Problems With RCDs

As far as users are concerned, the single greatest problem with RCDs is nuisance tripping. This problem will either be due to the design of the RCD or due to the installation. Nuisance tripping can be a frustrating problem for the user and there have been cases where RCDs were strapped out, removed from installations completely or replaced with RCDs with a higher rated trip current. All of these types of actions are very dangerous and should be avoided. Where the problem lies with the RCD, it is usually attributable to factors such as voltage spikes, surges, switching transients, noise, inrush currents, etc.

Over recent years, the problem of nuisance tripping attributable to the RCD itself have been addressed by IEC. New tests have been introduced into the product standards to ensure that RCDs have a higher immunity to nuisance tripping. Unfortunately, all RCDs do not comply with the new requirements. To minimise this problem, ensure as far as possible that the RCD has compliance to IEC61008 or EN61008 (RCCBs) or IEC61009 or EN61009 (RCBOs) and also to IEC61543 or EN61543 (EMC requirements for RCDs).

Over recent years we have seen an increasing use of RCDs on the market which are not compliant with relevant product standards but are marked as if they comply. Users should insist that only compliant products are used in their installations.

Sometimes the problem of nuisance tripping is attributable to the installation. IEC recommends that the standing earth leakage current on the installation should not exceed 30% of the rated trip current of the RCD intended to be used on that installation. This means that for a 30mA RCD, the standing leakage current should not exceed 10mA. Given that a 30mA RCD may trip anywhere from 15 - 30mA, a 10mA standing leakage current will virtually prime the RCD to trip. In general, RCDs cannot tell the difference between a standing leakage current and an earth fault current. The sum of these two currents is the residual current seen by the RCD and if this aggregate current is greater than its rated trip current, the RCD will trip.

The problem of standing leakage currents is increasing due mainly to two factors:

a) equipment manufacturers filtering internally generated noise to earth
b) fitting of RFI suppression to provide immunity to mains borne noise.

European Directives on EMC require manufacturers of products, appliances and equipment, etc. to contain within specified limits the levels of RFI type emissions produced by their products. Manufacturers often have to resort to the use of filtering circuits to meet these requirements. The filtering circuits can divert high frequency signals to earth, but can also result in significant levels of leakage current to earth at the standard 50Hz frequency. Under IEC rules, an electrical appliance may allow a standing leakage current of up to 3.5mA/50Hz to flow to earth. Several such appliances coupled with other sources of standing leakage current can give rise to nuisance tripping.

Washing machines, oil or gas burners and even immersion heaters can contribute to nuisance tripping. Temperature and humidity may also be factors in nuisance tripping. Other factors contributing to nuisance tripping may be poor earth terminations or surge suppressers. Before changing the RCD in response to a nuisance tripping problem, check the installation by measuring the standing leakage current and carrying out an earth loop test. Also try to identify equipment that is likely to contribute to standing or transient leakage currents. Dividing the installation into subcircuits with individual RCD protection is probably the most effective way to minimise nuisance tripping problems.