Rcd nuisance tripping

[19simon]

Called to investigate nuisance tripping. One 30ma rcd protecting tt installation. Insulation resistance just under 1 megohm with everything plugged in. Unplugged everything but tripping intermittently for no apparent reason. Rcd test all ok ramp test trip at 23ma. Is it possible for rcd itself to have intermittent fault? It's an old large grey wylex one. Could swear I heard fizzing in cupboard. But all breakers tight. It's a good metal merlin verdin one.

 

[LlandrilloSpark]

Just under 1meg ohm usually wouldn't trip RCD, did u say it was tripping when not under load, how about a clampmeter on the main earthing conductor see if you have leakage to earth, if so clamp around each cpc, it's also very possible the RCD has failed, I took one of those out last year because if was tripping occasionally, change the RCD it's s starting point?Good luck

 

[UKMeterman]

Hi,
Do a ramp test to confirm trip current, as some rcds can become more senstive with time.

Remind me you have done one, check all applainces, check that there are no NE shorts, note that class I appliances may leak up to 3.5mA each to earth, and class II may leak 0.25mA. also check imersion heaters and central heating pumps. Use an earth leakage clamp meter to measure the leakage current.

 

[malcolmsanford]

Very few domestic situations would utilise a S type RCD because of cost factors, they normally would be just a normal 100mA type. They were used in TT situations and in some cases TN-S situations because of the high Ra/Ze values and therefore the high Zs values.