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Author Topic: Switch for 3-phase power  (Read 921 times)

Cosmo

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Switch for 3-phase power
« on: July 09, 2014, 01:18:09 pm »


This is not sound-related, but I know some of the members here have way more experience with electricity than I do, so any and all suggestions are welcome.  I am a mechanical engineer and although I understand the basics of electricity, this is a bit out of my knowledge base.

What I am looking for is a way to switch on/off a large transformer at my day job.  Actually it is three transformers in a sheet-metal enclosure.  Each transformer is about the size of my head (one for each of 3 phases) and the enclosure is about the size of a mini-fridge.  It is used to step up our 240V 3-phase service to 480V in order to test some of our products.  The problem is that this testing only occurs infrequently.  With the warm weather we are getting now, the shop does not really cool down much at night because this transformer is on 24/7 and produces a fair amount of heat (it is hard-wired into our breaker box).  Even when we roll out the a/c unit (portable), it really just cancels out/keeps up with the heat from the transformer.  If we can shut off the transformer we could actually get the temperature in the shop down to a comfortable level.

When I first started looking online for switches, I came across several that were rather pricey ($2k-$3k).  While this may end up being what we need, I am looking for a cheaper solution.  And just to throw this out there, I donít really care if it meets code or is dangerous (3-gang knife-blade switch, anyone?).  I want to look at all options before presenting anything to the powers that be.  If itís cheap enough, I might even buy it myself!

The contacts should be rated (so I am told) at 125A, though that seems a little high to me.  What other questions need to be answered first?  Any help is appreciated.

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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.  Let him step to the music he hears, however measured or far away.  - H.D. Thoreau

TJ (Tom) Cornish

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Re: Switch for 3-phase power
« Reply #1 on: July 09, 2014, 01:30:01 pm »

This is not sound-related, but I know some of the members here have way more experience with electricity than I do, so any and all suggestions are welcome.  I am a mechanical engineer and although I understand the basics of electricity, this is a bit out of my knowledge base.

What I am looking for is a way to switch on/off a large transformer at my day job.  Actually it is three transformers in a sheet-metal enclosure.  Each transformer is about the size of my head (one for each of 3 phases) and the enclosure is about the size of a mini-fridge.  It is used to step up our 240V 3-phase service to 480V in order to test some of our products.  The problem is that this testing only occurs infrequently.  With the warm weather we are getting now, the shop does not really cool down much at night because this transformer is on 24/7 and produces a fair amount of heat (it is hard-wired into our breaker box).  Even when we roll out the a/c unit (portable), it really just cancels out/keeps up with the heat from the transformer.  If we can shut off the transformer we could actually get the temperature in the shop down to a comfortable level.

When I first started looking online for switches, I came across several that were rather pricey ($2k-$3k).  While this may end up being what we need, I am looking for a cheaper solution.  And just to throw this out there, I donít really care if it meets code or is dangerous (3-gang knife-blade switch, anyone?).  I want to look at all options before presenting anything to the powers that be.  If itís cheap enough, I might even buy it myself!

The contacts should be rated (so I am told) at 125A, though that seems a little high to me.  What other questions need to be answered first?  Any help is appreciated.
The big question is will you be switching under load or not.
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Tom Bourke

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Re: Switch for 3-phase power
« Reply #2 on: July 09, 2014, 01:35:25 pm »

What size breaker is feeding this transformer?  That, along with the information plate on the transformer will be used to determine the size disconnect.  A simple nonfused disconnect should run a couple of hundred dollars. I found this for under $200, Eaton DG324UGK.

What about using the circuit breaker feeding it to turn it off?  Some are switch rated for long term use.
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Cosmo

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Re: Switch for 3-phase power
« Reply #3 on: July 09, 2014, 05:03:50 pm »


What size breaker is feeding this transformer?  That, along with the information plate on the transformer will be used to determine the size disconnect.  A simple nonfused disconnect should run a couple of hundred dollars. I found this for under $200, Eaton DG324UGK.

What about using the circuit breaker feeding it to turn it off?  Some are switch rated for long term use.

@TJ - definitely not under load.  Good point.

The breaker "feeding" the transformer is what got me confused in the first place.  We are the end unit of a 5-unit building and the master circuit breakers (5) for all the businesses are located out back behind the center business.  But each one is a single-pole switch (or appears to be) with 100A stamped on the edge of the switch (circuit breaker) handle.  This confuses me in two ways: one, I thought 3-phase power needed a circuit breaker on each of the 3 legs and, two, I am pretty sure we have 200A service.  Somebody fill me in on how this works.

BTW, the transformer is hard-wired to the incoming bus to our breaker box inside.  We (I) have disconnected it before, but we have to shut off the master switch out back first, so that means after hours, computers off, etc.  Then, of course, our development guy will want the power the next day...

As far as getting inside the sheet metal to see what each individual transformer says on it, I'll do that if I have to, but that's harder than it sounds.

The Eaton switch referenced so far looks good and may be exactly what I want, but keep the suggestions coming.  Thanks!

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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.  Let him step to the music he hears, however measured or far away.  - H.D. Thoreau

Tom Bourke

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Re: Switch for 3-phase power
« Reply #4 on: July 09, 2014, 05:22:44 pm »

If the whole thing is on a 100@breaker then you could go with a 100A switch. What is the load on the transformer during testing?  You may be able to go with a smaller fused disconnect or even just a breaker in the existing panel.  Keep in mind 240V to 480V gives you double the current draw plus some extra for loss in the transformer on the input side.

I have seen many dual and triple pole breakers with only one handle.

Edit for some added info and a warning:
After thinking about your descriptions I think there is more going on than you are aware of.
You really need to have an industrial electrician deal with this.

The 100A breaker out back and you expecting a 200A service tell me there may be a 480 to 240 step down transformer in place.  It could even be the transformer your talking about.  The physical size and no local disconnect sounds about right for this to be the main transformer.

Have you measured the 240V?  If so, you do not have a current standard electrical service for an industrial building.  I would expect 208V hot to hot and 120V hot to neutral.  If you have a true 240V feed hot to hot with 120 hot to neutral then you also probably have a High-leg delta, aka wild leg, system as well.   The wild leg will be around 208V but can vary widely with system configuration and load.

A good industrial electrician should know the difference. I have worked with some bad ones who have only ever seen modern systems and have no clue about the older systems.

When dealing with this kind of current ratings you have the potential for LOTS of damage to your self and property.  A fused disconnect is probably the correct way to feed the test load. What feeds this disconnect is a question for an industrial electrician who can look over what you have in person.  I would even look into a large GFI if the person doing the testing needs to be hands on with the unit.
« Last Edit: July 09, 2014, 05:59:03 pm by Tom Bourke »
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I have a mild form of Dyslexia that affects my ability to spell.  I do use spell checking to help but it does not always work.  My form of Dyslexia does not affect my reading.  Dyslexics of the world untie! http://www.cwalv.com

Justice C. Bigler

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Re: Switch for 3-phase power
« Reply #5 on: July 09, 2014, 06:55:36 pm »

The bigger question is: How have you been a member here for 3 1/2 years, and racked up over 100 posts and not had your account locked for not posting with your real name?
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Ray Aberle

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Re: Switch for 3-phase power
« Reply #6 on: July 09, 2014, 07:12:53 pm »

The bigger question is: How have you been a member here for 3 1/2 years, and racked up over 100 posts and not had your account locked for not posting with your real name?

ooOoooo I don't know the exact back story, but I know that he is grandfathered in, in some fashion, that this has been acceptable. Mac will be able to speak more to it, if he's not PM'd you already-- but Cosmo is accepted.

-Ray
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Tim McCulloch

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Re: Switch for 3-phase power
« Reply #7 on: July 09, 2014, 09:57:25 pm »

The bigger question is: How have you been a member here for 3 1/2 years, and racked up over 100 posts and not had your account locked for not posting with your real name?

Cosmo is his real (and only) name.  The bouncer checked his ID....
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Bob Leonard

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Re: Switch for 3-phase power
« Reply #8 on: July 09, 2014, 09:57:32 pm »

Cosmo,

Why are you not using the breaker to the circuit and then tagging the breaker with a red flag? That's the only way I would move down line to work on the transformers and feel safe. If they aren't on their own breaker then put in a breaker.
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Ray Aberle

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Re: Switch for 3-phase power
« Reply #9 on: July 09, 2014, 11:30:55 pm »

Cosmo is his real (and only) name.  The bouncer checked his ID....

tim so smart.




I knew someone would chime in with a more authoritative answer. I knew he was legit, I just didn't know the precise circumstances...

-Ray "Can't wait for Cher, Sting or Fabio to join" Aberle
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