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Joined: Sep 2009
Posts: 3
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gscott Offline OP
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Years ago I had to remove all 480V controls from out motor control centers and push button locations. The subject has come up again and I can't find anything that showes this being against code. I have not found anything NEC, NESC or OSHA. Can someone en lighten me on the subject.


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Joined: Jul 2004
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I can't think of any reason why it would not be code conforming if they followed the rules. Bear in mind, this is the same voltage to ground as your 277 lighting and that is just a snap switch in a box.


Greg Fretwell
Joined: Sep 2009
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gscott Offline OP
New Member
For the most part these circuits are not fused, they come directly off the equipment fuse and can have fuses up to your larges horsepower. Ther has to be an arc flash issue.
I found this on www.msha.gov

30 CFR 18.47
Voltage limitation.

(a) A tool or switch held in the operator's hand or supported against his body will not be approved with a nameplate rating exceeding 300 volts direct current or alternating current.


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Is that 300v phase to phase or to ground?
480 wye is 277 to ground and I suspect that is where the 300v limit comes from.
I had the impression we were talking about panel mounted switches, not hand held. If this is a hand held dongle I can see your concern.
If people are not working on this equipment with the covers off, I don't think arc flash is that big an issue. I am assuming you are talking about the operator, not the maintenance staff.
The enclosure around equipment should contain the flash/fire. Isn't that part of the listing? The switch itself is not the motor starter is it? I do see that fusing the coil lead would be useful. If you don't you are bumping up against the fixture wire sizing rules.


Greg Fretwell
Joined: Mar 2003
Posts: 849
Y
Member
If its a 480 volt 3 phase motor odds are theres no 277 volt to ground (to switch) & even if there was you can't switch the grounded conductor.
Part VI in 2005 is area that deals with this as far as what I've found.
Yoopersup
intersting question I always took it for granted that 480volt CAN NOT be field switched, if so then limit switchs , ect????

Joined: Sep 2009
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gscott Offline OP
New Member
I was talking about push buttons, The operator is the one to protect. Limit switches and other devices I am not so concerned with. A 480v wye will be 277v to ground but a open delta will very and could be over 300 volts. sounds like it is ok to use 480v control on push buttons, but a control transformer to 120v just seams alot safer.

Gary

Joined: Jul 2004
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If you are changing out the motor controller contactors, why not just go to a 24v system? 120v is only marginally safer than 277.


Greg Fretwell
Joined: Dec 2000
Posts: 4,289
Member

The problem lies with the contact blocks behind the pushbuttons.
Most (not all mind you) are rated for 300 Volts.

It will probably pay to check

Joined: Jun 2004
Posts: 1,273
T
Member
208V / 480 V systems stink...

Too many problems with back-EMF in motor controls.

Give me 120V, 24V, 277V any day.

Back-EMF results in contactor chatter and other problems.

Ungrounded control circuits are a HEADACHE.


Tesla
Joined: Jul 2002
Posts: 8,426
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I wouldn't personally use "Mains voltage" for a control circuit, although in some cases you don't really have a choice.

I prefer something like 24V, fed from the secondary side of a double-wound transformer, as long as there are fuses in both sides of the winding, I don't see an issue.

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