DIY enuiry - replacing PIR security lightSecurity Light and rest of external are off a sepatate circuit breaker.
Light fused in LIVE wire.
On removing fuse and then checking for voltage between LIVE-NEUTRAL, LIVE-EARTH and NEUTRAL-EARTH, none measured.
Shorting LIVE to EARTH no effect.
Shorting NEUTRAL to EARTH causes circuit breaker to trip
Any idea of possible cause?
Many thanks,
Ivan Rutter
Hello Ivan...
Your fuse box has two sorts of breaker
1/ MCB'S: (Miniature Circuit Breaker)
These are like the old wired fuses.. they detect OVERLOAD, such as too much power going down a cable that could cause a fire.
Shorting 'N' to 'E' will not be causing these breakers to go.
2/ RCD: (Residual Current Device. AKA Earth Leakage Circuit Breaker 'ELCB')
These detect leakage, such as when a person is getting an electric shock, you only need a very small amount of current across your heart to kill you!
Shorting 'N' to 'E' will cause these breakers to go.
the way that fuse boxes are wired means:-
Several neutrals from different circuits share the same connection.
All of the earths from several circuits share the same connection point.
Due to costs of RCD's several circuits probably share the same RCD.
SO..
Your light circuit is absolutely dead with no power.
You have only isolated the LIVE wire.. but the Neutral and Earth still go back to the fuse box.
Any other circuits still switched on will have electricity going along their neutral wire, (which is joined to your neutral wire).
When you join the earth to neutral it allows some of this electricity to go from the neutral wire down to the earth wire..
It is this small current disappearing to earth that the RCD will detect and will turn that group of circuits off...
typically in well under half a second!
You joining 'N' & 'E' at the light fitting, is electrically no different to joining 'N' & 'E' at the fuse box.
So shorting the Neutral to Earth will be causing the RCD to trip not the MCB.
This is a perfectly normal action... where other appliances are still switched on somewhere else in the house.
Welcome to the forum.
BTW
:coffee
