To reduce nuisance RCD tripping that is not due to a fault, but the result of cumulative leakage currents from appliances, the 18th Edition Wiring Regulations now includes specific values for maximum protective earth currents (Regulation 531.3.2). This is a great help in identifying when division of the installation is required and was not part of the 17th Edition.

But what are acceptable limits for leakage current and how do you know when you are approaching the limits?

The 18th Edition now makes it clear, the accumulation of circuit protector currents/earth leakage currents that are present during normal operating conditions shall not be more than 30% of the rated residual operating current of the RCD e.g. a PE current of no more than 30% of 30mA which is just 9mA. This is necessary when you consider that it’s not unusual for a 30mA RCD to trip just above 15mA.

The trouble is how do you know when you are approaching 9mA? Computers, printers and other appliances can easily generate leakage currents in the range of 1mA to 3.5mA each.


Rather than estimate the total incorrectly, measuring the leakage current flowing in an existing installation before modifying it is probably the most effective approach to ensure the final installation is going to be fit for purpose. The alternative could be expensive call outs in months to come. Selecting the right leakage clamp meter is critical to getting reliable and repeatable results, many struggle below 10mA and can be prone to interference from other nearby conductors carrying higher currents.

Find out more in our video about how to make reliable measurements with the Martindale CM69 leakage current clamp, troubleshoot nuisance RCD tripping and other installation problems.