120V Industrial Machine

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Thread Starter

baberb

We have a small OEM machine that normally runs on 220v single phase, and we're adapting it to run in a plant with only 120v. General questions about disconnect, fusing, and a transformer to step-up to 220v for the heater.

When you have an industrial machine that runs on 120v, is it common to run both the hot leg (L) and the grounded conductor (N) through the main disconnect switch on the machine? The machine normally runs both legs of incoming 220v through the main machine disconnect, but for 120v can anyone confirm that both L and N should be run through the disconnect? Or should L be the only leg switched?

There is only one component (heater) on the machine that requires 220v, so we will have a small single-phase transformer to step-up the voltage. So I'll energize the primary of the transformer with 120v and only the hot leg fused, and on the secondary will be just an SSR and a heater cartridge. Should I fuse both legs of 220v on the secondary? Or should I ground one leg and fuse the other?
 
S
You must NOT switch the neutral or ground through the disconnect.

You should ground the transformer secondary somewhere. You could ground the center tap, if one exists, or one of the 220 legs. Any ungrounded conductor should be fused unless the transformer primary overcurrent protection is 125% or less. If you ground one leg, you won't need a SSR for that leg either, which will save cost and complexity.
 
They may not be required. But in the case where a ground fault occurs on the secondary, the fault current may be less than the pickup setting of the primary-side overcurrent device, so a fuse may come in handy.
 
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Bob Peterson

If it is designed properly, the fault current on the secondary side would be reflected to the primary side and the primary side fuse or CB will trip. This is specifically allowed by the NEC and UL508a provided some simple rules are followed.
 
S
Right. That was the source of the 125% limitation on primary overcurrent protection I mentioned. If you have secondary overcurrent protection, the primary can be quite a bit larger. Not sure whether that 125% number is still the same or not, but I believe that was the code limit awhile back.

> If it is designed properly, the fault current on the secondary side would be
> reflected to the primary side and the primary side fuse or CB will trip. This
> is specifically allowed by the NEC and UL508a provided some simple rules are followed.
 
Thanks for all of your input, I just have a few more questions...

> You must NOT switch the neutral or ground through the disconnect.

^ I understand that this is the usual way of doing it, but this machine is going to a plant in some rural area that in all liklihood may employ an uneducated electrician. So if they hook up the line and neutral of the 120v going to this machine backwards, or if the neutral is not grounded, what problems could arise? If someone working on the machine turns off the disconnect at the machine, the neutral may still be hot. Should I ground the neutral in the machine as well, to guarantee that if the electrician screws up the neutral it will blow the breaker?

This machine will be in a washdown facility, with lots of moisture and high likelihood of corrosion on the plant grounds. There is a high likelihood of somebody standing in a puddle of water in front of this machine, so if neutral is not sufficiently grounded, there could be risk of electric shock or fire, right? In that case, wouldn't I want to be able to disconnect all the incoming power lines on the machine's safety/emergency disconnect? Just like unplugging an appliance. It will still have the equipment grounding conductor connected to chassis.

From what I can see, the NEC does not expressly forbid the practice of running both L and N through the disconnect:

NEC 404.2 (B) Switches or circuit breakers shall not disconnect the grounded conductors
Except where all conductors are disconnected simultaneously or where the grounded conductor is disconnected after all ungrounded conductors
 
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Curt Wuollet

What I like to do is model the "normal" 240V power, that is, buy a transformer with dual secondaries and ground the center tap. It shouldn't make any difference, but I like it for a lot of the same reasons that the utilities do. No voltage higher than 120 V in relation to ground, and there is a definite relationship in regard to ground potential. You may even run into the odd machine where there are 120V loads although there shouldn't be. A sagging element will trip a breaker, etc. You do have to fuse both legs then, but you have less insulation stress.

Regards
cww
 
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