How to mitigate transferred voltage hazards from substation to process areas

[SouthSide08]

Trying to read up on transferred voltage hazards and cannot seem to understand what the correct solution on this issue. This is similar to thread238-413458. Without using expensive software is there a way to know how to solve the issue.

Typical scenario is an industrial application with the substation say 66kV. we would do calculation of GPR and step and touch voltages but within this substation only. this substation will then feed outdoor process area with 6.9kV motors say 50 feet away which would have a grid of their own. if there is a ground fault at the HV side of the substation how do we prevent or know if there is transferred potential to the process area. is it better to connect the ground grid for these separate areas or should both grid be isolated. or both method can be correct but calculation is needed using software like CDEGS.

 

[cranky108]

Maybe you don't know, but I believe substations are covered by NESC, and the process area by NEC rules.

 

[Kiribanda]

It is correct to say that since everything is bonded together, during an earth fault at the 66kV sub the GPR is transferred as a TRANSFERRED POTENTIAL to the
motor enclosures 50ft away.
1) Have you calculated the GPR for the maximum earth fault current at the 66kV sub?
2) What is your allowable GPR per your local codes?
3) If it cannot be met and very high, then you need to have a separate earth grid for the downstream loads of the 66kV sub
with a separate NGR.
3) The bottom of the new NGR should be INSULATED depending on the GPR value calculated for the 66kV side
4) By having two such separate earth grids the calculated GPR at the 66kV side is NOT transferred to the downstream.
5) Since the motors are fed by cables, there are no touch & step potential issues during an earth fault at the motor end. Hence no need to have earth grids for each & every motor.
6) All motors should be provided with a safety earth depending on the local codes.

 

[waross]

Cable fed motors, solid grounded neutrals and touch potentials.
Many cables used to feed motors contain a grounding conductor two AWG sizes smaller than the power conductors.
In the event of a motor winding shorting to the case, (or bad insulation on a splice in the motor junction box) and when the grounding conductor in the cable is the only ground return path, the supply conductor and the grounding conductor for a voltage divider across the line to neutral voltage.
Using the voltage drop tables in the CEC, that would result in a touch potential of 170 Volts to ground on the surface of the motor.
Such a fault will be cleared almost instantly by the protection and the odds are vanishingly small that someone may be in contact with the motor at the moment of failure.
This does present an ignition threat in hazardous areas.
All of the refineries and up-graders that I have worked in in the last 20 years, that use a solid grounded neutral add a bonding conductor from the motor
case to the supporting steel-work.
This bonding conductor is several AWG sizes larger than the power supply conductors.
This eliminates the possibility of a dangerous potential developing between a motor case and adjacent conductive surfaces.
The habit has carried through to a couple of NGR systems as well.

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