Ever had a problem with nuisance RCD tripping? It’s one of the most common faults, often leading to repeat call outs. A good quality leakage clamp meter and some background knowledge can save a lot of time and frustration in fixing the problem.

Modern appliances and plug-in power supplies, by the nature of their design, generate low levels of leakage current even when there is no fault present. This becomes a problem when there are several appliances connected to the same circuit and their leakage currents add up to exceed the trip threshold of the RCD protecting the circuit causing the power to cut out unexpectedly.

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 maximum values for 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 start tripping 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.

Direct earth leakage measurement with the CM69


Earth leakage clamps operate by clamping around both the live and neutral conductors and measuring the difference between the currents flowing, allowing the “leakage value” to be displayed. When troubleshooting it can be useful to clamp around the live and neutral of the outgoing circuits of the distribution board to track down the problem. Alternatively clamping around the supply earth conductor can be useful for determining the total earth leakage current, but errors can occur if parallel earth paths exist.

CM69 Peak Hold captures intermittent leakage currents


Selecting the right leakage clamp meter for the job 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. The Martindale CM69 accurately measures currents from 0.1mA to 60A with a resolution down to 0.001mA and includes a peak hold function for capturing the maximum value of leakage current over time which is useful for tracking down intermittent tripping problems. The general mutimeter functions and high safety rating also make it a handy electrician’s tool to have around at all times.

In addition to troubleshooting nuisance RCD tripping and ensuring compliance with the 18th Edition, high performance leakage clamps with switchable 50Hz filters can be used for periodic measurement of leakage currents to check for insulation deterioration without having to power down the circuit. Applications even extend to appliance testing to ensure compliance with Code of Practice for maximum acceptable appliance leakage currents. For industrial applications the True RMS capability of the CM69 means that it can be used for reliably measuring maximum triplen currents in neutral conductors of 3 phase systems to identify potential overload and overheating.

You can find out more in the Martindale video about how to make reliable leakage current measurements and troubleshoot nuisance RCD tripping and other installation problems.