neutral grounding resistor

[sindhu04]

How to size neutral grounding resistors for both low voltage and high voltage systems per IEEE standard or NEC code.

 

[Phil Corso]

Sindhu04...

The premise behind resistance-grounding of a system-neutral at any voltage level, is to mitigate the effects of ground-fault current, i.e., burned-insulation, and/or melted magnetic-iron.

A LV system-neutral can certainly be grounded through an impedance to limit fault-current magnitude (of the order of tens of thousands of kVA) but solid-grounding eliminates the need to install more sensitive ground-fault detectors on every feeder circuit-breaker. The phase-over-current protective device is usually sufficient. Repair or replacement of LV equipment is relatively easier to handle, both in material and cost!

Of course, any MV system-neutral can be solidly-grounded, but the resultant ground-fault current is much greater than LV systems (of the order of hundreds of thousands of kVA.) Also, if damage is kept low, repair is possible. If, however, current magnitude is very large, the magnetic-structure damage may preclude repair, but instead will require total replacement. Thus, resistance-grounding of a MV system-neutral is more of an economic choice. Almost with certainty, the Benefit-to-Cost Ratio justifies its implementation.

Regards, Phil Corso

 

[weressl]

Sindhu04...

The premise behind resistance-grounding of a system-neutral at any voltage level, is to mitigate the effects of ground-fault current, i.e., burned-insulation, and/or melted magnetic-iron.

A LV system-neutral can certainly be grounded through an impedance to limit fault-current magnitude (of the order of tens of thousands of kVA) but solid-grounding eliminates the need to install more sensitive ground-fault detectors on every feeder circuit-breaker. The phase-over-current protective device is usually sufficient. Repair or replacement of LV equipment is relatively easier to handle, both in material and cost!

Of course, any MV system-neutral can be solidly-grounded, but the resultant ground-fault current is much greater than LV systems (of the order of hundreds of thousands of kVA.) Also, if damage is kept low, repair is possible. If, however, current magnitude is very large, the magnetic-structure damage may preclude repair, but instead will require total replacement. Thus, resistance-grounding of a MV system-neutral is more of an economic choice. Almost with certainty, the Benefit-to-Cost Ratio justifies its implementation.

Regards, Phil Corso

Service continuity is (also) one of the main objectives of resistance grounding.

Low voltage systems are normally designed to limit the GFC to 10A or so. That gives plenty of threshold value for system charging current.

MV system are designed for 200-400A range for damage control, but if you have a system only with motors you may lower that for service continuity.

 

[rbalex]

To answer the OP's specific question IEEE 142 has a general discussion about the boundaries limits for resistances and other impedances. The IEEE IAS/PSE is in the process of updating the material, but the underlying concepts are not likely to change much if at all.

The NEC has no specific impedance values, but does outline the installation requirements in Section 250.36 for low voltages and 250.186 for systems greater than 1 kV.

I also endorse the two previous posts for evaluating specific applications.