introduction of earthfault current limiting resistors has become standard
practice for medium and low voltage electrical systems. These resistors are
either dry type with air insulation or wet type, usually oil immersed.
Liquid resistors are also used.
There has been
a recent increase in the failure rate of these earthing resistors, which
could result in catastrophic consequences, especially in the mining
industry. Also, the increase in copper theft has often resulted in the
removal of the earthing connection from the NER. The system becomes
condition, provided that no earth fault exists, the electrical system can
operate without a deliberate earth connection. The system has capacitance
to earth, which allows the system to “float” approximately at the same
potentials as if the NER was still connected.
any subsequent earth fault, two types of fault to ground can occur.
solid connection to earth will result in the system voltages shifting
compared to earth such that the faulted phase is at earth potential and the
other phases are at phase-to-phase potential with respect to earth. Usually
no tripping occurs and the earth fault remains undetected. When a second
fault occurs anywhere on one of the healthy phases, the result is a two
phase to earth fault (often called “cross country” faults i.e. faults at
different locations) generally with high fault current.2) Should
the fault be arcing in nature, the system capacitance in combination with
the system inductance can cause an increase in voltage to earth of up to
fives times system peak voltage. These overvoltages are likely to damage
the insulation of other equipment on the same system. They can therefore be
very dangerous to the system.
The RM 100 has
been developed to monitor continuously the resistance value of both low volt
and medium voltage earthing resistors and provide an alarm if the resistor
value goes outside these upper and lower limits. The designed range is from
10 to 150 ohms. The upper and lower alarm limits are fully adjustable and
the output relay contacts are delayed by at least 5 seconds to ensure that
no false alarms occur for possible transient conditions, e.g. a system earth
fault of say 2 seconds.