Grounding of Process Area

[NickParker]

How does grounding calculations(to arrive at the number of ground electrodes) in the Process area differ from the substation? I know for the substation, the standard IEEE-80 has to be followed?

What is the standard to be followed for the Process area grounding where Motors, pumps are located? The Motors are already supplied with EGC(Equipment Grounding Conductor) along with the Power cables.

 

[waross]

I have not been involved in grounding design of process areas but I have installed systems designed by others.
The American Petroleum Institute and engineering standards of oil companies far exceed minimum code requirements.
I don't remember the exact cable sizes, but a motor that has an NEC mandated #14 AWG equipment grounding conductor may have a #2 AWG or larger conductor grounding it to nearby conducting surfaces and to the ground grid.
Why?
Consider a motor fed with 480 Volts on #12 AWG copper conductors.
The NEC mandates a #14 AWG copper equipment grounding conductor.
Now consider a line to ground fault at the end of a long motor feeder.
The #12 AWG power conductor and the #14 AWG equipment grounding conductor will form a voltage divider across 277 Volts.
Until the circuit protection clears the circuit, there will be more than 1/2of 277 Volts or more than 138 Volts present on the motor frame.
This is an ignition hazard and a life hazard.
That is OK with the NEC but not with the API.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

There are a few physical differences between the substation and the Industrial process area where Motors, pumps, or other equipment are located.

* AIS HV Substations are usually developed in open land with individual separate foundations for power equipment with a ground grid interconnecting the equipment

* Process plant equipment is usually mounted on a concrete slab with reinforced steel, metallic pipes, conduits, and other steel members embedded on the concrete or grounded and bonded to the grounding system. In some cases are part of a large building or surrounded by a perimeter ground loop.​

The slab and rebar in contact with the earth are a natural electrode (Ufer ground) and most of the time, the building slab act as an equipotential surface that is intrinsically safe. Therefore, no additional ground grid or rods is required as it is in an HV substation.

 

[Kiribanda]

They are two different issues.
1)SUBSTATION GROUND GRID: This is infact is the system ground.
It is designed per IEEE GUIDE 80 to determine whether the calculated touch, step,
transferred potentials and GPR - mainly depends on the soil resistivity
and GF current - are within the safe limits during a ground fault inside the substation.
Because the return GF current is flowing through the soil back to the neutral of the remote Utlity
substaion transformer. STEP, Touch and grid resistance has to be measured before the commissioing
of the substation as a design verification. The recommended value per IEEE-142 is 5 Ohms
for a large substation.

2)PROCESS AREA GROUNDING SYSTEM: This is infact is the safety ground.
This safety ground is sized per local electrical codes such as CEC, NEC, BS7671 etc.
and consists of a set of copper electrodes (typically 3/4"x10ft) and bonding conductors
(typically 4/0AWG copper) scattered in the process area and all connected in paraell
thereby equivalent electrode resistance to remote earth is very low. All exposed conductive
parts such as motor frames, metallic frames of JBs etc and all extraneous conducting parts such as
building steel, pipe racks, pipes, support columns etc are all have to be bonded to this grounding system.

3) Finally, these two grounding (system + safety) systems have to be bonded together to mitigate potential
diffrence between them during a GF. But, it should be remembered that due to this bonding the
GPR of the main substation grid will be transferred as a transferred potential to the safety ground
and should be within safe limits.

 

[7anoter4]

I think there are 2 problems of hazard in Hazardous Area:
1-the short circuit arcing
2-the short-circuit heating
All live circuits -including surge protection-have to be well sealed in order to avoid flame delivered to atmosphere.
The heating occurs also in normal load [or overload] and the enclosure has to be so made to lower the temperature below the hazardous.
The grounding wire has to be so calculated in order to limit the temperature below the hazardous.
It depends on short-circuit current and the fault clearing time.
NEC 2017 Art.250.4 (5) for grounded system grounding and bonding wire states:
"It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur"
"safely" it means also-in my opinion- you cannot limit the conductor temperature to 250oC [for XLPE or EPR insulation] but to 70oC[short-time]-for instance if it is the limit permissible for the specific hazardous atmosphere.
Our practice was to provide also, a good foundation grounding which can equalize the potential all around and to bond all exposed metallic -conduits, pipes, tables and so on to the main grounding system.