80% from site during test and inspection, the other 20% are from Engineers who are preparing / planning a job. Having a basic understanding of the RCD tripping parameters defined in the product standards, for different types of residual currents, can save time on site when testing unfamiliar devices.

We have summarised the tripping characteristics for the various types of RCD detailed in 531.3.3 (see table further on). This can be used as a quick guide, for specific information for design purposes refer to the relevant product standard.

For example, a Type A RCCB (EN61008-1) when subjected to an AC sinusoidal residual current is designed to trip at >50%35%

We have come across examples of RCD Test Instruments that appear to have incorrect values programmed into the software. One example recently, testing on 30mA Type A setting ½ Pulse, recorded as fail - tripping at 12 mA i.e. 40%. The customer was not aware of the difference in the trip values, for AC and pulsating DC residual currents. He assumed the problem was associated with the RCD, not the software in the test instrument. If in doubt check with the Instrument Manufacture for software updates or your Trade Association, who may already be aware of any issues.

The RCD verification test for a 30mA RCD (5IDn relates to an AC sinusoidal residual current, irrespective of the Type of RCD. If the RCD appears to be tripping out of characteristic, when carrying out the test, check that you have the correct setting selected.

The standards define the tripping thresholds and times based on the residual current components (see table below). Note key differences between tripping thresholds for AC, Pulsed and Smooth DC.

RCDs for use by ordinary persons namely RCCBs (EN 61008) and RCBOs (EN 61009), have fixed time characteristics i.e. the above standards do not allow adjustable characteristics.  CBRs and MRCDs to EN 60947-2 Annex B & M can have adjustable characteristics, designed for use in installations under the supervision of Skilled Persons – See Regulation 411.4.4 Note 2.



Terminology can be confusing; we get regular requests for 30 mA Timed Delayed /Selective RCDs. This is not possible for safety reasons. However, to reduce nuisance tripping in circuits  associated with LED lighting, inverters, EVs etc, you can use a 30 mA Short time delay RCD. These devices have a built in 10 ms delay , but still meet the tripping requirements

The complexity of modern installations particularly those associated with special locations, due to the nature, characteristics and variation of the appliances, fixed loads and improved safety requirements, requires specific competences and knowledge. The IET resources web site gives access to a wide range of additional books and guidance for special locations. These publications contain additional information on RCD requirements, supporting the Wiring Regulations.

very interesting, as said before a minefield! It doesnt help with getting what you need though most wholesalers struggle with time delay and as soon as you ask for anything more than that the phone goes a bit quite..

Would it be a good idea if manufacturers of appliances and electronic devices, were made to build in RCD protection. Rather than a electrical contractor having to guess what type of RCD to fit. It would make life simpler to install Type B RCD,s at the front end. With devices that require special RCD’s have them built in?

Absolutely agree place the responsibility with the machine manufacturer. An installer should not have to waste time contacting a technical department and waiting up to a week for the component to arrive.

800a Moulded Case Circuit Breaker

agree with that totally. the manufacture would be the first to blame the installation when it came to warranties on the kit as well.

Hmmm Building RCDs into appliances would be great in an ideal world, to detect Faults within the appliance - But think about it, you would still need RCD protection upstream of the appliance!

The choice of Rcd type is irrelevant, if it’s not tested regularly. How many times do electricians do inspections, to find the Rcd doesn’t work as it hasn’t been tested in years. Now with the regulation change, a label tells customers to test even less. It would have been better to change test frequencies to monthly rather than six monthly. It’s a recipe for disaster.

Circuit Breaker, Solid State Relay, AC Contactor, Surge Protector - Kampa,https://www.kampaelectric.com/