Neutral earthing resistors are used to limit phase-to-earth fault current in order to protect both personnel and equipment. In LV and MV distribution networks the neutral points of power transformers and generators are earthed through a resistor.
When the limiting current is determined, the phase-to-earth short circuit current that would arise immediately adjacent to the transformer or generator concerned is calculated. A resistor value is then selected that typically limits the fault current to around 10% of that short circuit current. The limited fault current must also be high enough for the protection relays to detect reliably; for this reason the primary rating of the current transformer used in the resistor may be selected differently from the limited fault current value.
The objectives of the application are: to limit earth short circuit current to a level that will not damage the transformer or generator, maximising operational continuity and safety; to allow the fault to be detected by relays through the current transformer fitted in the resistor and to define the fault duration; to limit the transient voltage fluctuations that arise when the fault current is interrupted, thereby preserving the insulation levels of system equipment; to keep step voltage on site within safe limits, improving personnel safety; and to prevent overheating and mechanical stress in all equipment exposed to fault current.
The fault duration is normally selected as 5–10 seconds. In installations that cannot tolerate a sudden loss of supply it may extend to 30 seconds. In facilities where system continuity is critical — hospitals, data centres, textile, cement and injection moulding plants — the fault duration can even be selected as continuous, provided the fault current remains at a level that will not damage the system.
Two solutions exist on the earthing transformer side. In the single phase transformer and resistor system, a single phase earthing transformer is used together with a neutral earthing resistor. This arrangement is particularly suitable for earthing generators, because it normally behaves like an unearthed system yet limits the fault current when a phase-to-earth fault occurs. The primary winding of the earthing transformer connects to the system neutral and the neutral earthing resistor connects to its secondary winding.
The second solution is to create an artificial neutral point with a zig-zag transformer. In delta-connected systems that have no neutral point, or where the neutral point cannot be reached, an earthing transformer is used to create an artificial neutral and the system can then be earthed through it. Because most earthing transformers are designed to be exposed to fault current for less than about 10 seconds, they are physically smaller and less costly than a conventional three phase continuously rated transformer of the same class.
To quote a neutral earthing resistor, the single line diagram of the system and the basic system parameters are required wherever possible.
