Seperate Earth for instruments and power

Dear,

I asked the contractor to provide instrument earth for control power (24VDC) seperate from main power earth for control cabinets... he replied that he has to add more cost to do this...

i want to know for your experience if this is the common practice to do this (to have seperate power and instrument earth) which i find it safer, versus not having it and the implication to it. i have seen it several places not having this seperation.

thank you for your feedback,

charbel

if few month you will see the power and instrument will join together.

Dear osmanmom,

thanks, then I was wondering what is the benefit of having seperate earth for control and power in the cabinet?

follow the procedure/protocol

include the job in your PR/PO

others things beyond your control.

Earthing requires to be defined
normally the main earth is for carrying high fault currents.
An instrument earth is usually a shield ground to stop noise.
you need to search or google to see the differences

I would be wary of wary separate grounds. I was working a factory that bought a new feedline fro a stamping press that had its own ground separate from the press and they had a problem with ghost signals from the servo motor to the VFD. After a few days of troubleshooting they called in Allen-Bradley techs who said to run a daisy chain of ground wires to counter an Ecm difference between the 2 grounds that was creating the ghost signals.

Instrument earth is different to grounding

RULE of THUMB - solidly ground all metal partsto the main machine earth

Instrument earth is normally for low current communications

All big users need to be grounded, just for safety, so no worries about ground current etc.
however instrument ground used for shielding and reference should be on one central point, thus need a isolated ground rail, with only one point to the real ground (not need being same as safety ground(prefer not )
it must be possible to check for ground loops.

Dear,

i found something in the book of Brown at al about grounding.
please see attached.

this would make it clear i guess...
to connect all the shields of the instruments cables (analog) to the carrier shield bar
and connect the AC and DC ground to the isolated local ground reference.
then then take a connection ffrom enclosure frame ground to the isolated local ground reference.

Finally, connect the isolated local ground reference to the dedicated plant ground grid.

i appreciate any feedback.

I believe I would use a large ground bar kit or something similar and bond it directly to the panel and run one side of the transformer to it, making it the grounded neutral and also connect all the shields to that bar .

I would also make a connection from a separate "24 vdc common" buss bar on the dc supply to that ground bar kit

how much different is that from what you posted? ...not much... I just like the idea of one grounded buss bar for the transformer neutral and all the grounds ... I'm not sure why you call it an "isolated" local ground reference

For what reason, I don't know, but I think the dc common should have its own separate buss bar, but it should be connected to the neutral and ground buss bar

System earthed -24VDC is in same potential as earthed AC neutaral. So not needed separate "instrument earth", that is "pseudo science".

TE and PE must be different as follow where Cable shield is in TE-potential, not in PE system:

seppoalanen said:

System earthed -24VDC is in same potential as earthed AC neutaral. So not needed separate "instrument earth", that is "pseudo science".

TE and PE must be different as follow where Cable shield is in TE-potential, not in PE system:

Click to expand...

might be splitting hairs, and likely no one cares what I think, but I would not call that a ground as it is not grounded, but connected through a sort of "pull down resistor" grounding electrodes must by NEC have less than 25 ohms resistance

Also if there is no resistance in the ground loop, why would there exist a "high voltage potential difference"?

Finally, here's something I'd just like to throw out there for the heck of it because I found it interesting ... you all probably knew this already, but the resistance of the planet earth is zero. where the resistance enters in is the connection of the grounding electrode to the earth, and the resistance of the earth exists in "shells" around that electrode and the resistance of the entire earth is zero.
everyone here knows this, but often people say that electricity takes the path of least resistance... the fact is that it takes ALL the paths of All resistances. There are enough paths in the earth to make the resistance zero [\bloviation]