Have you ever wondered what the large resistors connected to the neutral on an medium voltage (MV) installation is? It can look out of place sometimes, however, it has a very specific function that prevents damage to supply generators and switchgear.

Neutral Earth Resistors 1
Image courtesy of National Resistors.

When an MV installation has an earth fault on the system, the fault current present can be exceptionally large. These fault currents will return to the supply transformer through the general mass of earth and through the neutral conductor. The impedance of the transformer and circuit depicts the size of the fault currents present. The size of the fault current returning to the transformer can be limited using resistors on the neutral conductor called Neutral Earth Resistors (NER). This reduction in fault current helps to protect the power transformer, switchgear and supply generators from damage.

If you find a NER bank on your installation and are either testing or performing maintenance checks on supply transformers, it is important to disconnect the NER from the transformer prior to testing. This will ensure correct and accurate testing results. If you require additional protection on your MV supply circuits, do not hesitate to contact us today for an analysis of all your system requirements.