Separate Instrument Earth Pit

Smart.

Thanks for the reference. So now I understand the NEC calls for bonding / connecting all ground electrodes at a facility together.

I was going through IEC 60364 and it has following to say

"The earthing arrangements may be used jointly or separately for protective and functional purposes according to the requirements of the electrical installation. The requirements for protective purposes shall always take precedence."

Does this mean that the "earth system for the shields (aka functional earth), i.e. all the way from shield to the earth electrode can be kept separate from the protective earth

You have earlier mentioned that NEC and IEC code differs to some extent. Is this the difference you have been referring to?

Regards
Omer

A few years back I wrote a paper with a presentation on a primer regarding instrument grounding. It covers much of what has been discussed here and some other aspects of grounding. If anyone is interested in the paper and presentation, drop me an e-mail and I will send your a copy.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
wlmostia [@] msn [.] com

"No trees were killed to send this message, but a large number of electrons were terriblyinconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.

One thing I forgot is that the ISA-90 standards committee on "Recommended Guidelines for Industrial Control System Power and Grounding" has been reactivated after a hiatus of about 20 years. If you are an ISA member and want to join, send an e-mail to [email protected]. You can to be a voting member (only one to a company) or you can be a informational member (you get all the e-mails and documents and you can submit comments or you can lurk), just no vote on the final product. Attending meeting is desirable but not necessary. Sometimes the committee may have conference calls which you can participate in if you wish.

If you want a say in developing this standard, sign up. Uncle ISA needs you!

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.

I'm not sure if anyone has pointed it out but your instrument shields must not be connected to ground except by the bonding conductor back to the main electrical point. They should terminate at the source on an insulated ground bar.

It's very hard to dissuade electricians from grounding the shield at the instrument which causes a ground loop and noise on the signal.
At some point you remove the bonding conductor and Ohm the bar to ground, it should be open. It only takes a short time to track down any grounded points.

What about remote panel (Eg remote PLC panel, vendor skid panels etc)? There is a shield earth bar in the panel. Should I run earth cable from this earth bar back to the single earth point which may be 100 to 200 meters away? Or should it be sufficient to connect to the earth grid cable running in the site for equipotential bonding?

Regards
Bhaskar

>What about remote panel (Eg remote PLC panel, vendor skid
>panels etc)? There is a shield earth bar in the panel.

I think it's safe to treat a remote PLC panel as a different job and as you say ground it locally.

For sure if the home run comms are on optical fiber.

I think RS485 will be isolated enough also.

If it was a local junction box on the end of a multiconductor cable I would connect the shields all the way through so you have one shield all the way from the analog input/output to the field instrument.

hello sir
this is vishal maharaj with you
i wanted to know how we will separate instrument earthing and electrical earthing in control room aria with plant