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 Post subject: IAC FOR Air Filled Cable Box on Transformers/Motors/Generat
PostPosted: Sat Nov 17, 2018 12:01 am 

Joined: Thu Nov 15, 2018 10:29 pm
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Internal Arc Classification is applied to LV & HV Switchgear Panels.However ,it is not applied to air filled cable boxes on oil filled & dry type transformers. The transformer standards like IEC,BS,IEEE,NEC, OSHA appear to have not advised on this safety issue.In case of any internal arc in the cable box, there should be Pressure Reducing Device on the cable box to relieve the pressure by venting internal air to the atmosphere.If not, due to over pressure, cable box may burst or it's covers would fly off endangering the occupants.

Reader is requested to comment & advise, if he/she has come across such occurrence and if any transformer standard exists for IAC for air filled cable box.

This should also be applied to air filled cable box on the motors/Generators.


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 Post subject: Re: IAC FOR AIR FILLED CABLE BOX ON TRANSFORMERS/MOTORS/GEN
PostPosted: Sat Nov 17, 2018 5:11 pm 
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SHAM KANITKAR wrote:
Internal Arc Classification is applied to LV & HV Switchgear Panels.However ,it is not applied to air filled cable boxes on oil filled & dry type transformers. The transformer standards like IEC,BS,IEEE,NEC, OSHA appear to have not advised on this safety issue.In case of any internal arc in the cable box, there should be Pressure Reducing Device on the cable box to relieve the pressure by venting internal air to the atmosphere.If not, due to over pressure, cable box may burst or it's covers would fly off endangering the occupants.

Reader is requested to comment & advise, if he/she has come across such occurrence and if any transformer standard exists for IAC for air filled cable box.

This should also be applied to air filled cable box on the motors/Generators.


First off, this issue is a variation on the idea of "just walking by" switchgear. The NFPA 70E Committee has repeatedly rejected various efforts similar to what you are describing to address the risk of "just walking by" electrical equipment. The likelihood of such a thing occurring even if it is non-AIC rated switchgear is in their words, "extremely remote". Mind you this is for switchgear and there are plenty of reasons for personnel to be present in many electrical rooms in many plants such as routinely performing lockouts and performing basic PM's and inspections including walk throughs, sweeping the floors, etc.

Prior to the IEC standard coming out a similar test existing in the IEEE/ANSI C30 tests for arc resistant gear. This test was specifically for metal clad switchgear only but many manufacturers started coming out with various "arc resistant" equipment based on running the test on equipment other than what it was intended for. No reason IEC can't be used the same way. In fact there is a company building dry transformers and other equipment (Becker) specifically to these kinds of standards. The ANSI test at least consists of hanging racks of 4 oz. cotton cloth around all the joints of the enclosure and then purposely setting off an arcing fault and subsequently inspecting the cotton cloth for damage. Obviously it's pretty easy to adopt this sort of test to just about anything. So a testing standard does exist, although it is not really intended to be applicable.

Can you produce documentation of an injury? I can't. There is a big difference between switchgear and the other equipment you mentioned. The major difference is that there is nothing to "operate" on or near a transformer or a motor termination enclosure so the likelihood of personnel in the area is vastly less. Thus the risk is extremely low. This is sort of a case of "if a tree falls in the woods and nobody hears it, does it still make a sound?" Even if a peckerheard flies off, and there is ample documentation that this does occur if the wrong grease is used in motors under the right conditions, personnel would have to be close enough and at the right position for it to occur. The odds of this happening are pretty low even ignoring the likelihood of the event occurring in the first place. I'm certified in leading risk evaluation teams with techniques such as HAZOP. One of the scenarios that came up is with fan blades in cooling towers in which for whatever reason a fan blade actually came off and shot through the fiberglass siding of the cooling tower and landed quite a distance away. When we looked at the likelihood of someone being in the line of fire and the frequency that this event actually occurred, we quickly reached a level of risk that is less than being involved in a fatal commercial airline crash....not very likely. I could use the exact same analysis for the door of the termination chamber of the motor operating the fan and come to the same conclusion.

The enclosures you are referring to, and particularly dry type transformers which are vented, are not constructed nearly as stoutly constructed or sealed as switchgear enclosures. The equipment inside is basically just passive connections with no moving parts so there is no need for the heavy framing seen in switchgear enclosures. So with the possible exception of case motor termination enclosures, they are either intentionally or unintentionally pretty well vented in the first place. So I'd challenge you to document evidence of such a hazard existing in the first place. The highest pressure I've managed to locate in any documentation on "arc blast" type effects is around 10 PSI in tests on sealed motor termination enclosures. If there is any gap or a flimsy panel that can bend and vent, or any air vents at all, the pressure simply won't get anywhere close to that. I've seen lots of sheet metal enclosures but I've never seen anything resembling a case enclosure on a transformer so the likelihood of the scenario you are painting seems pretty incredible to me.

The most recent example I can think of that even approaches your scenario is with a dry autotransformer rated around 10,000 kVA which was part of a motor test stand. As a result of running ungrounded an overvoltage developed which shorted out the plates in an oil filled surge capacitor. As the oil boiled it blew one of the bushings partly out, then sprayed mineral oil everywhere. Based on the fact that witnesses saw a "fireball" and there was a lot of smoke and soot damage everywhere plus an overpowering odor of oil and oil coating on everything after the fault what I believe probably happened is that the oil mist ignited and basically formed a "fuel-air bomb". A screw head off one of the sheet metal panels launched and hit a man about 15 feet away on the back of the head. With so little inertia there was no injury...just got his attention. The panels were all bent and thrown around but not much else happened, and this is a far more energetic and worse condition than any arc flash in terms of the amount of thermal and kinetic energy developed, yet the potential for injury was almost none. The guy who was "hit" was the manager over the motor shop so he was petty upset and responded emotionally to the situation so it's hard to explain to him that the force that blew the doors off the enclosure is very small so I gave up arguing. The panels were approximately 24" by 36" so they have a surface area of 864 square inches held on by about a dozen sheet metal screws. Even at 1 PSI the force on the panel would be 864 lbs which divided by 12 gives about 72 pounds per sheet metal screw, easily enough to rip them out. As soon as the panel "launches" the air expands around it and residual pressure quickly goes to little to nothing so the kinetic energy developed by the panel in this scenario would have to be developed over a very short distance resulting in more of a "toss" than much of any kind of force. Measurements recently published in IEEE ESW conference papers showed that the force measured outside the enclosure in an arc blast is far less than 1 PSI and theoretical calculations I've done show similar results. So there is more than ample force to rip through just about any and all fasteners on a door but not much force to develop a lot of kinetic energy.

E-Hazard does have a very graphic video showing the arm of a mannequin getting "ripped off" from a door propelled by an arc blast and I've seen that video. What I don't know is whether it was simply a cheap department store style mannequin used for effect, or an actual anatomical replica ("crash test dummy") where the arm would be fastened on much better. Again I have not seen any evidence of blunt force trauma injuries even with switchgear of the manner you are describing so the evidence here seems slim.

So for all these reasons my gut feeling is that arc blast itself isn't anything like the danger that it is purported to be and recent evidence seems to support this, along with even the most basic calculations converting the potential energy stored in pressurized air in the enclosure into kinetic (following Lee's Loss Prevention type methods), and that's even if the scenario you are describing actually occurred. But with essentially no moving parts in the equipment you are talking about we're looking at the likelihood of an electrical joint failure, which is very small. Chemical plant risk guidelines consider failures of electrical joints to be around 10^-12 or less per year likelihood as opposed to breaker failures which are closer to around 10^-5 to 10^-6. Thus these standards essentially ignore wiring failures due to the extremely low likelihood.

Mind you I'm in the maintenance and troubleshooting business. I don't get called except when there's a failure so from my perspective it's not a matter of if a piece of equipment will fail but when. I've seen a lot of destroyed and damaged equipment including bushings blown apart, transformers rupturing and igniting, motor terminations fail from poor installation practices or excessive equipment vibration and ripped apart in various ways, but I haven't seen a scenario like what you are describing in the first place, never mind actual injuries.


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 Post subject: Re: IAC FOR Air Filled Cable Box on Transformers/Motors/Gen
PostPosted: Thu Nov 29, 2018 1:57 am 

Joined: Sat Feb 07, 2009 1:05 pm
Posts: 2
Location: UK
Further to the extensive answer above and in particular relation to motors, you have to bear in mind that the incident energy level in a motor terminal box is likely to be less than that of the switchgear feeding it, due to the additional protection for the motor and the increased impedance from the supply cable.

The other issue you have to consider is that if you do install a venting device, where do you vent to that is safe? Anyone within the vicinity of the vent is more likely to be exposed to an arc flash and the energy will be more concentrated.

Like the previous responder, I have come across many a failure within motor termination chambers both up to 11kV, but nothing that has ever ruptured or blown off the termination box / lid.

Even the internal winding failures I have seen on air cooled generators up to 280MVA, only revealed small sooty marks around the seals of an inspection plate, otherwise externally you would not have known that there was a fault.

Kind regards


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 Post subject: Re: IAC FOR Air Filled Cable Box on Transformers/Motors/Gen
PostPosted: Sat Dec 01, 2018 11:55 pm 
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Hold on I didn't say I haven't seen peckerheads blown off.

If an arcing fault develops either within the peckerhead itself or within the motor in a well sealed motor such as TEFC designs, something's gotta give somewhere as the air pressure rises. But don't expect any kind of pyrotechnics approaching what you see in an arc flash testing video. In sheet metal boxes they're just going to warp and self-vent. A cast box is the most likely candidate for something like an arc blast to develop, but the odds of getting to very remote" likelihood are not very large.


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