18th Edition: New Requirements for Nuisance RCD Tripping

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For many years, Residual Current Devices or RCDs have been commonplace in the vast majority of electrical installations.

RCDs provide additional protection against electric shock by constantly monitoring the current flowing through Line and Neutral conductors within a circuit, or an individual item of electrical equipment. Under usual circumstances, the current flowing within the two conductors is equal.

However, under fault conditions (perhaps due to a fault in the circuit or an accident whilst using an electrical appliance), excess electrical current ‘leaks’ into the circuit protective conductor (CPC), causing an imbalance of current flow within the Line and Neutral conductors. This imbalance is detected by the RCD, which should automatically cut off the electrical supply before injury or damage can result.

Certain scenarios will cause an RCD to trip in unwanted circumstances due to their inherent nature of being able to detect minute amounts of fault current.

For example; it is commonplace for a 32A ring final circuit to power a large amount of electrical appliances within a dwelling. Even under normal operation, the cumulative earth leakage current from all of these appliances could be more than enough to cause an RCD to ‘diagnose’ a fault condition and trip, causing unwanted loss of power.

As such, BS7671:2018 now incorporates regulation 531:3:2 – ‘Unwanted tripping’. It states “in order to avoid unwanted tripping by protective conductor currents and/or earth leakage currents, the accumulation of such currents downstream of the RCD shall not be more than 30% of the rated residual operating current’.

A simple calculation determines that no more than 9mA of earth leakage current should be apparent on a circuit downstream of an RCD with a residual operating current of 30mA. However, with moveable, stationary and fixed Class I appliances all allowed a maximum of 3.5mA leakage current under normal working conditions, a 9mA limit could quickly be reached!

So how can we determine the amount of earth leakage current on a circuit that you are about to install? Fortunately, fairly simply. An ‘earth leakage clamp meter’ should be considered part of any electrical contractor’s test equipment arsenal for detecting small amounts of AC current within a circuit or installation. More common, conventional clamp meters are seldom sensitive enough to measure anything below 100mA, whereas some models of earth leakage clamp meters will measure AC current in quantities as small as 0.001mA (1 microampere).

We supply many earth leakage clamp meters, starting at £69+VAT. We also stock a wide range of MFTs, RCD testers and regular clamp meters.