the fuel valves of the system is dependent upon the sequential operation of relays.
In the systems disclosed in the patents referenced above, the energization of the pilot valve is effected in response to the operation of a control relay which can be energized only if the flame relay is deenergized. Thereafter, the energization of the main valve is effected in response to the operation of a flame relay when a pilot flame is established, but only if the control relay is energized.
While such interlock circuits guard against the welded contact failure referred to above, it appears that the a~

control (or flame) relay may be energized inadvertently following a failure of a sol:id sta-te con-troL device of the electronic circuits, allowing the main valve to operate in the absence oE a pilo-t flame. ~lso, in the patented systems, the flame sensing circuit is energized in response to the operation of a control device, such as a thermosta-t. Thus, in the event of a leak condition for a fuel valve of the system, which permits a flame to remain established after the system is deactivated, it would appear that under certain failure conditions, the flame sensing circuit would be ineffective to lockout the system before the control relay operated.
In the U.S. Patent 3,840,332 to Philip J. Cade, there is disclosed an automatic fuel ignition control system which effects lock out of the system whenever a flame is provided at a burner apparatus before the end of an ignition timing interval. However, the system will also be locked out following a loss of flame during a heating cycle or in the event of a line voltage interruption of a very short duration, ~herein the pilot flame is not extinguished before po~Jer is restored.
In my Canadian Patent Application Serial No. 26~,004 filed August 27, 1976, there is disclosed a proven pilot fuel ;
ignition system including a control arrangernent which provides ;
an interlock on start-up to prevent the energization of fuel valves of the system under certain failure conditions. The control arrangement also detects a leak condition for pilot and main valves of the system, effecting shut down of the system for a leak condition for either valve, but permitting recycling of the system following a momentary power loss or a flame out condition.

The control arrangement illcludes a delay circuit which effects operation o:E a control relay to allow energiza-tion of the pilo~ valve and a flame sensing circuit after a first delay following -the activation of -the system. If a fLame is established a-t -the time the Elame sensing circuit is ener-gized, the flame sensing circu.it causes the system to be locked out. Such tlming control is afforded by a timing circuit of the flame sensing circuit whi.ch requires absence of a flame for a time after the 1ame sensing circuit is energized before a flame relay of ~he flame sensing circuit is allowed to operate.
Under normal condi~ions, the delay circui-t is disabled under the control of the flame relay and effects energization of the main valve after a second delay interval after the pilot flame.
is established. The flame sensing circuit prevents operation of the main valve if a main burner flame is sensed duri.ng the second delay interval. An interlock circui-t prevents enabling of the delay circuit and the control relay at start up for certain failure conditions, including welded contacts of the flame relay.
SU~MARY OF T~IE INVENTIOrl The present invention has provided a control circuit for use in a fuel ignition system of the pilot ignition type which provides an interlock on start-up to prevent energization of fuel valves of the system under certain failure conditions.
The ~ystem includes a pilot valve operable when energized to supply fuel to a pilot outlet for ignition to establish a pilot flame, a main valve operable when energized to supply fuel to a burner apparatus for ignition by the pilot flame, and a control circui-t which controls the opera-tion of the fuel valves.
The control circuit inclucles in-terlock means in~
cluding Eirst switching means operable when enabled -to energize the pilot valve, flame sensing means for causing a second switching means to be enabled whenever a pilot flame is pro-vided, and delay means responsive to the second swi-tching means to effect the energization oE the main valve at the end oE a time interval, the flame sensing means preven~ing the energi-zation o-f the main valve and causing the pilot valve to be deenergized if a flame is provided at the burner apparatus during the time interval, indicative of a leak condition for the main valve. Also, the flame sensing means controls the second switching means to prevent the first switching means from being enabled, ~ocking out the system, if a pilot flame is provided at the onset of the trial for ignition interval, indicative of a leak condition for the pilot valve.
More specifically, in accordance with a disclosed embodiment, the pilot valve and the main valve comprise a redundant valve structure in which fuel is supplied to the main valve through the pilot valve only when the pilot valve is operated. The pilot valve is energized by the first switching means of the interlock means, which in turn is energized over a first circuit path in response to an activate meansl such as a thermostatically controlled switch, and operates to complete a second circuit path for energizing the pilot valve. The first switching means is energized over the first circuit path during a trial for iynition period def~ned by a timeout device of -the interlock means. The timeout device is operable to deenergize the pilo-t valve of -the redundant valve assembly if a pilo-t flame Eails -to be es-tablished during the trial for ignition interval, providing 1~0% shut oEE of fuel. The -timeou-t device is initially energized over -the first circuit path, and if such pa-th is in-terrupt:ed at start-up, the system cannot be activa-ted.
When a pilot flame is establishecl, the Elame sensing means, which is continuously energized, responds to the Elame to enable the second switching means which connects the delay means to the second circuit path for energization. ~he delay means includes third switching means which is operable when enabled to connect the main valve to the second circuit path for energization, and an enabling means which delays the enabling of the third switching means for a predetermined time after the delay means is energized. If a flame is provided at the burner apparatus during such delay interval, as due to a leak condition for the main valve, for example, the ignition cycle is terminated before the main valve is allowed to operate, and the system is locked out.
In accordance with the invention, the switching means which may comprise relays or some other contact operating switch device, afford a contact interlock protection which guards against unsafe failures including component failures, welded contacts, or a leak condition for either one of the valves. If for any reason the first switching means fails to operate before either the second or third switching means operates, a start-up of an ignition cycle i.s prevented, and ~5~

bo-th valves are maintainec1 deenerglzed. Also, once an ignition cycle is started, the system may be lockecl out, under certain failure conditions including a leak ~ondition for the main valve.
DESCR~PTION OF THE DR~WING
The single Figure is a schematic circui-t diagram for a control circuit provided by the present invention for use in a fuel ignition con-trol system of the pilot ignition type.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to the drawing, there is illustrated a schematic circuit diagram for a fuel ignition control circuit 10 provided by the present lnvention. The control circuit 10 is described with reference to an application in a heating system of the pilot ignition type which includes a pilot valve 12 and a main valve 14. The pilot valve 12 and the main valve 14 may comprise a redundant valve assembly in which fuel is supplied to the main valve 14 through the pilot valve 12, and thus only when the pilot valve 12 is operated. One redundant valve structure suitable for this application is disclosed in my Canadian Patent 1,044,130 issued December 12, 1978.
The pilot valve 12 is operable to supply fuel to an inlet of the main valve 14 and to pilot outlet 13 for ignition by sparks provided by an igniter circuit 16 to establish a pilot flame in the pro~imity of a burner apparatus 15. ~he main valve 14 is operable to supply fuel to the main burner apparatus 15 for ignition by the pilot flame. Due to the redundant valve structure, fuel can be supplied to the burner ,;,j apparatus 15 only when both the pilot valve 12 and the main valve 14 are operated.
The pilot valve 12 and the igrliter circuit 1~ are energiæed by an interlock circuit 18 which includes a first switching device or ac-tuator, embodied as a relay Rl, which is operable to complete an energizing pa-th for a solenoid 17 of the pilot valve 12 and the igniter circuit 16. The relay Rl is energized during a trial for igni-tion period initiated by the closing of thermostati.cally controlled switch contacts T~IS in response to a request for heat. The interlock circuit 18 in-cludes a timeout device, embodied as a warp switch ~S, which defines the trial for ignition peri.od and is operable to de-energize the pilot valve 12 of the redundant valve assembly if a pilot flame fails to be established during the trial for ignition period, thereby terminating the ignition cycle and providing 100% shut off of fuel to the burner apparatus 15.
The warp switch WS also enables an alarm device 11 to indicate :
that the system is locked out. :
The main valve 14 is energized under the control o~ a flame sensing circuit 20 and a delay circuit 22. When a pilot flame is established, the flame sensing circuit 20 responds to the flame to operate an associated second switching device or actuator, embodied as a relay R2, which completes an energizing path for the delay circuit 22. The delay circuit 22 includes a third switching device or actuator, embodled as a relay R3, which is operable to complete an energizing path for the main valve 14. The delay circuit 22 also includes an enabling circuit 24, which delays the enabling of relay R3 for a , predetermined time after -the delay circui-t 22 is energized. If a flame ls provided at the burner apparatus 15 during such delay interval, as due to a leak condition for -the main val~e 14, the ignition cycle is terminated before the ma:in valve 1~1 is allowed to operate, and -the system is locked out.
In accordance with the invention, relays Rl-R3 afford a contact interlock pro-tection which guards against unsafe failures including component failures, welcled relay contacts, or a leak condition for either one of the valves. If for any reason relay Rl fails to operate be~ore relays R2 or R3 operate, a start-up of an ignition cycle is prevented, and both the pilot valve and the main valve are maintained deenergized.
Also, once an ignition cycle is started, the system may be locked out, under the control of the warp switch WS, in the event of certain failure conditions including a leak condition for the main valve 1~.
More specifically, referring to the interlock circuit 18, the interlock relay Rl and a heater element 21 of the warp switch WS are energized over a circuit path which includes normally closed contacts R2A and R3A of relays R2 and R3, respectively. Thus, relay Rl and the warp switch heater 21 can be initially energized only when relays R2 and R3 are deenergized. When energized, relay Rl closes contacts RlA to complete an energizing path for the pilot valve solenoid 17 over normally closed contacts WSA of the warp switch WS, per-mitting the pilot valve 12 to open to supply fuel to the pilot outlet 13. The igniter circuit 16 is also energized to provide sparks for igniting the pilot fuel. Relay Rl also closes contacts RlB to prepare a holding pa-th to permit the relay Rl and -the warp switch heater 21 to be maintained energized when relay R2 opera-tes to open contacts R2A, contacts Rl~ bein~
connected in shunt wi-th contacts R2A.
Relay R2 is operated under the control of the flame sensing circui.t 20 when a pilot flame is es~ablished, to open contacts ~2A and to close contacts R2s~ which complete the holdi.ng path for relay Rl, the warp switch heater 21 being maintained energized over normally closed contacts R3A of relay R3, which is deenergized at this time. Relay R2 also closes contacts R2C to energize the delay circuit 22 and opens contacts R2D to disable the igniter circuit 16.
The flame sensing circuit 20 includes a controlled switching device, embodied as a programmable unijunction transistor 40, which together with associated timing networks 41 and 42, effect the enabling of a f~lrther controlled switching device, embodied as a silicon controlled rectifier 44, which controls the ope~ation of relay R2. The timing, or anode con-trol network 41, which includes a timing capacitor 46, deter-mines the potential at the anode electrode of the PUT device40. The timing, or gate control network 42, which includes redundant timing capacitors 48, determines the potential at the gate of the PUT device 40.
The PUT device 40 is enabled whenever the potential at its anode exceeds the potential at its gate by -~.6 volts.
Capacitor 46, which determines the potential. at the anode of the PUT device 40, is periodically charged over a resistor 47 by an AC signal which is continuously supplied to the flame sensing circuit 20 over an isolation transformer Tl.

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In the absence of a flame, capacitors 48 remain essen-tially -unchargedr such that the enabl.ing o.E the PU~ device 40 is effectively controlled by the charging of capacitor 46. When the PUT device 40 is enabled, capacitor 46 discharges into the gate of the SCR device 44. F~owever, in the absence of a flame, the discharge current is insufficien-t to enable the SCR
device 4~, and the re].ay R2 remains deenergized.
For the purpose of permitting relay R2 to be ener-gized when a flame is established, the flame sensing circuit 20 further includes a flame sensor, embodied as a sensing elec-trode 55, which is located in the proximity of the burner ap-paratus 15 and the pilot outlet 13 in a spaced relationship with the burner apparatus 15 defining a gap 56.
When fuel supplied to the pilot outlet or main burner apparatus is ignited, the flame bridges the gap 56, establishing a charging path for the capacitors 48, permitting the capacitors 48 to charge while capacitor 46 is charging.
Although capacitor 46 charges at a faster rate, the charging or capacitors 48 raises the potential at the gate of the PUT
device 40 so that a longer time is now required before the potential at the anode exceeds the potential at the gate by +0.6 volts. Thus, when the PUT device 40 is enabled, the capacitor 46 has been charged to a value which provides suf-ficient discharge current to enable the SCR device ~4, causing ~ :
the relay R2 to be energized.
It is pointed out that since the flame sensing cir-cuit 20 is continuously energized, relay R2 is operated whenever a flame is provided at the pilot outlet 13. Thus, ~ 6~
~hen the con-tro] circuit is deactivated by the opening of contacts THS at the end of each heating cycle, re:Lay R2 will remain operated with contac-ts R2A open in the event of a leak condition Eor -the pilot valve 12 which permits a Elame to re-main established a-t the pilot outle-t when the pilot valve is deenergized. Accordingly, when con-tacts THS close on -the next call for heat, the energizing path for relay Rl is interrupted and the system cannot restart.
The flame sensing circuit 20 further includes an oversignal clamping circuit 50, including transistor 51 which is normally disabled by normally open contacts R3B of relay R3.
Whenever a large flame is provided at the main burner appara-tus 15 before relay R3 operates, indicative of a leak condition for the main valve 14, the oversignal clamping circuit 50 causes the PUT device 40 to be maintained cutoff, causing deenergiza-tion of the relay R2 and allowing the warp switch ~S to effect the shut down of the system.
As indicated above, when relay R2 operates, its contacts R2C close to enable the delay circuit 22 and its con- ;
tacts R2D open to disable the igniter circuit 16. Brieflyj as described in more detail hereinafter, the igniter clrcuit 16 is of the capacitor discharge type, having a capacitor 62 which is perlodically charged by an ~C signal supplied over nor-mally closed contacts R2D of relay R2, and a controlled s~itching device, embodied as a silicon controlled rectifier 63, which is operable to cause the capacitor 62 to discharge over an ignition transformer T2 to effect the generation of spar~s between ignition electrodes 68. The ignition electrodes 68 are located adjacent to the pilot outlet 13 to ignlte the pilot fuel emanating therefrom.
When relay R2 operates -to open con-tacts R2D, the capacitor 62 ls permi-tted to be charged periodically for a time determined by a timing network 70 including a t:imlng capacitor 71, whereby the igniter circuit 16 continues to pro-vide sparks for a ti.me after relay R2 opera.-tes.
Referring to the delay circuit 22, the enabling cir-cuit 24 includes a controlled switching device, embodied as a programmable unijunction transistor 25, which is operable when enabled to complete an energizing path :Eor relay R3. The enabling circuit 24 further includes a timing network 26 in-cluding a capacitor 27, which controls the enabling of the PUT
device 25. The capacitor 27 is permitted to charge in response to activation of the delay circuit 22 following the closing of contacts R2C of relay R2 and, after a delay established by the charying time of capacitor 27, the PUT device 25 is enabled, causing relay R3 to operate to close contacts R3C, effectiny the enabliny of the main valve:14~ Also, contacts R3A open to deenergize the warp switch heater 21 and contacts R3B close enabling the o~ersignal clampiny circuit 50 of the flame sensiny circuit 20.
Contacts R3A of relay R3 permit the warp switch .,.
heater 21 to remain energized from the time that contacts THS close until after the delay established by the enabling circuit 24 at which time relay R3 operates to energize the main valve 14. Thus, in the event of an unsafe failure which prevents the operation of the relay R3 before the heatiny time ~ D~

of the warp switch heater 21, -the warp switch WS operates to open contacts WSA, deenergizing the pilot valve 12, eEfecting 100~ shut of of Euel supply to the burner and causing the system to be locked out. The warp switch WS also closes con-tacts WSB to enable the alarm device 11 to indica-te the lockout condition for the sys-tem.
Operation Briefly, in operation, when the thermostatically controlled contacts THS close in response to a request for heat, the warp switch heater 21 and the relay Rl are energized over the normally closed contacts R2A and R3A of respective relays R2 and R3. The energization of the warp switch heater 21 permits the system to be maintained activated for a trial for ignition period defined by the heatlng time of the warp switch heater 21. Relay Rl operates to close contacts RlB to prepare a holding path for the relay Rl and the warp swltch heater 21, and to close contacts RlA to complete an energizing path for the pllot valve 12 whlch operates to supply fuel to the pllot outlet 13. The igniter circuit 16 is also energized to generate sparks for igniting the fuel supplied to the pilot outlet 13.
When a pilot flame is establishe~ during the trial for ignition period, the flame sensing circuit 20 causes -the relay R2 to operate to close contacts R2C to energize the delay circuit 22 and to open contacts R2D to disable the igniter circuit 16. Relay R2 also opens contacts R2A inter-rupting the energizing path for the relay Rl and closes con-tacts R2B to complete a holding path for relay Rl.

After the t.ime delay es-tablished by the delay circuit 22, relay R3 is energlzed closiny contacts R3C to perm.it the main valve 1.~ to operate, and openlng contacts R3A to deenergize the warp switch heater 21. When the heating demand has been met and contacts TMS open, relays Rl and ~3 are deenergized and the fuel valves drop out, interrupti.ng the supply of fuel to the burner and the pilot outlet, causing the Elame -to be ex-tinguished. The flame sensing circuit 20 responds to the loss of flame to deenergize relay Rl which releases opening contacts RlA, and the circuit 10 is ready for the next heatlng cycle.
If a pilot flame fails to be established during the trial for ignition period, the warp switch WS operates to open contacts WSA deenergizing the pilot valve 12 thereby interrupting the supply of fuel to the pilot outlet 13 as well as to the main valve 14, causing the system to become locked out. The warp switch WS also closes contac-ts WSB to enable the alarm device 11 to indicate that the systern is locked out.
If relay R2 remains operated following the deactiva-tion of the system, such as due to a leak condition for the pilot valve 12 which permits a flame to remain established when the pilot valve 12 is deenergized, or for a malfunction o~ the flame sensing circuit 20 which causes rela~ R2 to be opera~ed in the absence of a Elame, then contacts R2A will remain open, preventing start-up on the next call for heat.
Also a leak condition for the main valve 1~ will permit fuel to be supplied to the main burner 15 before the main valve is energi~ed. Under such conditions the fuel wi.ll be ignited by the pilot flame causing the oversignal clamping circuit 50 to 9~
e~fect -the disabllng or relay R2, preventing -the enahling of relay R3. Accordingly, the warp swi-tch WS will -time out, de-activating the control circuit 10 with the pilot valve 12 deenergized, providing 100% shut off of fuel.
De-tailed Descrip-tion_ Considering the fuel ignition control c:ircui-t 10 in more detail, -the circui-t 10 has a pair of inE~ut terminals 81 and ~2 which are connec-table -to a 2~ V~C source for supp~ying power to the circuit 10. Terminal 81 is connec-ted over nor-mally open contacts THS and over normally closed con-tac-ts R2A
of relay R2 to a conductor Ll, and terminal 82 is connected to a conductor L2. The 24 VAC energizing signal may be supplied to the control circuit 10 over a -transformer T3, having a primary winding 85 connected to a source of power and a secon~
dary winding 86 connected across terminals 81 and 82.
Referring to the interlock circuit 18, the heater element 21 of the warp switch WS and an operate coil 34 of relay Rl are connected in series between conductors Ll and L2 over normally closed contacts R3A of relay R3 ~or energization in response to the closing of contacts THS and whenever relays R1 and R3 are disabled~ Relay Rl has normally open contacts RlB connected in a shunt path with contacts R2A to prepare a holding path for relay Rl when relay R2 operates to open con-tacts R2A, Relay Rl also closes contacts RlA to extend power to a conductor Ll' to permit energization of the pilot valve 12l the de]ay circuit 22 and the igniter circuit 16.
The operate coil 17 of the pilot valve 12 is connected over normally closed contacts WSA of the warp switch WS between .
~ . :
, .

conduc-tors Ll' and L2 and is energized whenever contacts Rl~
close, permi-tting -the pilot valve 12 to opera-te to supply Eue:l.
to the pilot outlet 13 and to the inle-t of the main valve 1~.
The warp switch WS also has normally open contacts WSB con-nected in series with the alarm device 11 between conductors Ll and 1,2.
The opera-te coil 19 of the main valve 14 is connected over normally open contacts R3C of relay R3 between conductors Ll' and L2 and is energized when relay R3 operates to operate the main valve 1~, supplying fuel to the main burner apparatus lS for ignition by the pilot flame.
Considering -the delay circui-t 22, the PUT device 25 has an anode control network including the operate coil 32 of .
relay R3 and a resistor 29, which serve as a voltage divider circuit to establlsh a potential at the anode of the PUT device 25 when contacts R2C close to extend AC power from conductor Ll' to the delay circuit 22. The operate coil 32 and resistor 29 are connected between conductors Ll' and L2 in a series circuit with normally open contacts R2C and a diode 31, which extends from conductor Ll' over contacts R2C, diode 31, and the winding : 32 to the anode of the PUT device 25, and over resistor 29 to conductor L2. A capacitor 30 is connected in parallel with winding 32 and resistor 29.
The PUT device 2S has a gate control network including capaci-tor 27 and resistor 28 which together with diode 31 form a unidirectional series charging path for capacitor 27 which ex-tends from conductor Ll' over contacts R2C, diode 31, capa-citor 27 and resistor 28 to conductor L2. The junction of capacitor 27 and rcsistor 2~ is connected t.o the gate oE the PUT device 25. The cathode o.E the PUT device 25 is co:nnec-ted directl.y to conductor L2, and -thus, the PUI' dev:ice 25 has i-ts anode-ca-thode circuit connected in series ~:i-th the opera-te coi:L 32 of relay R3 and diode 31 between conductoîs Ll' and L2 and is operable when enab].ed -to effect the energization of the relay R3. When energized, relay R3 closes contacts R3C to energize the main valve 14 and opens contacts R3A to deenergize the warp switch heater 210 Relay R3 also closes contacts R3B
to enable the over signal clamping circuit 50.
The PUT device 25 is enabled when the potential at its anode exceeds the potential at its gate by +0.6 volts.
Thus, when capacitor 27 has charged to a value that causes the potential at the gate of the PUT device 25 to be 0.6 volts less than the potential at the anode of the PUT device 25, the PUT device 25 is enabled, energizing the relay R3. Thereafter, the PUT device 25 is enabled durlng each positive half cycle of the AC signal, the relay R3 being maintained energized during the negative half cycles by capacitor 30.
Referring to the igniter circuit 16, capacitor 62 is charged and then discharged over the primary winding 74 oE the ignition transformer T2 during alternate half cycles of the AC
signal, to provide sparks between a pair of ignition electrodes 68 which are connected -to the secondary windiny of the trans-former T2. The ignition electrodes 68 are disposed adjacent to the pilot outlet 71 is a spaced relationship, providing a gap therebe-tween.
The igniter circuit 16 includes a voltaye doubler circuit including capacitor 6~ which suppLied a voltage to capacitor 62, enabling the capacitor 62 -to be charged to ap-proximately -twice the line voltaye supplied over conc1uctors Ll' and L2. Capacitor 6~ has a charying path which extends from conductor Ll' over a diode 65 and the capacitor 64 -to conductor L2. Capacitor 6~ is charged when conductor Ll' is positive relative to conductor L2 during positive half cycles of the AC line signal.
Capacitor 62 charges during negative half cycles of the AC line siynal, that is when conductor L2 is positive with respect to conductor Ll', over a path which extends from con-duc-tor L2 over capacitor 64 and a resistor 66 to one side of capacitor 62 at point 91, and from the other side of the capacitor 62 at point 92 over a diode 69 to conductor Ll'.
The SCR device 63 has its anode connected to conduc tor L2 over the primary winding 74 of transformer T2, resis-tor 66, and capacitor 64, and its cathode connected to conductor Ll' over diode 69. When relay R2 is deenergi2ed, a gate control circuit is provided for the SCR device 63 over normally closed contacts R2D of rela~ R2, a resistor 76, diode 77, and resis-tor 78 which are connected between conductor Ll' and point 92.
Capacitor 71 of the timing network 70 is connected in shunt with contacts R2D and resistor 76 to provide an alternate gate control circuit for the SCR device 63 when relay R2 is operated.
The gate of the SCR device 63 is connected to the junction of the cathode of diode 77 and resistor 78 at point 93 to permit the SCR device 63 to conduct whenever the potential at point 93 exceeds the gate threshold of the SCR device 63.

The flame senslng circuit is con-tinuously energi~.ed by an AC signal sup~plied to conductors L4 and L5 over -trans-former Tl. The transformer Tl has a primary winding 83 havlny one end connected over a conduc-tor L3 to -term.inal 81, ancl having its other end connec-ted -to conductor L2. The trans-ormer Tl has a secondary wlnding 8~1 connec-ted be-tween con-ductors L4 and L5.
The anode control network 41 :Eor the PUT device 40 includes capacitor 46 and a resistor 47 which are connected in series between conductors L4 and L5, providlny a chargincJ path for the capacitor 46. The anode of the PUT device 40 is con~
nected to the junction of resistor 47 and capacitor 46. A
diode 43 which is connected in parallel with capacitor 46, provides a bypass path for capacitor 46 during negative half cycles of the AC signal.
The gate control network 42 includes capacitors 48 and resistors 58, 49 and 59. Resistor 49 is connected between the yate of the PUT device 40 and conductor L5, and resistor 59 is connected between the gate of the PUT device 40 and -the ground point for the control circuit 10 a-t point 94~ which is also connected to conductor L2. Capacitors 48 are connected in parallel between point 94 and conductor L5 and a bleeder resistor 58 is connected in parallel with capacitors 48.
The flame sensing electrode 55 and resistor 54 form a portion of the gate control network 42 for the PUT device 40.
The sensing electrode 55 is connected over resistor 54 to conductor L4, with the electrode 55 being positioned in a spaced relationship with the ground reference point 94 for the 6~

control circuit lO, normally prov.iding a hiqh :resistance path, virtually an open circui-t, between conductor l,4 and the reference point ~. As indicated above, the ground reference point ~4 may be a metallic ground pro~ided by the. burner apparatus 15.
The sensing el.ectrode 55 i5 locat.ed adjacent to the pilot outlet 13 and one end of the burner 15 in -the region in which a 1ame is to be provided such that either a pilot or main burner flame bridges the gap 56 between the el.ectrode 55 ..
and the reference point 94, thereby lowering the resistance of the circuit path including resistor 54 over the electrode 55 between conductor L4 and the reference point 94 whenevex a flame is established. Thus, whenever a flame bridges the gap 56, the high resistance charging path is provided for capaci-tors 48, allowing the capacitors 48 to charge.
The cathode of the PUT device 40 is connected to the ..
gate of the SCR device 44 and over redundant resistors 39 to conductor L5. Thus, whenever the PUT device 40 is rendered conductive, a discharge path is provided for capacitor 4~ over the anode-cathode circuit of the PUT device 40 to the gate of the SCR device 44.
The PUT device 40 is rendered conductive whenever the -~
potential at its anode exceeds the potential at its gate by +0.6 volts, as determined by the action of networks 41 and 42.
For the condition where a pilot flame is not established~ the PUT device 40 conducts early in the positive hal.f cycles of the AC signal and before capacitor 46 is charged to a value sufficient to effect enabling of the SCR device 44 When a pilot flame is established, -the PUT device 40 conducts a-t a la~er time during -the ~ositlve llalf c~cles ~hen the capacitor 4& is charged to a value which is sufficien-t to render the SCR
device ~4 conductive.
The SC~ device 4~, which controls the energization of the relay R2, has l-ts anode connected -to one side of -the operate winding 45 of the relay R2, the other side of which is connected to conductor L4. The cathode of the SCR device 44 is connected to conductor L5 so -that when the SCR device 44 is enabledr the opera-te winding 45 of relay R2 is connected be-tween conductors L4 and L5, permit-ting the relay to oper~te.
The P~T device 40~ which controls the enabling of the SCR device 44, is pulsed into operation, providing an enabling pulse for 5CR device 44 ~or each cycle of the ~C signal during the tria]. for ignition interval. During the portion of the AC
cycle when the SCR device 44 is non-conducting, the relay R2 is maintained energized by capacitor 57 and free-wheeling diode 57' which are connected in parallel with the operate winding 45 of relay ~2.
Considering the over-signal clamping circuit 50, ;
transistor 51 is operable when enabled to limit the voltage swing at the gate of the PUT device 40 to a predetermined value whenever a flame is provided at the burner 15. The base of transistor 51 is connected to the junction of resistors 52 and 53, which are serially connected from the anode of the PUT device 40 to conductor L5, ~orming a vol-tage divider at the base of transistor 51, with the base potential being determined by the charging of capacitor 46. Transistor 51 has ~ 3~

i-ts collec-tor-emitter circuit connec-ted be-tweerl the gate of the PUT device ~0 and conductor L5 over norma:L:Ly open contacts R3B of relay R3. Whenever relay R3 is operated, then when the base-emi-tter turnon potential is exceeded, the transistor 51 cond~lcts, clamping the gate to PUT device 90 -to the potential on conduc-tor L5. The action of the over-signal clamping cir-cuit ~0 prevents pre-ma-ture Eiring of the E~UT device 40 when a main burner flame bridges the gap 56, which would otherwise result in the deenergization of relay R2.
Relays R2 and R3 are double-pole, double-throw relays. For relay R2, contacts R2A and R2B employ a common armature of the relay such that whenever contact R2A is closed, contact R2B is open. Also, should contact R2B become welded, contact R2A cannot reclose thereby preventing energization of the interlock circuit 1~ on the next call for heat.
In relay R3, contacts R3A and R3C employ a common armature such than when contact R3A is closed, contact R3C is open. If contact R3C becomes welded, contact R3A cannot re-close thereby preventing energization of the interlock circuit 18 on the next call for heat.
Operation When a 24 VAC power signal is applied to terminals 81 and 82 of the control circuit 10, the flame sensing circuit 20 is energized over transformer Tl o When contacts THS are open, and in the absence of a flame at the pilot outlet or the main burner, relays Rl-R3 are unoperated and the valves are deenergized.
When contacts THS close in response to a request for ; -2~-$~L

heat, the 2~1 VAC signal is appl.ied to concluctors Ll and L2 over normally closed contacts ~2A o~ relay R2 allcl since con-tacts ~3A are closed, hea-ter 21 of the warp switch ~S is energized, ini-tiating a -trial for igni-tion i.nterval. The interval, typically 15 to 20 seconds, is defined by -the heating time of the warp switch heater 21. Also, relay R1 operates closing contac-ts RlA, energizing the opera-te coil 17 of the pilot valve 12 of the redundant valve assembly, which opens -to supply fuel to an inlet of the main valve 14 and -to the pilot burner 13. The igniter circui-t 16 is also energized. Further~
con-tacts RlB close to prepare a holding path for relay Rl in shunt with contacts R2A.
With reference to the igniter circuit 16, when line L1 ' is positive relative to line L2, capacitor 64 is charged over diode 65 to a voltage of approxima-tely 37 volts. ~hen line L2 becomes positive relative to line I,l' during the nex-t ~:
negative half cycle of the AC line signal, capacitor 62 is charged over capacitor 64, resistor 66 and diode 69, with the charge on capacitor 64 being transferred -to capacitor 62, 20 such that capacitor 62 is charged to approximately 74 volts.
During the next half cycle when line Ll ' is again positive relative to line L2 and the AC signal begins to decrease from its maximum value, the voltage on capacitor 62 is greater than the supply voltage, permitting current to flo~ from the positive side of the capacitor 62 at point 91 through resistor 66, capacitor 64, the secondary winding 86 of the input trans-former T3, and over contacts R2D, resistor 76, cliode 77 and resistor 78 to the other side of capacitor 62, establishing a ~25-positive voLtage at the gate oE the SCR device 63 which then conducts.
Capacl-tor 62 then discharges over the primary winding 7~ of the transformer T2 and the SCR device 63, inducing a voltage in -the secondary wlnding 75 of the trans-former T2, ac-tiva-ting -the electrodes 68 -to generate an igni-tion spark. The igniter circuit 16 continues to operate in the manner described above, providing sparks until the relay R2 is operated in response to the flame sensing circuit when the ~uel is ignited.
When relay R2 operates and opens contacts R2D, capacitor 62 is charged and dischaxged, initially, over the timing capacitor 71. That is, when the voltage on capacitor 62 becomes greater than the supply voltage during the positive half cycles of the AC line signal, current flows from the positive side of the capacitor 62 through resistor 66, capacitor 6~, the secondary winding 84 of transformer T3, capacitor 71, diode 77 and resistor 78 to the o'her side of capacitor 62, proviaing a turnon voltage for SCR device 63 for permitting capacitor 62 to discharge over the ignition transformer T2 to generate a spark. The sparking continues until capacitor 71 becomes fully charged a-t which time current flow ceases and the SCR device 63 is not triggered, inhibiting further spark generation. In one em~odiment, the component values were selected to allow the igniter circuit 16 to provide sparks for a period of ten seconds after the operation of relay R2.
When power is applied to the flame sensing circui-t 20, and in the absence of a flame, then when conductor L4 -26~

~3$~

star-ts -to SWincJ positive, current fl.ows :E:rom concluctor :L.4 over resis-tor 47 and capacitor 46, charging the capac:i~or ~6. Ca-pacitor 46 charges durincJ each posi-tlve half cycle of khe AC
signal, providing an increasing potent:ial a-t the anode of the PUT device 40. In ~he absence o a pilot :~lame, capacitors 48 remain clischarged, and the PUT device 40 co:nducts early in the positive half cycles of the AC signal and be:Eore capacitor 46 has charged to a value sufficient to trigger the SC~ device 44 into conduction. Thus, relay R2 is maintained unoperated.
If a flame fails to become established during the trial :Eor ignition in-terval, the warp swi-tch WS deenergizes the pilot valve 12 locking out the system, and enables the alarm device ll.
When a pilot flame is established be~ore the warp switch WS operates, then during the next positive half cycle of the AC signal applied between conductors L4 and L5, when con-ductor L4 swings positive, current flows from conductor L4 through resistor 54, over sensing electrode 55 and the pilot flame to point 94, and over capacitors 48 to conductor L5, charging the capacitors 48. The voltage across capacitors 48 at point 94, which is connected over.resistor 42 to the gate of the PUT device 40, establishes a gate potential for the PUT
device 40.
During the same half cycle, capacitor 46 is charged over a path extending from conductor L4 over resistor 47 and capacitor 46 to conductor L5, establishing an anode potential for the PUT device 40. The values of capacitors 48 and 46 are selected such that some time before the peak of the AC line signal, during the first half cycle thereof, the anode to gate potential of -the PIJT device 40 exceeds -~0.6 volts so that the PUT device 40 conducts, per.~itting capaci.-tor 46 to dis-charge over the PUT device 40. At such time, capaci-tor 46 is charged to a voltage suf:Eicient to eEfect the enabling of -the SCR device 4~.
When SCR 44 conducts~ the operate coil 45 o:E relay R2 is energized, and relay R2 operates to close contacts R2C
to energize the delay circuit 22, and to open contacts R2D to disable the igniter circuit 16. Also, contacts R2A open and cont~cts R2B close so that relay Rl remains energized over the holding path over its contacts RlB, resistor 33 and contacts R2B.
Once the pilot flame has been established, the flame sensing circuit 20 provides enabling pulses to the gate of the SCR device 44 during positive half cycles of the AC signal.
During negative half cycles, the SCR device 44 is cutoff, and relay R2 is maintained energized by the energy stored in the relay magnetic field, resulting in current flow through the "free-wheeling" diode 57' and relay coil ~5 as the magnetic field decays.
When contacts R2C close, the 24 VAC signal supplied to conductor Ll' is e~tended to the delay circuit 22. During positive half cycles of the AC signal, current flow over diode 31, capacitor 27 and resistor 28 charges capacitor 27.
Initially the potential at the gate of the PUT device 25 is sufficiently greater than the potential at the anode of the PUT device 25 established by the voltage divider formed by the operated winding 32 of relay R3 and resistor 29. Thus, the PUT

device is maintained cut off. As capacitor 27 charges during successive cycles oE the ~C signal, the potential a-t the ga-te of the PUT device 25 decreases. ~he -time constant of resistor 28 and capacitor 27 is selected to provide a delay of about three seconds beEore the PUT device 25 is enabled to effect the operati.on of relay R3.
During the delay for the delay ci:rcuit 22, a check is made for a leak condition for the main valve 14. After relay R2 operates to enable the delay circuit 22, reIay R3 is maintained deenergized for the three second delay period.
Accordingly, contacts R3B remain open so that the over-signal clamping circuit 50 is disabled. If the main valve 14 is leaking, fuel is supplied to the main burner 15 and is lit by the pilot flame producing a large flame. Such condition re-duces the impedance of the current path over the flame sensing electrode 55 causing increased current flow to the gate of the PUT device 40. Thus, for the condition where such leakage ~
occurs after relay R2 has operated, but before the end of the ~ ~ .
three second delay time, the PUT device 40 is maintained cutoff, causing relay R2 to release. When contacts R2C open, the timing circuit 22 is deenergized keeping relay R3 deenergized. After the heating time of the warp switch heater 21, the warp swltch .
operates contacts WSA and WSB to deenergize the pilot valve 12, providing 100% shut off of fuel, and the alarm device 11 is enabled.
: It is apparent that shou].d the main valve 14 be leaking prior to the energization of the circuit 10, -then when the pilot valve 12 is operated and a pilot flame is established fuel. leaking to the maill burne~ ls lit by t:he pllot Elame, and -the over-signal c:lampinq circuit 50 is effec-tive -to mai:ntain the :PUT device 40 cu-toff. Thus, xelay R2 is deenergized and the sys-tem is deactiva-ted in the manner described a~ove.
Assuming there is no leakage from the main val.ve 14, the dela~ circuit 22 energiæes relay R3 after the three second delay, and contacts R3A open to deenergize the warp switch heater 21, and contacts R3C close to energize the main valve 14.
Also, contacts R3B close to enable the over-signal clamp.ing circuit 50.
When the over~signal clamping circuit 50 is enabled, thenl as capacitor 46 charges during each positive half cycle of the AC signal, the potential at the base of transistor 51 rises, causing the transistor to conduct when the base-emitter turnon potential is reached. When transistor 51 conducts, a discharge path is provided for capacitor 46, which discharges.
As capacitor 46 discharges, the gate potential for the PUT
device 40 decreases until the anode-gate potential is 0.6 volts at which time the PUT device 40 conducts, discharging the capacitor 46 and pulsing the SCR device 44 into conduction. `~-Accordingly, relay R2 is maintained energized.
When the heating demand has been met, contacts THS
open, deenergizing the circuit 10, causing -the main valve 14 and the pilot valve 12 to drop out, and causing relays Rl and R3 to be deenergized. When the main valve 14 and the pilot valve 12 drop out, the main burner flame and the pilot flame are extinguished. ~lowever should a leak develop in the pilot valve 12, then when the circuit 10 is deenergized, the pilot ,: :

flame remains establishecl and xelay R2 is main-tained operated by the flame sensing circuit 20. ~ccordingly, -the next -time the circuit lO is activated in response to operation oE contacts T~IS, contacts R2A are open and the system cannot restar-t. The same effect is provided for a failure of the flame sensing circui-t 20 where the relay R2 is permitted -to be operaked in the absence o a flame.
For a flame out condition, relay R2 drops out deenergizing relay R3 and enabling the igniter circuit 16, and the system recycles as indicated above with the warp switch WS
deenergizing the pilot valve 12 if the flame is not reestab-lished within the timeout of the warp switch.
In the event of a fast line interruption, the interlock arrangement and the delay circuit 22 permit the system to recycle without lockout if a flame is reestablished before the timeout of the warp switch WS. Followin~ such power interruption, and assuming a flame re~ains establlshed, then when power is restored, the relay R2 is operated before relay Rl operates, interrupting the energi~ing path for relay Rl and maintaining the pilot valve deenergized. Thus, fuel cannot flow to the pilot outlet or the main burner and, in the absence of a leak condition for the pilot valve, the flame will be extinguished. The system then recycles as above as soon as relay R2 drops out following loss of flame.
In the event ofa failure condition following a successful start up~ such as the welding together of contacts R2C which control the enabling of the delay circuit 22, then when the circuit 10 is deactivated with the opening of contacts THS, contacts R2~ cannot reclose. Thus, the nex-t time -tha-t contacts THS close, the energiZinCJ path Eor -the interlock circuit 18 is interrupted and -the sys-tem cannot restart.
Should contacts R3C, which control the energ.izing of the main valve 14 become welded, con-tacts R3A cannot reclose and the interlock circuit 18 cannot be energized on the next call for heat. In either case, the pilot valve 12 is maintalned de-energized, preventing fuel supply to either the pilot outlet or the main burner.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a fuel ignition system including a pilot valve operable when energized to supply fuel to a pilot outlet for ignition to establish a pilot flame, and a main valve oper-able when energized to supply fuel to a burner apparatus for ignition by the pilot flame to provide a flame at said burner apparatus, a control circuit comprising interlock means in-cluding first switching means operable when enabled to energize said pilot valve and to prepare an energizing path for said main valve, activate means operable in response to a request for heat to enable said first switching means, second switching means, flame sensing means for causing said second switching means to be disabled in the absence of a flame at said pilot outlet to permit said first switching means to be enabled by said activate means and to be enabled when a pilot flame is established at said pilot outlet to prevent said first switching means from being enabled by said activate means, and delay means responsive to said second switching means to effect the energization of said main valve at the end of a time interval initiated by the operation of said second switching means, said flame sensing means preventing the energization of said main valve in the event that a flame is provided at said burner apparatus during said time interval.

2. A system as set forth in Claim 1 wherein said interlock means further includes timeout means energized in response to said activate means to define a trail for ignition interval and to cause interruption of the supply of fuel to at least said pilot outlet if a pilot flame fails to be established during said trial for ignition interval.

3. A system as set forth in Claim 2 wherein said pilot valve and said main valve comprise a redundant valve structure wherein fuel is supplied to said main valve over said pilot valve only when said pilot valve is operated, said time out means deenergizing said pilot valve at the end of said trial for ignition interval in the absence of a pilot flame to prevent fuel from being supplied to said pilot outlet and said burner apparatus.

4. A system as set forth in Claim 2 wherein said delay means includes third switching means operable when enabled 1: :
to energize said main valve, and enabling means responsive to said second switching means to enable said third switching means, said enabling means including timing means for delaying the enabling of said third switching means for said time inter-val after said second switching means operates.

5. A system as set forth in Claim 4 wherein said flame sensing means includes means for causing said second .
switching means to disable said delay means and to cause said timeout means to deenergize said pilot valve if a flame is provided at said burner apparatus before said third switching means is enabled.

6. A system as set forth in Claim 4 wherein said third switching means is operable when enabled to deenergize said timeout means.

7. A system as set forth in Claim 4 wherein said second and third switching means include means for providing an energizing path for said interlock means to permit enabling of said first switching means, the enabling of said first switching means being prevented whenever said first circuit path is interrupted when said activate means operates.

8. In a fuel ignition system including a pilot valve operable when energized to supply fuel to a pilot outlet for ignition to establish a pilot flame, and a main valve operable when energized to supply fuel to a burner apparatus for igni-tion by the pilot flame to provide a flame at said burner apparatus, a control circuit comprising interlock means in-cluding first switching means operable when enabled to energize said pilot valve, second switching means, flame sensing means continuously energized for causing said second switching means to be disabled in the absence of a flame at said pilot outlet to permit said first switching means to be enabled and for causing said second switching means to be enabled whenever a pilot flame is provided at said pilot outlet to prevent said first switching means from being enabled, and delay means re-sponsive to said second switching means to effect the energi-zation of said main valve at the end of a time interval initiated by the enabling of said second switching means, said flame sensing means preventing the energization of said main valve in the event that a flame is provided at said burner apparatus during said time interval.

9. A system as set forth in Claim 8 wherein said flame sensing means includes a controlled switching device and timing means for controlling said controlled switching device to effect enabling of said second switching means when a pilot flame is established and to prevent enabling of said second switching means in the absence of a pilot flame.

10. A system as set forth in Claim 9 wherein said flame sensing means includes limiting means operable when enabled to permit said controlled switching device to maintain said second switching means enabled when a flame is provided at said burner apparatus, said limiting means being disabled during said time interval preventing said controlled switching device from maintaining said second switching means enabled when a flame is provided at said burner apparatus during said time interval.

11. In a fuel ignition system including a pilot valve operable when energized to supply fuel to a pilot outlet for ignition -to establish a pilot flame at said pilot outlet, and a main valve operable when energized to supply fuel to a burner apparatus for ignition by the pilot flame to provide a flame at said burner apparatus, a control circuit comprising interlock means including first switching means, activate means for enabling said first switching means over a first circuit path to complete a second circuit path to energize said pilot valve, second switching means, flame sensing means for causing said second switching means to complete said first circuit path in the absence of a flame at said pilot to permit said first switching means to be enabled and for causing said second switching means to interrupt said first circuit path whenever a pilot flame is provided at said pilot outlet thereby preventing enabling of said first switching means, and delay means including third switching means and enabling means operable when connected to said second circuit path to enable said third switching means to connect said main valve to said second circuit path for energization thereover, said second switching means connecting said enabling means to said second circuit path after a pilot flame is provided, said enabling means including timing means to delay the enabling of said third switching means for a time interval after said enabling means is connected to said second circuit path, said flame sensing means including limiting means for preventing the energization of said main valve in the event that a flame is provided at said burner apparatus during said time internal.

12. A system as set forth in Claim 11 wherein said interlock means includes timeout means connected in said first circuit path, said timeout means being energized over said first circuit path in response to operation of said activate means to define a trial for ignition interval during which said pilot valve is energized, and to deenergize said pilot valve at the end of said trial for ignition interval in the absence of a pilot flame.

13. A system as set forth in Claim 12 wherein said third switching means comprises a switch device having first normally closed contacts connected in said first circuit path and operable to override said timeout means when a pilot flame is provided during said trial for ignition interval, and second normally open contacts for connecting said main valve to said second circuit path, said first and second contacts employing a common armature of said switch device whereby should said second contacts become welded together, said first contacts cannot reclose, thereby preventing the enabling of said interlock means.

14. A system as set forth in Claim 13 wherein said switch device has third normally open contacts connected to said limiting means to override said limiting means during said time interval and to enable said limiting means after said time interval.

15. A system as set forth in Claim 11 wherein said second switching means comprises a switch device having first normally closed contacts connected in said first circuit path and second normally open contacts for connecting said delay means to said second circuit path, said first and second contacts employing a common armature oil said switch device whereby should said second contacts become welded together, said first contacts are prevented from reclosing thereby pre-venting the energizing of said interlock means.