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Neutral Grounding Resistors

I-Gard’s neutral grounding resistors have unique properties that make them better. It starts with the element. A better grade of stainless steel with a lower coefficient of resistivity ensures that the over-current relay works – it responds when it should. Not all stainless steel does that. Read more about “The Element Difference”

Add the optional SIGMA Monitoring Relay – a combination neutral grounding resistor (NGR) monitor and ground fault relay, and you have a complete system of protection – a better NGR.

For Low Resistance Grounding
Neutral grounding resistors are similar to fuses in that they do nothing until something in the system goes wrong. Then, like fuses, they protect personnel and equipment from damage. Damage comes from two factors, how long the fault lasts and how large the fault is. Ground fault relays trip breakers and limit how long a fault lasts. Neutral grounding resistors limit how large the fault is.

A resistor is placed between the neutral and ground to limit the fault current to a desired value for a length of time. This is mainly done to limit the damage at the point of fault and to allow coordination of the electrical system in order to isolate only the faulted circuit.

There are a couple of items that you need to consider when using low-resistance grounded systems.

The first is what to limit the fault current to? You must consider your system protection and determine the minimum value of fault current you can detect. If for example the smallest breaker you have on your system is a 600 A. Frame with 600:5 A. Current Transformers. The minimum pickup on the protection relay for ground fault is 20%. Then the minimum ground fault you can detect is 120 A. However if the main breaker is 2000 A. with 2000:5 A Current Transformers then 400 A. is needed to trip the Main.

The second thing to consider is the type of steel you request for the grounding resistor. Many people today request stainless steel for the resistor element, and that it should comply with IEEE32. Of the stainless steels used today, they are similar but not identical. The thing you have to be aware of is that most stainless steel has a temperature coefficient of resistivity. This is an indication of how the resistance changes with respect to temperature. IEEE32 allows the temperature of the resistor element to change 760° C for a short duty resistor.

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