Abstract:
An internal fault indicator for an electrical device is triggered by a sudden increase in pressure as occurs when an insulation failure creates an electric arc. The heat released in the arc is transferred onto the surrounding volume causing localized overheating, vaporization and decomposition of the insulating material. The resulting pressure surge moves a diaphragm. The movement of the diaphragm releases a spring driven plunger from a barrel which extends through the housing of the electrical device. Prior to activation the plunger is held in an “armed” position by a retaining pin. Upon triggering, the plunger is pushed by the spring until it protrudes from the housing to provide a visual signal of the internal fault. A pressure relief valve may be integrated with the internal fault indicator.

Description:
TECHNICAL FIELD 
     This invention relates to indicators for signalling the occurrence of internal faults in oil-filled or gas-filled electrical equipment, such as transformers, reactors, capacitors and the like. The invention has particular application to electrical components used in electrical power distribution systems. In particular, the invention relates to internal fault indicators which display a visible indicator when an abnormally fast increase in pressure is detected within the housing of an electrical device. 
     BACKGROUND 
     Electrical power distribution grids use electrical components, such as transformers, capacitors, and reactors. Potentially dangerous conditions can be created in such devices when aging or operating stresses cause the insulation system to fail. A short circuit within such a device can release a large amount of energy within a fraction of a second. In the worst case the device can explode due to rapid internal pressure buildup from the vaporization of insulating oil and the decomposition of the oil vapor into combustible or volatile gases. 
     Nearly all pole mounted distribution transformers are protected by a cutout which includes an expulsion fuse or some other fast acting protective device. Such cutouts can minimize damage by disconnecting a faulty device from its source of electric energy so as to interrupt arc current in the event of an overload or internal fault. Service personnel can also use cutouts as manual switches for energizing or disconnecting particular circuits. If there is an overload in the system and the cutout operates, then service personnel can easily spot the open cutout and know that the transformer disconnected by the open cutout is out of service. If the fault is downstream of the transformer then, once that fault has been corrected, it is a simple matter for service personnel to re-fuse the cutout to re-energize the circuit. 
     If the fault is in the transformer then closing the cutout before the transformer has been repaired will likely produce arcing within the transformer. A device that has failed once is certain to fail again if it is re-energized before the internal damage caused by the arcing has been corrected. Arcing can leave carbonized paths within the device and may impair the mechanical integrity of the device&#39;s housing, or “tank”. This increases the risk that the device will fail catastrophically if it is re-energized. In extreme cases the transformer may explode. This could cause property damage and serious injury to service personnel and any members of the public who happen to be close by. To avoid this possibility service personnel must perform careful inspections and take special precautionary measures before attempting to re-energize any electrical apparatus found disconnected from the power system by its protective device. 
     Unfortunately, an internal fault can occur in a device without leaving any obvious visible cues that the fault has occurred in that device. Unless service personnel can tell that a particular device has failed they may reapply power to the device without detecting that the electrical device has failed. This may cause catastrophic failure of the electrical device, as noted above. 
     It is known that there is a transient surge in pressure inside oil-filled electrical devices, such as transformers, when the devices suffer from an internal arcing fault. This happens because arcing produces a marked increase in temperature which vaporizes some of the oil. Some electrical devices are filled with electrically insulating gases such as SF 6 . In such gas-filled devices arcing causes pressure surges in the gas. 
     There exist fault detectors capable of providing a visual indication that a device has failed. Such fault detectors accelerate the restoration of services while minimizing the possibility that a failed device will be re energized as a result of a human error. U.S. Pat. No. 5,078,078, invented by Cuk, who is also the inventor of this invention, describes a device for detecting transient surges in pressure within the housing of a transformer or similar device. The device fits in an opening in a casing of the transformer. A moveable piston senses rapid pressure surges which result from internal arcing faults within the transformer. The piston has at least one aperture in it so that slow increases and decreases of pressure within the transformer do not cause significant motion of the piston. An indicator attached to the piston changes appearance when the piston has moved a predetermined distance and retains the indication until reset. A disadvantage of the Cuk device is that the change in appearance of the indicator may not be readily apparent, especially from a distance. Furthermore, there is no easy way to prevent false triggering during transport and installation. 
     U.S. Pat. No. 5,623,891 discloses another device for detecting transient surges in pressure within the housing of a transformer. The device has a diaphragm which is subjected to internal pressure surges within the transformer. The diaphragm carries a trigger retainer which engages a trigger on an indicator shaft mounted for rotation within the housing. An indicator is mounted on the indicator shaft beneath a lens which is visible from the outside of the housing. A bias spring biases the indicator shaft toward rotation relative to the housing when the trigger is engaged with the trigger retainer. When the diaphragm moves in response to a pressure surge in the transformer, the trigger retainer moves away to release the trigger, and the bias spring rotates the indicator shaft and indicator so that a warning section on the indicator is positioned for viewing through the lens. 
     The device disclosed in U.S. Pat. No. 5,623,891 has a number of disadvantages including: 
     The inner surface of the indicator lens can become fouled by fumes generated during an internal fault; 
     The outer surface of the indicator lens can be obscured by ice or snow; 
     The device provides no way to prevent false triggering during transport and installation; 
     It is typically necessary to mount the device o n the top surface of a transformer, in a position where the lens is not readily visible from the ground; 
     The device may be falsely triggered by oil splashing or the like; and, 
     If the device is to be mounted on the side of a transformer housing then there must be a larger than usual air space in the transformer housing or the diaphragm will be partially submerged. 
     There is a need for internal fault indicators for electrical equipment of the type used in electrical power distribution which provides a clear visual indication that a device has experienced a fault and which avoids some of the disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     This invention provides a warning indicator for oil-filled electrical equipment, such as power transformers or the like. The warning indicator detects pressure surges created in the housing of the electrical device by an internal electrical fault and yet is insensitive to pressure changes due to normal temperature variations. The indication may be used to positively identify an electrical device which has suffered from an internal fault so that service personnel will be warned not to undergo potentially hazardous attempts to re-energize a faulty piece of electrical equipment. 
     Accordingly, one aspect of the invention provides a fault indicator for indicating the occurrence of a rapid pressure surge within a housing of an electrical device. The fault indicator comprises: a barrel capable of being mounted in an opening in a housing of an electrical device; and an actuating mechanism. The actuating mechanism comprises: a chamber within the housing, the chamber having at least one orifice communicating between interior and exterior surfaces of the chamber; and, an actuating member movable in responses to a pressure differential between the interior and exterior surfaces of the chamber. The fault indicator also has a plunger within a bore of the barrel. The plunger is biassed outwardly in the barrel and is normally retained in an armed position by the actuating member. When the pressure differential is positive, the actuating member is moved and thereby permits the plunger to move outwardly in the bore to a triggered position. 
     Preferably the chamber comprises a diaphragm and the actuating member is attached to the diaphragm. The actuating member preferably comprises a trigger pin which projects from the diaphragm and engages a trigger notch in the plunger when the plunger is in its armed position. The diaphragm and the barrel are both preferably oriented generally horizontally. This makes the fault indicator compact. 
     Another aspect of the invention provides a fault indicator for indicating the occurrence of a rapid pressure surge within a housing of an electrical device. The fault indicator comprises: pressure surge detecting means for moving an actuating member in response to a rapid rise in pressure within a housing of an electrical device; indicator means actuated by the pressure surge detecting means, the indicator means comprising a plunger movably disposed within a bore, the plunger movable outwardly in the bore from an armed position to a triggered position upon movement of the actuating member; and, retaining means for preventing the plunger from becoming separated from the fault indicator. 
     A still further aspect of the invention provides a method for indicating the occurrence of a rapid pressure surge within a housing of an electrical device. The method comprises: providing a chamber within a housing of an electrical device, the chamber comprising an enclosed volume and an orifice communicating between the enclosed volume and an air space within the housing and providing a plunger having a hidden portion which is hidden from view; allowing a rapid pressure surge within the housing to displace a wall portion of the chamber inwardly; in response to motion of the wall portion releasing a plunger; and, moving the plunger so that the portion of the plunger which was hidden from view is exposed. 
     Further features and advantages of the invention are described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In figures which illustrate non-limiting embodiments of the invention: 
     FIG. 1 is a partially cut away view of an electrical power transformer equipped with an internal fault indicator according to the invention and connected to an energy supply; 
     FIG. 2A is a section through an internal fault indicator according to the invention; 
     FIG. 2B is a detailed sectional view of a portion of the internal fault indicator of FIG. 2A with a shipping lock in place to prevent premature triggering of the internal fault indicator; 
     FIG. 2C is a section through an internal fault indicator according to an alternative embodiment of the invention wherein a coil spring is used to provide a bias force on a trigger pin; 
     FIG. 3 is an exploded view of the internal fault indicator of FIG. 2; 
     FIG. 4 is a perspective view of the internal fault indicator of FIG. 2 before activation; 
     FIG. 5 is a perspective view of the internal fault indicator of FIG. 2 after activation; 
     FIG. 6 is a perspective view of the internal fault indicator with its shipping lock in place; and, 
     FIG. 7A is a schematic view showing one possible arrangement for preventing the rotation of a barrel of an internal fault indicator according to the invention in an aperture in a housing of an electrical device; and, 
     FIG. 7B is a schematic view showing another possible arrangement for preventing the rotation of a barrel of an internal fault indicator according to the invention in an aperture in a housing of an electrical device. 
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 List of Reference Numerals 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 pole 
                 12 
                 crossarm 
               
               
                 14 
                 power lines 
                 16 
                 transformer 
               
               
                 18 
                 fused cutout 
                 20 
                 housing 
               
               
                 22 
                 internal fault indicator 
                 24 
                 aperture 
               
               
                 26 
                 actuator mechanism 
                 28 
                 indicator mechanism 
               
               
                 30 
                 shipping lock 
                 32 
                 pressure relief valve 
               
               
                 40 
                 chamber 
                 42 
                 orifice 
               
               
                 44 
                 diaphragm 
                 44A 
                 face of diaphragm in 
               
               
                   
                   
                   
                 chamber 
               
               
                 44B 
                 face of diaphragm in 
                 44′ 
                 gas barrier (FIG. 2C) 
               
               
                   
                 housing 
               
               
                 45 
                 stiffener (FIG. 2C) 
                 46 
                 compliant substructure 
               
               
                 48 
                 spiral spring 
                 49 
                 splash cover 
               
               
                 50 
                 rod 
                 54 
                 trigger pin 
               
               
                 56 
                 barrel 
                 56A 
                 outer end of barrel 
               
               
                 56B 
                 threaded shoulder 
                 56C 
                 bore of barrel 
               
               
                 561 
                 flange 
                 56E 
                 inner end of barrel 
               
               
                 57 
                 gasket 
                 58 
                 nut 
               
               
                 59A 
                 flat 
                 59B 
                 flat 
               
               
                 59C 
                 notch 
                 59D 
                 projection 
               
               
                 60 
                 plunger 
                 60A 
                 inner end of plunger 
               
               
                 60B 
                 outer end of plunger 
                 60C 
                 side of plunger 
               
               
                 60D 
                 cylindrical extension 
                 62 
                 eject spring 
               
               
                 62A 
                 tails of eject spring 
                 64 
                 trigger notch 
               
               
                 65 
                 guide opening 
                 67 
                 sealing ring 
               
               
                 68 
                 chamber/splashguard 
                 69 
                 groove 
               
               
                   
                 assembly 
               
               
                 69A 
                 edges of groove 
                 70 
                 valve member 
               
               
                 72 
                 valve seat 
                 74 
                 pressure relief valve spring 
               
               
                 76 
                 spring retainer 
                 78 
                 vent cap 
               
               
                 80 
                 spring (FIG. 2C) 
                 79 
                 ring 
               
               
                 82 
                 flange 
                 84 
                 shipping lock holding groove 
               
               
                 85 
                 control signal generator 
                 86 
                 transmitter 
               
               
                   
               
             
          
         
       
     
    
    
     DESCRIPTION 
     The invention will now be described using the example of an internal fault indicator for a power transformer. It will be appreciated that the invention has application to high power electrical devices generally and not just to transformers. FIG. 1 shows a typical distribution pole  10  with a crossarm  12  supporting power lines  14 . A transformer  16  is mounted on the pole  10  and is connected via a fused cutout  18  to one of the lines  14 . When the cutout opens, it hinges down as illustrated in dashed outline in FIG.  1 . This breaks the circuit between transformer  16  and line  14 . 
     Transformer  16  has a housing or “tank”  20 . An internal fault indicator  22  (which could also be called an internal fault detector) is mounted in an aperture  24  (best shown in FIG. 7A) in a side wall of housing  20 . Aperture  24  is preferably a small hole. Aperture  24  may, for example, be a hole about 1 inch (25.4 mm) in diameter. Housing  20  contains electrically insulating oil (or gas). Internal fault indicator  22  is located in an air space above the level of oil in housing  20 . 
     As shown in FIG. 2A, internal fault indicator  22  comprises an actuator mechanism, indicated generally by  26 , which detects transient pressure surges within housing  20 , and an indicator mechanism, indicated generally by  28 , which changes appearance when the actuator mechanism has detected a transient pressure surge. Preferably internal fault indicator  22 . also comprises a shipping lock  30  which, when installed, prevents indicator mechanism  28  from being triggered. Internal fault indicator  22  can also conveniently include an integral pressure relief valve  32 . Shipping lock  30  may prevent pressure relief valve  32  from being actuated. 
     When there is a breakdown of the insulation surrounding the energized or “active” components of transformer  16  an electric arc is created. The electric arc dissipates large amounts of energy. The sudden dissipation of energy within housing  20  causes a sharp rise in the pressure within housing  20 . Even at levels of short circuit current on the order of 100 amperes, or less, the pressure within housing  20  rises at a rate which is distinctly higher then any other pressure rises that are reasonably expected to occur in normal operation of transformer  16 . This rapid pressure rise is detected by actuator mechanism  26  which triggers indicator mechanism  28 . 
     If the pressure rises to a value which is greater than the set point of pressure relief valve  32  then pressure relief valve  32  opens until the pressure has been relieved. The pressure within housing  20  may rise to a level capable of opening pressure relief valve  32  as a result of normal fluctuations in ambient temperature and loading. Service personnel may also manually operate pressure relief valve  32 , as described below, to equalize the ambient pressure inside housing  20  with the air pressure outside of housing  20 . 
     As shown best in FIGS. 2A and 3, actuator mechanism  26  comprises a chamber  40  which is open to the interior of housing  20  only by way of a small orifice  42 . A thin diaphragm  44 , which functions as a gas barrier, supported by a compliant substructure  46  forms one wall of chamber  40 . In the illustrated embodiment the compliant substructure comprises a spiral spring  48 . Spiral spring  48  is preferably configured to lie on a spherical surface when it is at equilibrium. Diaphragm  44  has one face  44 A in chamber  40  and a second face  44 B exposed to the ambient pressure within housing  20 . Chamber  40  is preferably roughly semi-spherical so that it can occupy a reasonably small space within housing  20 . Diaphragm  44  preferably has a reasonably large area so that pressure differentials across diaphragm  44  will generate sufficient forces to trigger indicator mechanism  28 . Diaphragm  44  may, for example, have a diameter of 3 inches or more. For maximum reliability and sensitivity, diaphragm  44  should face downward toward the surface of the oil in housing  20 . 
     Because air can enter or leave chamber  40  by way of orifice  42 , the air pressure within chamber  40  will track slow changes in the ambient pressure within housing  20 . Such changes might occur, for example, when the temperature of transformer  16  changes. On the other hand, if the pressure within housing  20  increases very suddenly, the air pressure within chamber  40  will take some time to increase because of the small size of orifice  42 . Preferably diaphragm  44  moves far enough to reliably trigger indicator mechanism  28  in response to pressure surges which are more rapid than about 0.5 to 1.5 psi over 5 ms and diaphragm  44  is insensitive to fluctuations in the ambient pressure within housing  20  which occur more slowly than about  1  psi per second. During this period the pressure on face  44 B of diaphragm  44  will temporarily significantly exceed the pressure on face  44 A. Diaphragm  44  is pushed toward chamber  40 . This would occur if an electrical fault in the active components of transformer  16  caused an electrical arc within housing  20 . A splash cover  49  dampens the effects of oil splashing onto diaphragm  44  as might occur, for example, if housing  20  was shaken by an earthquake. 
     An axial guide rod  50  extending from spiral spring  48  projects into orifice  42 . The location of the end of guide rod  50  projecting through orifice  42  can be used to verify that spiral spring  48  has been properly located within chamber  40  during assembly. The movement of diaphragm  44  triggers indicator mechanism  28 . In the illustrated embodiment a trigger pin  54  projects from face  44 B of diaphragm  44 . Trigger pin  54  may be press fit into a hub located in the central portion of spiral spring  48 . Under normal operating conditions chamber  40  is exposed to various mechanical vibrations and shocks including seismic tremors. To avoid false triggering by such mechanical vibrations, and to permit rapid operation, the mass of diaphragm  44  and spiral spring  48  should be small. Diaphragm  44  can comprise a thin layer of an air impermeable material such as 5 mil polyethylene film. Spiral spring  48  may be fabricated from a thin sheet of a suitably resilient plastic. 
     Indicator mechanism  28  comprises a barrel  56 . A flanged outer end  56 A of barrel  56  projects through aperture  24 . An all weather gasket  57  is captured between outer end  56 A and the outer surface of housing  20 . A nut  58  threaded onto a threaded shoulder  56 B on barrel  56  is tightened against the interior wall surface of housing  20  to ensure the integrity of the seal around aperture  24 . Barrel  56  should be prevented from rotating in hole  24 . This may be accomplished, for example, by making aperture  24  D-shaped with a flat  59 B in aperture  24  which engages a corresponding flat  59 A on shoulder  56 B (see FIG.  7 A). FIG. 7B shows an alternative construction which prevents rotation of barrel  56  relative to aperture  24 . In the embodiment of FIG. 7B, a projection  59 D on housing  20  engages a notch  59 C in shoulder  56 B. 
     Preferably barrel  56  is small enough to fit into an aperture which is approximately 1 inch in diameter. Barrel  56  is made of non-conductive material so that barrel  56  does not provide a conductive path through the wall of housing  20 . Barrel  56  may, for example, be fabricated from fiber-reinforced polypropylene with additives to provide resistance to degradation by the action of sunlight. 
     A plunger  60  is located within a bore  56 C of barrel  56 . Plunger  60  is urged outwardly by an eject spring  62  which is compressed between plunger  60  and an inwardly projecting flange  56 D at an inner end  56 E of barrel  56 . Preferably eject spring  62  is received within a cylindrical extension  60 D of plunger  60 . Eject spring  62  is preferably attached both to barrel  56  and to plunger  60 . This may be accomplished by providing tails  62 A on either end of eject spring  62 . Tails  62 A positively interlock with mating features on plunger  60  and barrel  56 . 
     Until internal fault indicator  22  is triggered, plunger  60  is prevented from being ejected from barrel  56  by the engagement of trigger pin  54  in a trigger notch  64  in plunger  60 . Trigger pin  54  passes into bore  56 C through a chamfered guide opening  65 . Spiral spring  48  provides a slight spring force which tends to seat trigger pin  54  in trigger notch  64 . Plunger  60  preferably has a flanged outer end  60 B which bears against a sealing ring  67  near the outer end of bore  56 C. This seals opening  24  while plunger  60  remains in its armed position within bore  56 C. 
     The side surface  60 C of plunger  60  is brightly colored, and preferably has a color which has high contrast to the colors typically found in the environment of a transformer  16 . Preferred colors are blaze orange, and bright yellow. It can be appreciated from the foregoing description that, upon a rapid pressure rise within housing  20 , diaphragm  44  is displaced away from barrel  56 . This pulls trigger pin  54  out of trigger notch  64 . Eject spring  62  then pushes plunger  60  out of bore  56 C of barrel  56 . Plunger  60  is pushed at least far enough outward in bore  56 C that a rear end of plunger  60  is past the location of trigger pin  54  and side surface  60 C, which was previously hidden from view within bore  56 C is revealed, as shown in FIG.  5 . 
     Preferably, after plunger  60  has been pushed outward in bore  56 C, the outer end of plunger  60  extends significantly beyond the outer opening of barrel  56 . This provides a highly visible indication that a fault has occurred in transformer  16 . The shape of internal fault indicator  22  is changed after plunger  60  has been ejected. Furthermore, after plunger  60  has been ejected its brightly colored outer surface  60 C is exposed to view. 
     Shortly after plunger  60  has been pushed outward in barrel  56  the pressure in chamber  40  will equalize with the ambient pressure within housing  20 . This causes diaphragm  44  to resume its normal position. When diaphragm  44  has resumed its normal position, trigger pin  54  projects into bore  56 C. Trigger pin  54  thereby blocks plunger  60  from being pushed back into bore  56 C. This prevents transformer  16  from being put unknowingly back into service without having passed an internal inspection. In general, whenever an electrical device has malfunctioned in a way that has triggered internal fault indicator  22 , the device should be opened and inspected before it is put back into service. 
     As best seen in FIG. 3, the illustrated embodiment of internal fault indicator  22  can be assembled by first affixing barrel  56  in aperture  24  as described above and then chamber  40  can be attached to barrel  56 . In the illustrated embodiment a combined chamber splashguard assembly  68  has a groove  69  on its lower surface for receiving barrel  56 . The outer edges  69 A of groove  69  are resilient and can be snapped over the outer surface of barrel  56 . When barrel  56  is received in groove  69 , groove  69  engages and grips barrel  56 . With chamber/splashguard assembly  68  installed on barrel  56  (as shown in FIG. 2A) trigger pin  54  passes into guide opening  65  on barrel  56 . 
     Pressure relief valve  32  may be made integral with plunger  60 . The pressure relief valve comprises an axially movable valve member  70  which is biased into engagement with a valve seat  72  by a low rate spring  74 . If the ambient pressure within housing  20  exceeds the atmospheric pressure outside of housing  20  then there is a net outward force on the end of valve member  70 . When the this force exceeds a predetermined value, for example, a force corresponding to a pressure differential of 5 psi, spring  74  will compress and allow gases to vent from housing  20 . Valve member  70  protrudes through a spring retainer  76  to a vent cap  78 . As valve member  70  moves axially outwardly, gases can escape from housing  20  by way of a venting gap between vent cap  78  and the outer end  60 B of plunger  60 . A ring or other graspable member  79  may be attached at the outer end of valve member  70  to permit manual venting of housing  20 . Combining an internal fault indicator and a pressure relief valve in a single device avoids the need to provide two apertures in housing  20  and conserves space within housing  20 . 
     The outer end  56 A of barrel  56  can receive a shipping lock  30 . FIG. 6 shows a fault indicator  22  with a shipping lock  30  installed. Shipping lock  30  attaches to outer end  56 A of barrel  56  and blocks plunger  60  from moving outward in bore  56 C. Shipping lock  30  can be kept in place until after transformer  16  has been installed. After transformer  16  has been installed, and before transformer  16  has been put into service, shipping lock  30  is removed. 
     In the illustrated embodiment, shipping lock  30  comprises a member having a pair of inwardly directed flanges  82  which engage grooves  84  (best seen in FIG. 2B) on outer end  56 A of barrel  56 . In the illustrated embodiment grooves  84  are defined between a stepped flange on the end  56 A of barrel  56  and the outer surface of housing  20 . Preferably lock  30  must be broken to remove it from the end of barrel  56 . 
     Internal fault indicator  22  optionally includes a facility  85  for generating a control signal when the internal fault indicator is triggered. This facility may comprise one or more sets of electrical contacts which close or open when internal fault indicator  22  is triggered. The electrical contacts may be operated to generate the control signal, for example, by the passage of plunger  60  in bore  56 C, or by the motion of trigger pin  54 . The electrical contacts may be in a first position (either closed or open) when plunger  60  is in its armed position. As fault indicator  22  is triggered the electrical contacts are switched so that when plunger  60  is in its triggered position the contacts are in a second position (either open or closed). Facility  85  may comprise other mechanisms such as fiber optics for communicating a control signal indicating to transmitter  86  that internal fault indicator  22  has been triggered. A transmitter  86  generates a fault signal such as a radio signal in response to the control signal. 
     It can be appreciated that the internal fault indicator depicted in the accompanying figures has a number of advantages over prior art fault indicators. The ability to provide a single device which functions both as a pressure relief valve and as an internal fault indicator provides significant advantages over prior devices. It simplifies the construction of housings for electrical devices since a single opening in the housing can service both a fault indicator and a pressure relief valve. It also provides more latitude in arranging parts within the electrical device housing. Space is at a premium inside the housing of a typical electrical device. This is especially the case in the top portion of the electrical device. A pressure relief valve and an internal fault indicator should both be in the air space at the top of the housing. In typical electrical devices power leads also enter the housing through the upper air space region. 
     Providing a plunger  60  which is expelled from bore  56 C when fault indicator  22  is triggered results in a visual indication that a fault has occurred in a device which can be seen much more clearly and unambiguously than has been previously possible. Both the apparent “shape” and color of the fault indicator change upon actuation. Further, because the fault indicator can be mounted in a side wall of a housing  20 , it can display its indication in a location which is more readily visible from the ground than previous fault indicators which are mountable only in the top surface of a housing. 
     The construction of internal fault indicator  22  which includes a sensor diaphragm and an indicator element wherein, upon actuation, the indicator element moves in a direction generally parallel to a plane of the diaphragm provides a compact internal fault indicator  22  which can be mounted in the air space at the top of housing  20  and yet has a large enough diaphragm area to provide good sensitivity to pressure surges inside housing  20 . 
     Providing an indicator element which cannot be returned to its initial position after internal fault indicator  22  has been triggered without opening housing  20  reduces the likelihood that, through human error, an electrical device will be placed back into use before it has been properly inspected and serviced. 
     A diaphragm assembly which includes a spiral spring, for example, the spiral spring  48  shown in the Figures, has the advantage that it is self-centering and allows easy axial movement of trigger pin  54 . 
     Internal fault indicator  22  can be made so that it projects from housing  20  by only a minimal amount. Thus there are no surfaces to which snow and ice are likely to adhere. 
     Where internal fault indicator  22  will be used in electrical apparatus, the fault indicator should be designed and constructed to provide longevity, and high reliability under all expected operating conditions. Further, the components of internal fault indicator  22  should, as much as possible be made from non-conductive materials so as to interfere as little as possible with the distribution of electric fields in the device. As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example: 
     the single orifice  42  shown in the drawings could be replaced with a number of smaller orifices or some other construction which limits the rate at which the pressure within chamber  40  can follow fluctuations in the ambient pressure within housing  20 ; 
     the shape of orifice  42  may be annular, as illustrated on FIG. 2 or some other shape; 
     compliant substructure  46  is preferably a spiral spring but is not necessarily a spiral spring; 
     compliant substructure  46  could be integral with diaphragm  44 ; 
     in the illustrated embodiment trigger pin  54  prevents plunger  60  from being reinserted into bore  56 C after internal fault indicator  22  has been triggered. A separate pawl or other one-way ratchet mechanism could be provided so that internal fault indicator  22  can be reset only from inside housing  20 ; 
     While it is desirable that ejection spring  62  be attached to both plunger  60  and barrel  56  a separate retainer cord could be provided to prevent plunger  60  from falling completely away from internal fault indicator  22  upon actuation. Ejection spring or a retainer cord constitute “retaining means” which function to prevent plunger  60  from becoming separated from fault indicator  22  by being attached to both plunger  60  and transformer  16 ; 
     various mechanical linkages may be used to release plunger  60  in response to motion of diaphragm  44 ; 
     Instead of using diaphragm  44  or a compliant support member, such as a spiral spring  48  to bias pin  54  toward plunger  60 , a separate bias means, such as a spring  80  could be used to bias diaphragm  44  toward plunger  60  as shown in FIG.  2 C. In FIG. 2C, a thin gas barrier  44 ′ is supported by a lightweight stiffener plate  45  from which pin  54  projects. A coil spring  80  urges pin  54  into engagement with plunger  60 . 
     In place of a chamber  40  closed on one side by a flexible diaphragm, actuator mechanism  26  could comprise: 
     A chamber closed by both a relatively high mass piston and a relatively low mass piston. The two pistons may be concentric with one another and are connected to springs having the same spring constant. The inertia of the large mass piston prevents the large mass piston from moving in response to sudden pressure surges. The large mass piston and the small mass piston can both move in response to slow pressure fluctuations. Relative motion of the large mass and small mass pistons can be used to release indicator mechanism  28 ; 
     Chamber  40  may comprise the interior of a bellows having rigid end faces joined by a flexible cylindrical wall. Relative motion of the rigid end faces can trigger indicator mechanism  28  by way of a suitable mechanical linkage. One or more openings in the bellows will prevent the end faces from moving in response to slow fluctuations in the ambient pressure within housing  20 ; 
     In non-preferred embodiments of the invention, diaphragm  44  could be replaced with a rigid or semi-rigid movable piston which is displaced toward chamber  40  in response to sudden pressure surges within housing  20 ; 
      a chamber  40  closed on one side by a diaphragm, as described above, for example, or any of these alternative mechanisms constitute “pressure surge detecting means” which respond to surges in pressure within housing  20  by moving a portion of a wall of a cavity with a force sufficient to operate an indicator mechanism  28 ; 
     plunger  60  may have a different shape from the shape described above, for example, plunger  60  could comprise a flag, rod, plate, or the like having hidden portions which are hidden from view within bore  56 C when plunger  60  is in its armed position and are revealed when plunger  60  moves to a triggered position. A plunger  60  as described above, and any of the alternatives described herein for displaying an indication that internal fault detector has detected a fault, constitute “indicator means ”; 
     The locking device could attach to housing  20  or device  22  in a manner different from that illustrated herein. The locking device could be a different kind of member which prevents plunger  60  from accidentally moving to its triggered position before internal fault indicator  22  is put into service. For example, the locking device could comprise a pin (not shown) which passes through an aperture in plunger  60  and therefore prevents plunger  60  from moving longitudinally in barrel  56  until the pin is removed. The locking device could also comprise, for example, a sliding or pivoting or break-away member at the outer end of plunger  60  which blocks plunger  60  from moving outwardly in barrel  56 . 
     Ejector spring  62  could comprise an extension spring arranged to pull plunger  60  outward in bore  56 C in place of the illustrated compression spring. 
     Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.