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 Post subject: HI voltage testing
PostPosted: Tue Dec 13, 2011 3:36 pm 
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I’ve been reading up on article 130 in regards to PPE. The question that came across my desk was “What’s the exception for arc flash protection,” in the context of Hi-pot testing. From my experience the hi-pot may produce an arc, but nothing to the levels that this code is trying to cover.

I think I should be good with a 10ft clearance to prevent personnel from incidental contact with the 15kv (max) @ 4A system and gloves for incidental contact. I actually have 2 Hi-pots a small one 10kV@.3A and a larger one that I think can put out 15kv@4A. In any case from my previous work in switchgear this code is not relevant to these devices, but we’ve had part of the safety personnel use the table under 130.7 in relation to the Hi-pot and subsequently to the megger.

These are current limited devices the megger is battery powered and the HI-pot is fed from 120Vac@15Aac. I don’t have a copy of NFPA70E and from what I’ve read in 2009 I would have fallen under the exception of a 125KVA transformer.

I do realize .3 can kill a person and voltage jumps, but I think that the call for a arc flash study or full blue suit while working on this equipment is a bit excessive.

Chris


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PostPosted: Tue Dec 27, 2011 9:36 am 
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Very good question. The 125KVA transformer exception doesn't do it for you because that exception is used in conjunction with a requirement of voltage level being <240V. I would not wear arc flash PPE for hi-potting because that seems ridiculous; however, I do not know that as written 70E gives you a defined exception for this. The same goes for telephone circuits at the customer's location. It is widely accepted that they don't have enough energy to electrocute people, but if you are playing around with them when the phone rings you can have 120V in your hands. Yes that's stupid, but I was younger then. I don't think that 70E gives you a written exception for this either - at least not in the 2009 edition.


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PostPosted: Tue Dec 27, 2011 9:45 am 
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I agree arc flash PPE is overkill for hi-pot testing, but can't prove otherwise (I thought there was something regarding this in the 2012 70E but can't find it, perhaps it was just a proposal).

As far as shock protection, I wrote OSHA about this about 10 years ago and they replied saying all of the same requirements apply for the test voltages used as if it were an energized conductor at that same voltage. (Don't have the letter anymore, lost it when my computer dies a few years back)


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PostPosted: Tue Dec 27, 2011 5:22 pm 
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You can calculate the arc flash energy for your Hi Pot. It's pretty simply to do. The major difference is that the bolted fault current is equal to the output of the hi pot which is pretty low. Trip time probably won't apply so you'll have to assume 2 seconds unless the capacitor or battery is drained of charge in that amount of time (check mAh rating). The fact that the current is so low will limit arcing fault energy to an extremely low value.


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PostPosted: Wed Dec 28, 2011 8:31 am 
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So long as the people doing the testing don't forget to place some kind of an "Electric Shock Hazzard" warning sign at both ends of the cable and some ribbon to keep people not involved in the test out of the area...Some knowledge of what they are hi-potting would help too...
A friend of mine was about 1/2" away from knocking the teeth out of a guy who was testing a cable when he got hi-potted... The tester was taking a short cut and had no idea what he was testing...


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PostPosted: Thu Dec 29, 2011 10:58 am 
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Thank you for the responses. I actually just found NFPA70E: Standard for Electrical Safety in the Workplace®, on line, and I was debating what formula to use. For the most part they are all for 3 phase systems, except for D.8.
I am using the Formulas in Annex D. I think the formula under D.6 for calculating incident energy comes the closest to what I’m looking for. Again the only problem is it’s for 3 phase, and I don’t know what impact that would have on my actual numbers. Just doing the calculation the operator would have to be about 2.5” from the arc point to be in the arc flash boundary. With the 4A(max inrush) 15kV system, this is the largest one that we use on site. So the 10ft rule from OSHA should suffice so that our operators don’t have to put on a space suit.
Again that was using a 3 Phase calculation, and I can see some resistance with the validity of my numbers at the different distances. Now that I have access to the code, I can say if I am under 4cal/in2 I don’t have to worry about the arc, it’s just using the right formula at this point.

Thanks again and have a happy new year,

Chris Witry


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PostPosted: Sun Jan 08, 2012 5:05 am 
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MKEEET wrote:
For the most part they are all for 3 phase systems, except for D.8.



I don't recall exactly where I've seen it but the results are very similar and the recommendation is to use the same formulas. Assuming the arcing fault never advances to a 3 phase bolted fault condition (this can happen with low level arcing faults), then the formulas would be identical.

The actual source for the calculations regardless of how you go about it is IEEE 1584. 70E is supposed to produce the same results but on occasion in the past the formulas in the appendix of 70E have been different from those in IEEE 1584. In the past if IEEE 1584 changed, then there would be a delay before NFPA 70E changed. That is why for instance they "removed" the 125 kVA transformer exception in the 2012 version.

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Again that was using a 3 Phase calculation, and I can see some resistance with the validity of my numbers at the different distances. Now that I have access to the code, I can say if I am under 4cal/in2 I don’t have to worry about the arc, it’s just using the right formula at this point.


You most certainly do have something to worry about. At 1.2 cal/cm^2 using the generally accepted 2 second cutoff then there is a 50% chance of a second degree burn. There is no accepted engineering data showing what happens above that point (there is no "fatality threshold") but some rough geometric calculations I've done and posted in the general discussion section of this forum show that it is not very much of an increase over the second degree burn threshold.

That being said, working in PPE that gives adequate protection up to 4 cal/cm^2 isn't a big deal. We have an entire division of several thousand people at my company that wear this stuff as everyday work wear, and many utilities require this.


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PostPosted: Tue Jan 31, 2012 7:58 am 
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It more the incident energies that I am calculating are in the range of .46 cal/cm2 @12in, and the safety breaker fails and I’m left with an arc that lasts more 2-3 cycles(I used 2 seconds) before the CPT de-energizes. For the most part I'm more worried about shock or incidental contact, more than the burn.
Right now I’m looking at article 340.5(3), because we a lot of constant current devices such as welders and according to this if we are under 60VDC it is considered safe. Unless you have broken skin, I live in the mid-west so it’s cold and dry at the moment. So my hands are cracking, and I assume/know that everyone gets a knuckle buster once and while.
That being said would blue nitrile gloves be acceptable to meet the spirit of the law? That it’s the resistance of your skin that protects you, and if compromised with a cut, or gash, working with that voltage is unsafe.
The problem that I’m having is I’m not seeing the other side of the “if statement”. If a workers hand is cut do they have to ware class 0 electrical gloves, disposables, or a band aid? I’m thinking the nitrile gloves, because they should raise the skins resistance beyond what it normally is. So we will fall back into the unbroken skin category.


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PostPosted: Tue Jan 31, 2012 8:45 pm 
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See IEEE Red Book among other places that has a table showing the risk of shock and also gives tables of typical resistances.

Rubber gloves are basically tested by filling them with water, immersing them in a tank of water, and "hi potting" them. Below around 50-100 volts it's almost impossible to get an arc so only resistance matters (almost any air is a perfect insulator). 70E doesn't quite give you a "free pass" below 50 volts and plenty of welders can tell you stories about getting between the ground and being wet and how "safe" it is. This is an area where judgment plays a factor.


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