Dealing with Filtering Effectively
Electrical filtering can pose a serious problem for RCDs because of the levels of leakage current that can be diverted to ground by the filters. This can give rise to nuisance tripping and possible blinding. Such filtering is usually fitted in electrical equipment and appliances, e.g. washing machines.
The low voltage directive sets out requirements for manufactures of all electrical appliances and in addition to meeting the requirements with regard to the relevant product standards they must meet EMC directives.
EMC directives set limits on the emissions, for example noise, that appliances can generate and either radiate over the air or induce back on to the main supply.
A solution that is used by many manufactures of appliances such as washing machines is to fit filters, either common mode or differential mode. A common mode filter will effectively pass noise to ground where a differential mode will provide filtering between the live and neutral or two live conductors for example. The first purpose of filtering is to provide a path for the noise to escape, removing it from the equipment and into the ground so that it is not conducted back though the main supply. The alternative purpose of filtering is to attenuate the noise to an acceptable level.
In the event of an RCD being fitted upstream of an appliance such as a washing machine the RCD might see the current in the filter as a residual current, detect it and if it is above the trip threshold of the RCD it will be unable to prevent itself from tripping and removing power.
When we are designing our products it is paramount that we use an RCD that is not adversely affected by equipment that is fitted with filtering. At Western Automation we provide optimised design solutions that ensure that the RCD is immune to an acceptable level of EMC noise and performs to the best possible industry standards.