rule on plc power in control panel

M

Thread Starter

M griffin

I have a control panel which contains motor starters and a PLC. The panel feed is 600 VAC to a main disconnect in the panel. The supply for the plc power is from a step down xfmr. The primary of the xfmr is wired to the line side of the disconnect and not the load side. So when the disconnect is switched off, the plc still has power. Is there any code violations with this?
 
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Patrick Allen

There has to be some means of turning off the power to the PLC. The designer I work with feels this is a very unusual set up, and would likely not be to code. I'm guessing you're in Canada (600V). I do have the newest Ontario Code book, and I'll see if I can find something in it.

Patrick
 
M

Michael Griffin

At 15:04 11/07/01 -0400, M griffin <[email protected]> wrote:
&lt;clip>
>I have a control panel which contains motor starters and a PLC. The panel
>feed is 600 VAC to a main disconnect in the panel. The supply for the
>plc power is from a step down xfmr. The primary of the xfmr is wired to
>the line side of the disconnect and not the load side. So when the
>disconnect is switched off, the plc still has power. Is there any code
>violations with this?
&lt;clip>
You didn't say where this machine will be located, therefore it is a
bit difficult to determine what code applies. However, I have seen a few
machines wired this way, but with large warning signs, and a transparent
shield over the section of the panel which is still live.
The problem with this solution is that you still need *another*
disconnect switch so that you can disconnect and lock out the entire panel.
This second switch is the real disconnect, and the first one seemed to be
there more or less for decoration.


**********************
Michael Griffin
London, Ont. Canada
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D
If I remember correctly from my days of perusing the NEC and NFPA79, I don't
think your in violation of those. Local codes may be a different matter.

One important consideration is the wire color. It should be yellow to
indicate the presence of a foreign voltage - i.e. a voltage that is
present when the main equipment disconnect is off. You should also have
some means of switching off the secondary of the transformer so that you
can work on the low voltage side.

Dean R.
 
R

RAUL BENCOMO

Yes, what you need is a disconnect and protection mean;in the primary
side of your transformer (could be fuses or CB) after the main
disconnect. All the transformers should have primary protection.

Raul Bencomo
 
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