The mine had NER’s installed on site and they were doing annual periodic maintenance checks to verify NER resistance and integrity during shutdowns.  They had identified that with this type of testing you can only be confident of the NER integrity at the time you check.  The NER can fail at anytime after a check and before the next but you will not be aware of the failure. This was of concern to the Electrical Engineers on site and the Fuseco team were engaged to provide some advice.

What is a Neutral Earthing Resistor (NER) Monitor?

Three phase power systems are most often solidly earthed, however by using a Neutral Earthing Resistor (NER) a power system can be resistance earthed. This is done so that if an earth fault were to occur the amount of fault current will be limited by the NER resistance. One potential problem with a resistance earthed system is; what happens if the NER goes open circuit due to damage? If this were to happen the power system has now changed from a resistance earthed system to a system with no earth reference (floating). Now the NER is no longer restricting the amount of fault current and other devices that rely on an earth reference in order to operate, cannot do so. By using a NER monitoring relay the resistance and integrity of a NER can be continually monitored to detect for any variance in the NER resistance or integrity.

By having a relay that is checking NER integrity 100% of the time there was no need for annual checks that were time consuming and not effective. The Dugald Zinc Mine Engineering team were very happy to replace manual checks with a relay solution.