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By: Test-a-relay  11-11-2011
Keywords: resistors

Description

The 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 effectively “unearthed”.

In this 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.

However, for any subsequent earth fault, two types of fault to ground can occur.

 1)     A 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.

Keywords: resistors

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11-11-2011

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