Can someone advise under what condition should earth and neutral wires be connected in a typical PLC control panel. Is there any standard that govern this?

 

> Can someone advise under what condition should earth and neutral wires be connected in a typical PLC control panel. Is there any standard that govern this?

National Electric Code Article 250 is the safety standard you want.

The earth and neutral should only be connected in the PLC control panel if it has a separately drived source such as an isolation transformer.

There was a very good series of articles on this subject in Power Quality magazine last year.

If you have any questions contact me or the NFPA at http://www.nfpa.org/

Ralph McDonald

Spicer Group

 

> Can someone advise under what condition should earth and neutral wires be connected in a typical PLC control panel. Is there any standard that govern this?

By the National Electrical Code, you have to provide an equipment grounding ("green wire")for safety to prevent against shock hazard.
The grounded (neutral) conductor is NOT bonded to the grounding ("green wire") conductor at the load (in this case a PLC). The only place that the NEC provides connection of grounding and grounded conductors is at the separately derived source (transformer) supplying loads.

 

I will assume that you are powering your PLC with clean (isolated) power. In this case you should use a 4 wire system. The hot (1) and nuetral (2) wires are tied back to the x1 and x2 connection on your isolation transformer. The green ground wire (3) is tied to the grounding terminal on your PLC power supply and RUN DIRECTLY back to the ground lug on your isolation transformer - do not tie to back panel. FYI: if you are distributing power from your back panel, the ground bus MUST be isolated from the back panel. The ground wire from the ground lug of the back panel must be tied directly to the ground lug on the isolation transformer (the 4th wire). Of course the ground lug of the isolation transformer is tied directly to earth/mill ground. This is a star grounding configuration and provides the necessary safety grounds for both the PLC and backpanels. It offers the advantage eliminating a ground loop between the PLC and backpanel. If another seperate ground rod is driven, the ground rod must be tied to ground lug on the isolation transformer. Another backpanel ground can also be tied to this ground rod (but it still must be tied seperately to the ground lug on the isolation transformer. The advantage here is that it places the backpanel and cabinet at a slightly lower ground potential than the PLC and acts as an EMF shield. This was all verified through the Chief Electrical Inspector for the city of Dallas/Fort Worth. At the time he was the foremost authority on NEC code.