The earthing resistor is based on a standard design with large flexibility for different installations. Long life and a minimum of maintenance have highest priority.
Earthing resistors are normally designed according to IEEE32. This is originally a North American standard and today it is also used globally.
There is no equivalent IEC standard for earthing resistors. Applicable IEC standards are IEC60529 for ingress protection and IEC60071 for insulation coordination.
The resistor elements are made of temperature resistant stainless steel or of a nickel-chrome alloy. Modern material guarantees long and reliable life even if the temperatures exceed 760°C, specified by IEEE32. Higher working temperatures result in more compact and cost effective solutions.
Today’s most common connection time is 10 s. This gives a good safety margin since most earth faults are detected and switched off in less than 1 s. Earlier longer connection times were specified, mainly due to the use of liquid cooled resistors. The shorter connection time, the more compact design.
Most earthing resistors are supplied enclosed for outdoor use, ingress protection is normally IP23 or IP54. Some resistors are supplied as open type, to be built-in in switchgear or similar.
Earthing resistors connected to transformers or generators are most commonly used together with other components like isolators, vacuum circuit breakers, current and voltage transformers. They are often located in the switchgear. One alternative is to combine these with the earthing resistor and form a complete earthing system.
There are many advantages to instead combine them with the earthing resistor, which will form a complete earthing system. Above all, installation and commissioning are simplified when the earthing system is supplied as one connected and factory tested unit.
Common combinations are resistors with disconnectors, manually operated isolators or vacuum circuit breakers as well as current and voltage transformers for metering.
The resistor can also be combined with dry type transformers, Zn type or single phase type when a low voltage earthing resistor is required.