Randon RCD tripping

[markadams]

Hi,

I visited a property suffering from random tripping of the 100ma RCD. I found several very loose terminals in the CU neutral and CPC screws. I tightened those and the problem disappeared for a couple of days. Originally it was happening every couple of hours.

This morning I get a call saying it has reoccurred.

The CU has all circuits except lights on a 30ma RCD and three lighting circuits on the 100ma RCD side (main switch).

The 30ma RCD never trips.

I have tested Zs and IR readings for all circuits and all fine with IR above 200. RCD Trip times are also ok but ramp test on the 30ma side was 15 ma before tightening screws and 18ma after.

Also neither RCD will turn back on until ALL MCBs are off.

Any suggestions from previous similar problems?

 

[hightower]

Clamp meter on the CPCs of the circuits only on the 100 mA side to find which circuit it is?

 

[markadams]

This does not highlight a fault as most of the time a large enough earth fault is not present.

Mark

 

[hightower]

Surely got to be one of the lighting circuits at fault though - probably a case of trying to work out what was on at time of tripping and trying to find the pattern. Any external lights that might be coming on with pir?

 

[marc8]

is the 100ma trip at the start time delayed or just a standard unit?

 

[AndyL]

Full insulation resistance test of the lighting circuits, being sure to switch all switches. Is there any outside lighting? Sensor controlled lights etc?

 

[markadams]

The RCD is an 80A 100ma standard type (not time delayed).

The tripping started again today when the house was empty and no lights were on, they turned it back on, went out again and same again. It as two PIR lights which are in a covered area but it is tripping in the day so the PIR is not activating.