Resistance and short-time thermal rating for a resistance-grounded MV system, sized backward from a target ground fault current.
Standard method — stated scope limits apply
System & Target
10 s is the common short-time rating for MV NGRs — confirm against your protection clearing time.
Enter system voltage and target fault current.
About This Calculation
A neutral grounding resistor (NGR) deliberately limits ground fault current on a resistance-grounded system to a controlled, known value — typically tens to a few hundred amps rather than the thousands of amps a solidly-grounded fault would produce — trading some sensitivity in ground fault detection for dramatically reduced arc-flash energy, equipment damage, and step/touch voltage hazard during a ground fault. Sizing an NGR means choosing that target fault current (based on system voltage, protection sensitivity needs, and equipment withstand) and then working out the resistance and short-time power rating the resistor must have to sustain it for the protection clearing time.
Frequently Asked Questions
Why deliberately limit ground fault current instead of maximizing protection sensitivity?
A solidly-grounded system’s ground fault current can be nearly as large as its three-phase fault current — that much energy dumped into an arcing fault causes severe equipment damage and a genuine arc-flash hazard to personnel. Resistance grounding accepts a somewhat less sensitive (but still entirely adequate) ground fault detection scheme in exchange for keeping fault energy and hazard dramatically lower.
What is the difference between low-resistance and high-resistance grounding?
Low-resistance grounding typically targets tens to a few hundred amps and is designed to trip on the first ground fault, common on medium-voltage industrial systems. High-resistance grounding limits fault current to just a few amps, deliberately allowing continued operation through a single ground fault (common on low-voltage systems where an unplanned trip is very costly) while alarming so the fault can be found and cleared in a controlled shutdown.
How is the NGR’s short-time rating determined?
It must absorb the full I²R heating from the target fault current for at least as long as the protection system takes to clear the fault, with margin — resistors are typically rated for a specific time (e.g. 10 seconds or 1 minute) at their rated current, and that time must exceed the actual expected fault duration including any coordination margin.
Does NGR resistance value affect ground fault relay settings elsewhere in the system?
Yes, directly — the whole point of choosing the NGR’s target current is that it sets the maximum ground fault current the system will ever see, which in turn sets the pickup range every ground fault relay downstream must be coordinated within; changing the NGR without re-checking those settings can leave the ground fault protection scheme mis-coordinated.