22 September 2013

What to do with a nuisance tripping RCD


ONE WAY of dealing with nuisance tripping is to fit an automatic resetting or recloser RCD. This was the subject of my post, The RCD that automatically resets itself.

In the vast majority of cases the fault is downstream, somewhere in the property. Invariably the trips are caused by deteriorating insulation on heater elements in water heaters, cooker elements, electric kettles, tubular heaters, or even the defrost cycle on the fridge. Inevitably the tripping is caused from the ingress of damp or water into the electrical system. Although regarded as a nuisance the fault is with the deteriorated element and not the RCD, replacing the offending element or appliance will resolve the problem. 

Like any mechanical device, a RCD can wear out over a period of time and become weakened. This can result in nuisance tripping, replacing the device can cure the problem.

Across Spain it is common for the whole installation to be on a single RCD. This can cause secondary safety problems with loss of lighting and defrosting of food if a trip occurs. The cause of the nuisance tripping can be due to accumulated or burden currents caused by items with lowered insulation resistance. This may occur due to older equipment or even wiring in buildings where prolonged damp and rain conditions can cause the insulation resistance to lower due to the ingress of moisture. The individual items may each be electrically safe but a large number of small burden currents accumulates and reduces the tripping level. Dividing the circuits and fitting additional RCDs can solve the problem.

Given certain conditions outside the property, upstream faults can cause nuisance tripping. While voltage and current on the earth line is usually fault current from a live wire, this is not always the case, thus there are other situations in which an RCD can nuisance trip. When an installation has two connections to earth, a nearby high current lightning strike will cause a voltage gradient in the soil, presenting the RCD sense mechanism with enough voltage to cause it to trip. If you are experience tripping during lightning storms it is recommended to fit a Surge Protection Device.

If the installations earth rod is placed close to the earth rod of a neighbouring building, a high earth leakage current in the other building can raise the local ground potential and cause a voltage difference across the two earths, again tripping the RCD. 

For upstream faults, due to the logistics involved in tracing the problem, the most practical solution is to fit a recloser RCD.