CONNECTING PE TO CLEAN EARTHSYSTEM

[ShineElec]

thread238-453233

In one of our sites, we have installed a separate clean earthing system (with three earth pits in a triangular formation)for INSTRUMENTATION in the substation building. And around the substation we also have installed several nos. of ELECTRICAL earth pits and LIGHTNING earth pits and connected them together and formed the main grid. The clean earth system is more than 5m away from this main grid. But inside the SIS(satellite Instrument shelter) room in the substation, the ELECTRICAL EARTH(PE)is connected to the CLEAN EARTH BAR with 1no. 70 sq mm cable.

It has been justified as to minimize the high-voltage protentional difference between the SAFETY EARTH and CLEAN EARTH system.

can somebody explain this???????

 

[che12345]

'...The clean earth system is more than 5m away from this main grid. But inside the SIS(satellite Instrument shelter) room in the substation, the ELECTRICAL EARTH(PE)is connected to the CLEAN EARTH BAR with 1no. 70 sq mm cable. It has been justified as to minimize the high-voltage protentional difference between the SAFETY EARTH and CLEAN EARTH system'
1. "Clean earth" is a misconception.
2. Your electrical earth and lightning earth are bonded together is correct.
3. Now, bonding "clean earth" and the electrical earth is correct. It MUST be connected together forming ONE earthing system. It does NOT matter whether the clean earth is >5 m away.
4. "It has been justified as to minimize the high-voltage protentional difference between the SAFETY EARTH and CLEAN EARTH system" is CORRECT.
Che Kuan Yau (Singapore)

 

[waross]

I have worked on sites where the main bonding grid was comprised of tens of thousands of pounds of steel and copper.
There is no way that three rods will be a better or cleaner earthing point.
This will still not avoid an earth potential rise in the event that an earth fault elsewhere causes a ground potential rise.
Think: Step and touch potentials being transferred to instrument circuits..
That said, it is common practice to install an instrument grounding bus in the control room.
It is often common practice to install an equipment grounding bus connected to the main grid for all non-instrument grounds.
The two bus bars are then connected by a heavy gauge jumper.
This ensures that all of the instrument circuits remain at the same potential regardless of any earth potential rise and holds the instrument circuit grounds at the same potential as the equipment mounting frames.
Any earth potential rise due to external earth faults will be a common mode voltage to both the instrument circuits and to the mounting frames.
I have seen more than once, cases where an instrument technician has misdiagnosed a problem as a "Dirty Ground" when the actual problem was eventually found to be unrelated to the grounding system.
Don't forget the 4/0 AWG jumper between the main ground bus and the instrument ground bus.
It's easier by far to install the three rods in the "Magic Triangle" than to argue with the instrument tech's.
That too is standard practice in some industries.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

CLEAN EARTH was an old idea when the manufacturers were not able to explain some erratic behavior
of process control units. It is no longer valid now. Bond them together to make an equipotential.
Also by separating them 5m in the soli do you think that it is possible to stop influencing the other?

 

[dpc]

The "clean ground" concept has been discredited for decades. Also, check with your local electrical codes. It could be a code violation.

 

[7anoter4]

NEC 2013 250.94 Bonding for Communications Systems.
(A) The Intersystem Bonding Termination Device. An intersystem
bonding termination (IBT) for connecting intersystem bonding conductors shall be provided external to enclosures at the service equipment or metering equipment enclosure and the disconnecting means for any buildings or structures that are supplied by a feeder or branch circuit.
In my opinion, compared with "clean grounding" IBT is a new name of the old grounding.
If no IBT is available, then one has to proceed as recommended in these articles:
770.100(B),(2), 800.100(B)(2), 810.21(F)(2), and 820.100.

 

[ShineElec]

I have worked on sites where the main bonding grid was comprised of tens of thousands of pounds of steel and copper.
There is no way that three rods will be a better or cleaner earthing point.
This will still not avoid an earth potential rise in the event that an earth fault elsewhere causes a ground potential rise.
Think: Step and touch potentials being transferred to instrument circuits..
That said, it is common practice to install an instrument grounding bus in the control room.
It is often common practice to install an equipment grounding bus connected to the main grid for all non-instrument grounds.
The two bus bars are then connected by a heavy gauge jumper.
This ensures that all of the instrument circuits remain at the same potential regardless of any earth potential rise and holds the instrument circuit grounds at the same potential as the equipment mounting frames.
Any earth potential rise due to external earth faults will be a common mode voltage to both the instrument circuits and to the mounting frames.
I have seen more than once, cases where an instrument technician has misdiagnosed a problem as a "Dirty Ground" when the actual problem was eventually found to be unrelated to the grounding system.
Don't forget the 4/0 AWG jumper between the main ground bus and the instrument ground bus.
It's easier by far to install the three rods in the "Magic Triangle" than to argue with the instrument tech's.
That too is standard practice in some industries.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Can you please brief what will happen if the bonding/jumper is not provided between Instrumentation(Clean) and Main earth Grid??

 

[7anoter4]

The Clean Grounding get the source potential with respect to Main Grounding

 

[stevenal]

Can you please brief what will happen if the bonding/jumper is not provided between Instrumentation(Clean) and Main earth Grid??

Possibly a fatality during a ground fault when two grounds at separate potentials are bridged by a human. See the graphic in the post above.

 

[waross]

During a ground fault, heavy currents and the associated voltage drop may be present on the grounding conductors.
In the event that instruments were connected to different points on the grounding grid, and are separated by a voltage drop, the voltage difference between them may be dangerous and may interfere with the function of the instruments.
This condition may also interfere with the signal quality and accuracy, and cause mis-operation of the controlled equipment.

Best practice is to use a dedicated ground buss for all instruments, and then connect this bus to the main ground grid at one point, so that any touch potentials on instrument cases will be mitigated to a large extent and any voltage rise on the instrument grounds will be a common mode voltage that will not affect the function nor accuracy of the instrument signals.

It may be more correct to replace the terms "Clean Ground" and "Isolated Ground" with the term "Dedicated ground Bus".
By the way, the "Magic Triangle" composed of three ten-foot rods driven in a triangle,is "Mostly Harmless*".
Those rods are rendered superfluous by the jumper connection to the main grounding grid.
(*"Mostly Harmless"; borrowed from HitchHiker.)