Arc flash protection at 476 volts

As I will sometimes test used 460v equipment, using my 240 -> 476v, 45 kVA transformer, I would like to somewhat enhance my safety, in regards to "arc flash" and other such problems.

What I want to do is this:

1) Wear heavy cotton clothing, like a work jacket 2) Wear a welding helmet 3) Stand on a piece of wood 4) Operate stuff with one hand only 5) As I turn something on, have my employee stand by and hold a handle of a disconnect leading TO the transformer. 6) If he sees or hears anything untoward, he would be instructed to immediately turn the disconnect off.

Does this make sense?

i

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Ignoramus30685 wrote:

Wow, you need to do some reading on these hazards. I don't think your 45 KVA transformer can really deliver the kind of currents that can lead to the worst sorts of arc flash disasters, you could probably still get something much worse than you imagine.

One hand only while standing on a piece of wood is fairly good for 120 V, but is pretty inadequate for 460. You ought to look into a pair of electrical gloves with the rubber insulation inside. You don't need the multi-KV sort, just the general low-voltage saftey kind, so they shouldn't be very expensive.

Cotton is not going to be much protection against an arc flash, it might ignite, even when a number of feet away. How about a welding jacket to go with your welding helmet? I don't know what they do to those, but I assume they have some sort of material and/or treatment to make them less flammable.

Make sure the disconnect will actually disconnect and quench the arc under fault conditions. You may want to check into current limiting fuses and such to make sure the available fault current is minimized.

Jon

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Yes and no.

Better than nylon but Nomex or other FR clothing is better

IF it is autodarkening it might work, otherwise how will you see.

Does nothing for arc flash, may do little for anything else, except slip and land you on your keister. Use a proper rubber mat

Always a good idea, but does nothing for arc flash

A gfi and a set of current limiting fuses is a far better idea.

jk

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Not completely.

The problem with arc flash is not really electrocution, it's the thermal radiation pulse from the flash cooking all exposed body parts in an instant. All this happens far too fast for a human to react, so for instance item 6 above is hopeless. All that employee can do is call 911. Should he still be able.

In round numbers, the fastest a human can react to the arrival of an expected event is about 50 milliseconds.

Circuit breakers are also too slow, and so don't cut the arc off fast enough, greatly increasing the integrated thermal load. Use fuses intended for the purpose.

Provide physical barriers to the flash radiation. Welding equipment can help a great deal, but industrial-scale arc flash can overwhelm even that. For one thing, the flash can be too fast for the auto-dark circuit to react in time. One should be able to "press the button" without being able to see the button.

And the use of remote actuation, such as a long fiberglass pole or a cord, or a contactor temporarily replacing the button, is a good idea.

As others have said, a little research is in order.

Joe Gwinn

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Thjere are many safety guidelines out there regarding arcflash protection Check into the equipment supplied by some of the safety equipment manufactuers for items you need,

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On 1/15/2012 5:50 PM, Ignoramus30685 wrote:

May I suggest you test the equipment at a lower voltage as a first step. Connect to your 208/3 and if you don't find an immediate problem, then rewire for the higher voltage.

Paul

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If I rewire it for 230, why would I test it at 460. The problem is that some things rae hard to rewire. Say, right now I have a 4 HP Series II Bridgeport I need to test.

i

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Not as hard as the Dr. is going to have putting you back together.

Chuck P.

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Here's a little update. I tested both the Series II Bridgeport, as well as the dual headed Quincy compressor.

"Nothing happened", both equipments seem to be OK.

The 45 kVA transformer seems to have no trouble starting a 10 HP motor.

i

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Ignoramus23559 wrote:

It certainly shouldn't. Yes, there will be a bit more dip in the line voltage than if this was a real 460 V service, but it should have no problem starting those motors, probably all at once, even. The Bridgeports at work are on 460, and the spindle motors start instantly. There is no acceleration sound at all, just instantly from zero to full motor speed. My Bridgeport on a VFD starts a lot more gradually.

Jon

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The 10 HP Quincy, with big pulleys and everything, had no problem either.

I am extremely happy with this transformer.

I think that I will replace the unfused disconnect, on the secondary side, with a fused disconnect, and will use appropriate fuses when testing stuff.

i

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On 1/16/2012 3:50, Ignoramus30685 wrote:

Get a proper arc fault switch..

The energy from the arc melts the clothing to your skin, right before cooking your skin.. The pieces of the hardware from the arc site fly through your body, which is blown away by the shockwave from the arc..

Here are videos of actual occurrances:
http://www.youtube.com/watch?v=P00WE7z9tu4

http://www.youtube.com/watch?v=dPJtknGmsys

http://www.youtube.com/watch?v=pCZax3vIslo

http://www.youtube.com/watch?v
lBLQjOAJI
http://www.youtube.com/watch?v=-iClXrd50Z8

Not nice..

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Have you looked at tests prior to just throwing 480 across machinery from some junk pile? You should see how timid techs are that work with even lower voltage equipment can be when it comes to powering up questionable sources.

A hipot tester, not from a junk pile might be good to have.

I worked at a place that insisted on getting large hardwired electrical equipment from scrap dealers. The prices to get the stuff recertified as well as fixed prior to full power up was always more than the equipment itself. The amount of exploded and burned up power control boards coming out of "removed from service, worked fine" stuff was pretty amusing. Even the "wimpy" boards that drove the power semiconductors would burn up- as in holes straight though fiberglass circuit boards before the fuses would blow.

Luckily you don't generally get injured when you make mistakes or get bad advice when it comes to computers, and recipies for pasta sauce, or steps on fixing a stereo or the other stuff that people chat about on usenet.

480 is no joke.

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On Jan 15, 7:50pm, Ignoramus30685 <ignoramus30...@NOSPAM. 30685.invalid> wrote:

I heard, and saw the aftermath of a 480 Volt arcflash. Loudest THUNDERCLAP!! I Ever heard, and I was a building away. Shorted through a set of test leads... The copper Vaporized, and shot out of the wire side ways. The molten copper shot out of the wire leads and imbedded into whatever surrounded the leads. The rubber test leads looked normal, but were completely empty of conductors. It went right out through the (newly formed) pores in the rubber covering.

The person who was holding the leads, had the hair burned off of his head, face, and arms. He looked like a racoon, from the shadow of his glasses. He was deaf for quite a while...

NO TIME to think or blink. He was blown away, and burned, and imbedded with copper slag in a MONSTER explosion.....

Interesting educational experience... Being a LONG ways away, and still feeling the concussion.

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-I heard, and saw the aftermath of a 480 Volt arcflash. -Loudest THUNDERCLAP!! I Ever heard, and I was a building away. ...

I came back after being out sick for a day to find the machine I had been building black with soot, and on the edge of a perhaps 20 foot black circle on the floor. Some new hire engineers had been attempting to find the correct phasing for a 480V power supply by holding the wires in place by hand, and apparently one of them slipped- - - - - .

jsw

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And, what happened to those engineers?

Also, how big of a transformer this was on?

What I do not fully understand, also, is what role do fuses play in this. Would a fuse not blow very quickly under such fault conditions?

i

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Ig,

A fuse only protects against an over-current condition, and that for a pre-determined time, depending upon the type and open-delay of the fuse.

What the silly story implied was that the injuries/deaths/immolations that cause a TWENTY FOOT char mark on the floor was cause by arcing, not a short.

An arc can be sustained for as long as the fuse's current limit is not exceeded, and there are no arc-suppressors in the line.

FWIW... check out the physics on how long the arc can be in air at 480V.

It's not the size, but the heat (energy dissipation). A 480V arc ain't all that long.

LLoyd

LLoyd

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wrote...

I never saw them again.

100A, it ran the whole factory. I accidentally touched one phase and didn't get a shock, only a small burn that felt like a sliver.

Speculation is that the 480 lead may have touched a large electrolytic capacitor. I cleaned up my machine and didn't hear any more about the incident afterwards.

Motor fuses are usually either dual element slow-blow or 3x larger than the run current.

jsw

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You need better research than can be done on a newsgroup. Try finding NFPA docs on arc flash and how to protect from them. Email me (I don't get here often anymore) if you want a DOE document on electrical safety that has some good background info in it and is easy to understand. You might also visit Mike Holt's site - http://www.mikeholt.com /. It's for professional electrical engineers and electricians so tread carefully in the forums.

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Mikeholt is a great forum, indeed!

i

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