Dear Mr Ali-rashid
Q. Can we use an earth leakage relay instead of an insulation monitoring device? Do they provide
the same protection?
A. An earth leakage relay is used to detect earth-leakage (fault) in a system where the [source
neutral is earthed]. A [trip signal] is issued when the earth-leakage fault current exceeds the
current set value (usually not exceeding 5A) and the time delay (usually not exceeding 0.5s).
An insulation monitoring device is used in an isolated neutral [neutral is isolated from earth
or system without a neutral] systems. An insulation monitoring device usually indicated the system
[insulation resistance] in Mohm. When the insulation resistance is lower than the set Mohm value
(usually not lower than 1 Mohm in LV systems), the device closes a N.O. contact to [activate the
(external) alarm]. Low insulation (fault) usually does not required {an immediate tripping}, but
must be attended to ASAP.
Note: In an isolated neutral system, the [first line] shorted to the earth does not cause any over-current and the system operates normally, but must be attended to (urgently !) in order to prevent the [second line] shorted to the earth; which will result to a line-to-line short.
Che Kuan Yau (Singapore)
