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Micro PLC in MV Switchgear

12/14/2016 12:27 PM

We have Micro PLC in the MV Panel. This Micro PLC has got integrated digital module and separate analog Port. Also connected to it is HMI with separate display.
The question is how the earthing to this Micro PLC is done. Since it is mounted inside the MV Panel, there are electrical interference to it.
If the PLC is earthed in the ground bus of MV Switchgear (6kV), what are the problems could occur?

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Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#1

Re: Micro PLC in MV Switchgear

12/14/2016 12:58 PM

British Standard 7671? Indian Standard 3053? The National Electrical Code (USA)? Get the idea?

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Guru

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#2
In reply to #1

Re: Micro PLC in MV Switchgear

12/14/2016 2:01 PM

NFPA 70e.

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Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#3
In reply to #2

Re: Micro PLC in MV Switchgear

12/14/2016 2:43 PM

Thanks. It is worrying that MV switchgear is being designed by individuals who are completely unaware of national electrical codes.

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Guru

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#5
In reply to #3

Re: Micro PLC in MV Switchgear

12/14/2016 4:17 PM

It is especially troubling when you consider that not only are they unaware of codes, but that they come to an anonymous forum expecting absolute correctness of the guidance sought here to the point that they are willing to risk injury and even death to perhaps one, several or even hundreds of innocent human lives based on what they read on the internet.

Unethical if not illegal.

That is why I give many of them a verbal drubbing when they ask these dangerous questions.

That is also why I do not like anonymous posts. No way to identify the repeat offenders.

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Guru
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#4
In reply to #2

Re: Micro PLC in MV Switchgear

12/14/2016 4:05 PM
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Power-User

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#6

Re: Micro PLC in MV Switchgear

12/15/2016 9:44 AM

The possible problems (not related to safety and fire) are that the devices connected to your electronics do not stay at the same potential as your PLC during transient faults or switching events. There is an IEEE SWC (472) specification that should protect the components, assuming the PLC is earthed according to the manufacturer's instruction. (Assuming it was tested to that standard also)

If your analog & digital components external are not tied to the same potential (earth) as the PLC, then you could fry (technical term) your PLC I/O components.

The PLC is likely designed to handle the EMI & RFI from the high voltage sources, providing it is earthed to the same potential earth as the source. The equipment designers then have a known starting point to account for the extra energy & shielding techniques.

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#7

Re: Micro PLC in MV Switchgear

12/18/2016 2:24 PM

Does it even need to be earthed (some don't) or are you talking about the screens of the PLC data cables?

Have you downloaded the appropriate PLC manual for installation guidance on this (apparently) new installation under construction?

And as others have said, don't forget to check appropriate wiring standards and local codes for further application-specific guidance.

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