US3472981A

US3472981A - Pressure responsive protective means for vacuum type circuit interrupters immersed in liquid

Info

Publication number: US3472981A
Application number: US570484A
Authority: US
Inventors: Orin P Mccarty; Francis S Urbanek
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1966-08-05
Filing date: 1966-08-05
Publication date: 1969-10-14
Anticipated expiration: 1986-10-14
Also published as: JPS4512765B1; BR6791240D0; AT282772B; GB1187447A; CH470108A

Description

Oct. 14, 1969 Q p MCCARTY ETAL 3,472,981

PRESSURE RESPONSIVE PROTECTIVE MEANS FOR VACUUM TYPE CIRCUIT INTERRUPTERS IMMERSED IN LIQUID 2 Sheets-Sheet Filed Aug.

INVENTORS. OR/N P. M0 CARTY FRA/vc/s fly/wag BY ATTORNEY Oct; 14, 1 969 RMCCARTY ET AL 3,472,981

PRESSURE RESPONSIVE PROTECTIVE MEANS FOR VACUUM TYPE CIRCUIT INTERRUPTERS IMMERSED IN LIQUID 2 Sheets-Sheet Filed Aug. 5, 1966 Vl EN O S O/P/N P McC/ v; FEM/cw J. UREA/.IEF, 5y m ATTORNEY United States Patent U.S. Cl. 200-144 9 Claims ABSTRACT OF THE DISCLOSURE In a transformer tap changing apparatus, a vacuum type interrupting switch immersed in oil is biased to contact engaging position by differential pressure and provided with a push-off spring to separate the contacts in the event of lossof vacuum. An actuating mechanism providing resilience (as in a wipe spring) or lost motion permits the contacts to be separated by the push-off spring even though the actuating mechanism is in switch closing position.

Our invention relates to means for preventing damage to vacuum type circuit interrupters in the even of significant loss of vacuum while immersed in a body of liquid, and particularly to protective means for liquid-immersed vacuum type circuit interrupters associated with tap changing equipment for electric transformers or the like.

Vacuum type current interrupting devices of high interrupting capacity are useful in electric switches and circuit breakers of all types, and are. presently available in voltage and current ratings capable of interruptingpower flow at voltages commonly used in commercial and industrial distribution and transmission systems. In most applications the vacuum interrupting device is likely to be located in an ambient atmosphere of air, so that even a large leak in the evacuated envelope can do no more than till the envelope with air or gas at ambient pressure. Under such conditions the device may fail to interrupt current, but'the contained air or gas is not likely to build up sufficient pressure to fracture the envelope. In certain special applications it is desirable to immerse a vacuum interrupting device in a surrounding ambient liquid. In such locations itis possible that a leak of appreciable size and time duration will admit to the switch envelope a pool of liquid sufiicient, if heated excessively by an arc, to develop a bursting pressure.

One special application where it is desirable to immerse a vacuum interrupter in a surrounding ambient liquid is in load tap changing apparatus for power transformers, reactors or the like. Such transformers are usually immersed in a dielectric liquid such as oil or askarel, and the tap changing equipment is also liquid immersed. However, the tap changer ordinarily includes an arcing duty current interrupting switch has been located in a separate liquid-filled compartment to avoid contamination of the main body of transformer oil as a result of arcing at the switch. To accommodate the mechanical interconnection of the arcing switch with the transformer contacts and other switches in a tap changing apparatus, it has been customary to locate the entire tap changing apparatus in the separate liquid-filled compartment. This requires that'all the tap leads from the transformer winding be brought out from the main body of the transformer to the tap changing compartment. In high voltage apparatus this in an expensive procedure in both labor and material costs.

"ice

in the main body of transformer oil or other dielectric liquid. This economical arrangement does become possible when the arcing interrupting switch is of the vacuum type. With the entire tap changing apparatus so located, the tap leads need not be brought out of the tank, and no separate tap changing compartment need be provided. Even if a separate compartment is otherwise desirable, a vacuum interrupting device is advantageous in a load tap changer in order to avoid contamination of the oil in that compartment and the consequent frequent maintenance. On the other hand a vacuum interrupting device so located in the dielectric liquid of an associated electrical apparatus should not be permitted to develop any leak sufiiciently large to admit liquid to the switch envelope. Even in the absence of further damage, a fractured interrupter would so contaminate the dielectric liquid that expensive and time-consuming maintenance work would be required.

Accordingly, it is a principal object of our invention to provide improved leak responsive protective means for vacuum type circuit interrupters associated with electric tap changing equipment.

It is another object of our invention to provide leak responsive protective means for preventing recycling of a cyclically operated vacuum interrupting device immersed in liquid.

A further object of our invention is to provide means for preventing the development of excessive pressures as a result of leaks in liquid-immersed vacuum type circuit interrupters associated with tap changing equipment for electric transformers and the like.

Still another object of our invention is the provision of liquid-immersed vacuum type are interrupter apparatus for transformer tap changing equipment having means to prevent fracture of the interrupter in the event of liquid leakage.

It is a particular object of our invention to provide, in an electric tap changing apparatus of the type including a liquid-immersed arcing duty interrupter of the vacuum type, means for disabling the interrupter in open circuit position in the event of a significant loss of vacuum in the interrupter.

In carrying out our invention in one preferred embodiment, we utilize a tap changing apparatus of the type including two movable tap selector switches connected in parallel circuit relation to a common line terminal and adapted to be moved in sequential step-by-step manner from any one tap position to an adjacent position. The parallel branch circuit connections of the selector switches preferably include current limiting reactors and are adapted to be interrupted sequentially prior to movement of each associated tap selector switch. For such interruption we utilize a vacuum type circuit interrupter, either one in each branch circuit or a single interrupter connected to be switched into the branch circuits sequentially during the interrupting interval of each circuit.

A vacuum interrupter itself is usually a normally closed contact device by reason of the fact that the movable contact is connected to a flexible, or deformable, wall portion of the envelope (such as a metallic bellows), and external pressure on such wall portion urges the movable contact to engaged position. In our protective apparatus, however, we provide a push-off or biasing spring for-normally overcoming such pressure bias and providing a net contact bias to open position. In our tapchanging circuit, however, the vacuum interrupter is normally held closed in the rest position of a cyclic switch operator, and between the operating means and the movable contact of the vacuum switch we provide a wipe spring sufficient, when partially compressed, to overcome the net opening bias. In the event vacuum is lost in the envelope to a significant extent, the pressure bias on the flexible wall is diminished and the net opening bias becomes sufiicient to overcome the wipe spring and thereby preclude closure of the vacuum switch contacts.

It will be understood by those skilled in the art that the wipe spring merely adds to the contact pressure in closed position and that, if desired, it may be omitted. In this event, the push-off spring force should normally be less than the closing pressure bias of the bellows so that net bias is in the closing direction. Thus if the pressure bias is diminished due to leakage, the push-01f spring will predominate and open the contacts.

Our invention will be more fully understood and its several objects and advantages further appreciated by referring now to the accompanying detailed specification taken in conjunction with the attached drawing in which:

FIG. 1 is a schematic illustration of a transformer load tap changing equipment including a vacuum type are interrupting apparatus embodying our invention, and

FIG. 2 illustrates a similar tap changing apparatus in which alarm and lockout means are provided for operation upon response of our vacuum loss protective mechanism.

Referring to the drawing, we have illustrated a high voltage auto-transformer of the liquid-immersed type, including a tank or enclosure substatnially filled with insulating oil or other dielectric fluid 11, and having positioned therein a winding 12. The transformer winding 12 is grounded at its low voltage end, as by connection to the tank 10, and at its other end it is connected to a high voltage line conductor 13 which passes out of the tank through an insulating bushing 14. The high voltage end of the auto-transformer winding 12 is provided with a plurality of selectable voltage taps each having a terminal contact 15. A selected one of the tap terminals 15 is adapted to 'be connected through a load tap changing apparatus 16 to a secondary line conductor 17 disposed in an insulating bushing 18. The load tap changing apparatus designated generally by the reference numeral 16 includes a pair of movable tap selector contact fingels 20, 21, associated current limiting

reactors

22, 23, a pair of

selector switches

24, 25, a vacuum type circuit interrupting switch 26 and a motor-driven cycle timer (designated generally as 28) for controlling the operation of the foregoing switches.

The tap selector circuit of the apparatus 16 is of a well-known type. This circuit provides two parallel current paths, or branch circuits, between any selected tap terminal 15 and the common secondary line conductor 17, each parallel path including one of the tap selector fingers 20, 21, in series with one of the

reactors

22, 23, and one of the

selector switches

24, 25. The arcing duty vacuum interrupting device 26 is connected between the parallel branch circuits with the

selector switches

24, 25 in series circuit relation directly across the arcing contacts of the vacuum switch.

The cycle timer 28 is selectively operable in either direction to effect sequential stepping movement of the tap selector fingers 20, 21 progressively in either direction along the group of transformer tap terminals 15. The cycle timer is driven selectively in either direction by a motor 30 through a reversing switch 31 from a suitable source of control voltage shown as a tertiary winding 32 on the transformer 12. When energized the timer starts from a normal rest position and operates for a single complete cycle to eifect a single stepping movement of both contact fingers and 21. The motor energizing circuit includes a normally open starting switch 33 which, for the purpose of illustration, may be considered to be manually operable. The cycle timer includes a motor holding and limit switch 35 connected in parallel circuit relation across the starting switch 33 and arranged to deenergize the motor at the end of a single cycle of operation in either direction.

The cycle timer 28 comprises a pair of cams 40 and 41 arranged to open the

selector switches

24 and 25 respectively in sequential relation in either direction of operation, and a cam 43 arranged to open the arcing contact switch 26 at two spaced-apart intervals during a cycle. A pair of Geneva

gears

44 and 45 driven by the cycle timer is connected to actuate the tap selector fingers 21 and 20 respectively in sequential relation at appropriate times during a cycle of operation in either direction.

A typical single cycle of operation of the tap-changing apparatus described above is as follows. If the starting switch 33 is closed momentarily to energize the motor 30, the cycle timer begins to rotate in a selected direction (assume clockwise) as determined by the reversing switch 31. Immediately upon initiation of operation, the holding and limit switch 35 closes to maintain motor energization for a full cycle. During this cycle of operation the cam 40 first opens the selector switch 24, thereby to substitute the contacts of the arcing contact switch 26 for the switch 24 in the branch circuit through the tap selector contact finger 20 and reactor 22. Following opening of the selector switch 24, the contacts of the arcing vacuum switch 26 are opened by the cam 43, thereby to interrupt current through the tap selector contact finger 20. Thereafter the Geneva gear 45 moves the tap selector contact finger 20 to the next adjacent tap 15 in the selected direction. Following this operation the cam 43 again closes the contacts of the arcing vacuum switch 26, and cam 40 closes selector switch 24, and thereafter the cam 41 opens the selector switch 25. After opening of the switch 25 the cam 43 again opens the arcing vacuum switch 26 thereby to interrupt current through the tap selector contact finger 21 and the reactor 23. While the contact finger 21 is deenergized, the Geneva gear 44 moves that contact finger to the next adjacent tap 15 to which the selector contact finger 20 was previously moved. Thereafter the arcing contact vacuum switch 26 is again closed by the cam 43, and the selector switch 25 is reclosed by the cam 41. This completes a single cycle of operation, and the limit switch 35 thereupon deenergizes the motor 30. It will be evident to those skilled in the art that operation in the reverse direction is entirely similar.

Reference will now be had more particularly to the arcing contact vacuum interrupter 26 connected between the parallel tap selector circuits. This circuit interrupter comprises an evacuated envelope 50 within which are disposed a pair of relatively movable contacts 51, 52. As shown, the contact 51 is stationary and includes a fixed conducting rod or stem 53 which extends through and is sealed to a wall of the envelope 50. The moving contact 52 of the vacuum interrupter is carried upon a rod 55 reciprocably mounted in a bearing 56. To provide a vacuum-tight seal between the moving contact and the envelope 50 a flexible metallic bellows 57 is connected atv one end to the envelope and at the other end to the moving contact rod 55.

The bearing 56 does not provide a hermetic seal, so that the interior of the bellows 57 is exposed to the ambient pressure outside the switch envelope 50. In the example illustrated this is the pressure, usually near atmospheric, of the surrounding liquid body 11. This external pressure, being greater than the very low pressure within the highly evacuated envelope 50, exerts upon the flexible, or deformable, bellows 57 a force tending to expand the bellows and move the switch contact 52 to closed or engaged position. To overcome this force and provide a net opening bias upon the movable contact rod 55, we provide externally of the envelope 50 a compression spring 58 interposed between the envelope 50 and a shoulder 59 on the contact rod 55. The spring 58 is designed to provide an opening force greater than the closing force exerted by the bellows 57. As a specific illustration, a bellows having an end surface of approximately two square inches and thus a closing force of about 30 pounds may appropriately be opposed by a spring 58 which exerts an opening bias of approximately 50 pounds, the range of movement of the spring 58 being relatively small so that its bias force does not change significantly between closed and open positions of the movable contact rod 55.

For moving the vacuum switch contact between open and closed positions, we provide an actuating rod 60 biased to open circuit position by an actuating spring 61 and disposed to be moved between open and closed circuit positions by the cycle timer cam 43. The actuating rod 60 has a lost motion connection with the movable contact rod 55, and a contact wipe spring 62 is interposed between these elements to take up the lost motion in open position and provide a contact wiping pressure in closed position. To this end we have illustrated a cage 63 fixed to the actuating rod 60 and slidable upon the contact rod 55. To limit the lost motion a shoulder 64 on the rod 55 is disposed within the cage 63 and a compression wipe spring 62 is interposed between the cage and the shoulder 64. The wipe spring 62 may be substantially fully extended when taking up the lost motion (i.e., contacts open) but it must exert sufficient force when partially compressed (i.e., contacts closed) to overcome .the net opening bias of the bellows 57 and spring 58. In the example illustrated the normal wiping pressure of spring 62 may be about 40 pounds.

While we prefer to use a wipe spring because it provides more definite opening action upon loss of vacuum, it will be understood that, if desired, it may be omitted. In that even the push-01f spring 58, even when compressed, must exert an opening bias less than closing bias resulting from normal pressure differential on the bellows 57. In such case the net bias on the contact rod 55 is still normally in the closing direction, but if the bellows force is lost due to leakage, the push-01f spring 58 will become effective to separate the contacts. The operating mechanism, of course, must be such that it does not positively hold the contacts in closed position.

The actuating tension spring 61 may have any amount of force suflicient to hold the actuating rod 60 in cam following relation with the cam 43.. Preferably, however, the actuating spring 61 is sufiiciently strong (and the cam 43 or other mechanical release so shaped) that the vacuum switch contacts will be opened at an optimum high speed. A vacuum switch actuating mechanism of this character is more fully described and claimed in a patent application, Ser. No. 570,485 filed on August 5, 1966, by Howard A. Fohrholtz and assigned to the same assignee as the'instant application.

It will now be understood by those skilled in the art thatin the normal operation of the vacuum interrupter tap-changing apparatus which we have illustrated the cycle timer 28 is normally stationary in the rest position illustrated until such time as a tap change is desired, In this normal rest postion the timer cam 43 holds the vacuum switch 26 in closed circuit position against its bias, If in this condition of the apparatus vacuum is lost in the switch envelope 50 to a sufiicient extent to permit contraction of the bellows 57, the vacuum switch contacts 51, 52 will separate. It will be understood that such action will occur only when the differential pressure upon the bellows diminishes (due to increasing pressure within the envelope 50) sufficiently that the net opening bias force (i.e., spring '58 less the opposite bellows force) overcomes the force of wipe spring 62. Since the closing force exerted by the bellows (i.e., the differential pressure) may in the extreme be reduced to Zero, it is apparent that the biasing spring 58 must be able to overcome at least the force of the wipe spring 62, that is, the closing force of the wipe spring 62 must be less than the opening force of the biasing spring 58. If such contact separation occurs in the rest position of the cycle timer 28, current interruption will occur at the selector switches 24, 25 in the next tap-changing cycle. Such interruption, being under oil, will generate gas and the defective operation maybe detected by suitable protective means such as that described and claimed in Patent 3,296,580-McCarthy. In this way repeated destructive arcing in the vacuum switch 26 may be prevented.

The combination of biasing forces applied to the movable contact rod 55 of our vacuum switch 26 is such that if the significant loss of vacuum (i.e., increase in internal pressure) occurs while the contacts 51, 52 are separated, the increased net opening bias described will prevent reclosure of the contacts. In practice this action is most likely to occur if liquid has entered the envelope 50 before the contacts were opened. In such case the internal pressure will be too high to permit interruption of any are drawn between the contacts, and with any liquid present the arc will vaporize liquid to rapidly increase pressure and preclude reclosure.

The leak detection apparatus of our invention need not be sensitive to such relatively small pressure increase in the switch envelope 50 as will preclude arc interruption. In a switch of the type described, a high vacuum of the order of about 10- millimeters of mercury is usually necessary to assure interruption. While a very small leak may raise internal pressure above this value by several orders of magnitude, it is unlikely that fluid will be admitted unless the leak and thus the pressure increase is considerably greater. In our apparatus we intend to detect primarily pressure increases of a magnitude sufiicient to signify that fluid may be present in the envelope.

It will be evident to those skilled in the art that in our tap-changing apparatus a current which is not interrupted by the vacuum device 26 will be interrupted at the tap selector fingers 20, 21. This is not objectionable for a few short operations, and such defective operation may be otherwise detected, as described, for example, in Patent 3,206,569McCarthy. If, however, the failure to interrupt is due to conditions where liquid may be present in the interrupter 26, repeated operation must be precluded promptly to avoid vaporization of the liquid to such a point that the envelope 50 may be fractured. It will be evident that, while small changes in gaseous pressure may not actuate the bellows 57, even a small amount of liquid in the envelope 50 will cause an abrupt pressure increase when any arcing occurs. This avalanche effect will assure prompt loss of the bellows closing force and consequent disabling of the vacuum interrupter contacts in open position independently of the position of the actuating rod 60 thereafter.

In the embodiment of our invention as described at FIG. 1, the mode of operation is such that if the vacuum loss detection mechanism does function to open or preclude closing of the vacuum interrupter contacts, the tap changer may still continue to operate with arcing ocurring either at the selector switches 24, 25 or at the tap selector fingers 20, 21. As described above, the tap selector fingers will interrupt a current which the vacuum contacts fail to interrupt. On any subsequent tap changing cycle the selector switches will interrupt current because the vacuum switch contacts remain separated. It is desirable to prevent recycling under such abnormal conditions.

At FIG. 2 we have shown an embodiment of our invention in which means have been provided to prevent recycling of the tap changer after the vacuum loss detector operates without, however, interrupting any cycle within which that operation may occur. The apparatus shown at FIG. 2 includes all the apparatus at FIG. 1, and like parts have been assigned the same reference numerals. At FIG. 2, however, we have shown in addition an alarm and lockout circuit associated with the vacuum loss detector and designated generally by the numeral 70.

The alarm and lockout circuit at FIG. 2 includes a relay 71 having an actuating winding 72 in the circuit 70 and a pair of normally closed contacts 73 in series circuit relation with the cycle timing motor 30. In order to prevent interruption of the motor circuit by relay 71 at an intermediate position in any cycle, the contacts 73 are shunted by a holding contact 74 actuated by a motor cam 75 and arranged to be open in only the normal rest position of the timer shaft. To give a visual indication of vacuum loss, an indicating lamp 76 is connected across the relay Winding 72 and will be energized whenever the relay 71 is actuated.

The circuit 70 itself is a series circuit arranged to connect the relay coil 72 and lamp 76 to a suitable source of electric current supply, shown as battery 77, whenever the vacuum loss detecting mechanism previously described has responded to a significant pressure increase within the vacuum switch envelope 50. The loss detecting mechanism described operators to so modify the net bias force 'on the moving contact rod 55 that the rod is moved to or held in its switch-opening position even though the actuating rod 60 is in its switch-closing position. The series lockout circuit 70 is completed only when the movable switch rod 55 has so responded to vacuum loss. To this end the circuit includes in series circuit relation an interlock contact 80 coupled to the switch rod 55 and an interlock contact 81 coupled to the actuating rod 60. The contact 80 is closed only when the movable interrupter rod 55 is in open position, while the contact 81 is closed when ever the actuating rod 60 is in its switch-closing position. In normal operation the

contacts

80 and 81 are never closed at the same time. For example, in the timer rest position shown, switch 80 is open and switch 81 closed. When the actuating rod 60 moves downward to opening position the contact 81 opens before the contact 80 on rod 55 closes. In the even that the vacuum loss detector responds to internal pressure in envelope 50, the contact 80 closes even though the actuating rod 60 and contact 81 remain in the rest position shown. If the vacuum loss response of rod 55 occurs with the vacuum contacts 51, 52 open, then the contact 80 remains closed after contact 81 is reclosed by rod 60. With the lockout circuit thus completed the lamp 76 and relay coil 72 are energized. As previously described, opening of relay contacts 73 is not immediately effective to deenergize the motor 30 if it is in operation at the time, but it will preclude initiation of a new cycle after the holding cam switch 74 is opened.

Thus it will be evident that in protecting a liquid-immersed vacuum switching device itself against loss of vacuum sufficiently great to signify the admission of liquid to the device, it is not necessary to detect those small gaseous pressure increases which will prevent are interruption. It is necessary only to preclude arcing with liquid present. Even such arcing can be tolerated for short intervals insufficient to cause excessive of the liquid. By permitting such arcing for short intervals and using a pressure responsive protective device, we assure positive operation of the device if any liquid is, in fact, present or if a sufficient amount of gas is present to respond to are heating.

While we have illustrated a preferred embodiment of our invention by way of example, various modifications will occur to those skilled in the art. We therefore Wish to have it understood that we intend in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention.

What we claim as new and desire to secure Patent of the United States is:

1. In a tap changing apparatus for an inductive electric winding, an enclosure containing a body of dielectric liquid, a plurality of tap terminals immersed in said fluid and connected to said winding, a pair of tap selector contact fingers mounted in said housing to engage said tap terminals, said selector contact fingers being both in contact with a common one of said tap terminals in any selected tap position and being movable sequentially to an adjacent ta-p contact in a tap changing operation, a pair of parallel branch circuits connecting said tap selecby Letters tor fingers to a common line terminal, an arcing duty vacuum switch device immersed in said body of liquid and comprising fixed and movable contact members separable within an evacuatedenvelope, said envelope including a flexible wall portion connected to said movable contact member and biased inwardly by external ambient pressure, means connecting said vacuum switch device to interrupt at least one said branch circuit during a tap changing operation, cyclic switch operating means loosely coupled to said movable contact member and operable to actuate said movable contact member from a normally closed position to openand reclose said vacuum switch device, and spring means coupled to said movable contact member and opposing the inward bias of said movable wall portion, said spring means having sufficient force to maintain said contact members disengaged independently of said operating means in response to significant increase of pressure within said evacuated envelope.

2. A tap changing apparatus according to claim 1 in which said spring means biases said movable contact member to disengaged position and said switch operating means is resiliently coupled to said movable contact member through a wipe spring acting to engage said contact member, said wipe spring exerting less force on said movable contact member than does said spring means.

3. A tap changing apparatus according to claim 2 wherein said operating means engages said movable contact member through said wipe spring in contact engaging operation and engages said movable contact member directly in contact disengaging operation.

4. A tap changing apparatus according to claim 1 wherein said cyclic vacuum switch operating means includes a contact wipe spring engaging said movable contact member in contact engaging operation, said flexible wall portion of said evacuated envelope biasing said movable contact member toward a contact-engaging position, and said spring means provides force sufficient to overcome said wipe spring when the diiferential pressure bias on said movable wall portion is significantly diminished by loss of vacuum in said envelope.

5. A tap changing apparatus according to claim 1 and including in addition means for disabling said cyclic switch operating means, and a pair of interlocking switches controlled respectively by said cyclic switch operating means and said movable contact member, said interlock switches being interconnected to render said disabling means normally ineffective.

6. A tap changing apparatus according to claim 5 wherein said interlock switches are connected in series circuit relation and each is normally open when the other is closed.

7. A tap changing apparatus according to claim 3 and including in addition electroresponsive means adapted to be energized upon significant loss of vacuum in said envelope, an energizing circuit for said electroresponsive means including a pair of interlock contacts in series circuit relation, and means connecting said interlock contacts respectively for actuation by said movable contact member and said cyclic switch operating means, said interlock contacts being each normally open when the other is closed and said movable contact when open holding one said interlock contact closed.

8. In combination, a vacuum switching device irrimersed in a body of liquid and including fixed and movable contact members separable within an evacuated envelope, cyclic switch operating means for actuating said movable contacts between open and closed circuit positions, said operating means including an actuating member having a lost-motion connection with said movable contact member, a contact wipe spring interposed between said actuating member and said movable contact member, means responsive to significant increase inpressure in said envelope to hold said movable contact member in open circuit position independently of the position of said actuating member, electroresponsive means, an energizing cir- -cuit for said electroresponsive means including a pair of interlock contacts in series circuit relation, one said interlock contact being closed only when said movable contact member is in its open circuit position and the other said interlock contact being closed only when said actuating member is in its circuit-closing position, said interlock contacts being closed simultaneously and said electroresponsive means energized only when said movable contact member is held in said open circuit position by said pressure responsive means and said wipe spring is abnormally compressed.

9. Apparatus according to claim, 8 wherein said electroresponsive means is connected when energized to disable said cyclic switch operating means and means controlled by said cyclic switch-operating means as provided to render said electroresponsive means ineffective except at the terminal point in a cycle of switching operation.

References Cited UNITED STATES PATENTS 2,472,625 6/ 1949 Smith. 3,038,980 6/ 1962 Lee. 5 3,206,569 9/ 1965 McCarty. 3,244,843 4/ 1966 Ross. 3,404,247 10/ 1968 Glassanos.

FOREIGN PATENTS 10 5/1960 Great Britain.