Arc flash protection at 476 volts

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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

Re: Arc flash protection at 476 volts
Ignoramus30685 wrote:

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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

Re: Arc flash protection at 476 volts

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Yes and no.

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Better than nylon but Nomex or other FR clothing is better
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IF it is autodarkening it might work, otherwise how will you see.
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Does nothing for arc flash, may do little for anything else, except
slip and land you on your keister.  Use a proper rubber mat
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Always a good idea, but does nothing for arc flash
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A gfi and a set of current limiting fuses is a far better idea.



jk

Re: Arc flash protection at 476 volts

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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

Re: Arc flash protection at 476 volts
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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,

Re: Arc flash protection at 476 volts
On 1/15/2012 5:50 PM, Ignoramus30685 wrote:
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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

Re: Arc flash protection at 476 volts
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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

Re: Arc flash protection at 476 volts

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

Chuck P.

Re: Arc flash protection at 476 volts
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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

Re: Arc flash protection at 476 volts
Ignoramus23559 wrote:


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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

Re: Arc flash protection at 476 volts
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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

Re: Arc flash protection at 476 volts
On 1/16/2012 3:50, Ignoramus30685 wrote:
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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=AFlBLQjOAJI


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


Not nice..

Re: Arc flash protection at 476 volts
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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.





Re: Arc flash protection at 476 volts
On Jan 15, 7:50=A0pm, Ignoramus30685 <ignoramus30...@NOSPAM.
30685.invalid> wrote:
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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.
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.

Re: Arc flash protection at 476 volts

-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



Re: Arc flash protection at 476 volts
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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

Re: Arc flash protection at 476 volts

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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

Re: Arc flash protection at 476 volts

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I never saw them again.

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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.

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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



Re: Arc flash protection at 476 volts

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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.


Re: Arc flash protection at 476 volts
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Mikeholt is a great forum, indeed!

i

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