RCDs/RCBOs – Common Issues and Misconceptions

There is some lack of understanding of RCD protection issues as seen on Facebook and during PowerLogic talks with electrical workers. Some of the common ones are:


Many fail to recognise the difference between a plain old RCD and a RCBO. RCBOs include earth leakage and overcurrent protection.

Excuses to Fit RCD Protection

Electricians are often looking for excuses and exemptions not to fit RCD protection. At PowerLogic, we are always saying not to look for excuses but look for reasons to fit them. Many old installations with porcelain fuses and circuit breakers need to be upgraded. Talk to the residents and suggest these should be graded. They will always ask “how much”. But how must value do they put on their loved one lives?

Test Button

Many think the test button is mechanical check only of an RCD’s operation. This is incorrect as the test button brings a resistor into the circuit that draws approximately twice the residual current e.g. a push button on a 30mA RCD seeks to operate at approximately 60mA. It is therefore, a very good test.

Faulty RCDs

RCDs do fail and therefore it is important to regularly test them. Additionally, RCDs form essential safety equipment. If one fails, the circuit needs to be isolated there and then. Clients need to be aware that you may need to isolate circuits without little warning. One of our clients was prosecuted for failing to do this. His client, the owner of the site, failed to replace the RCD and an employee was almost electrocuted when things went wrong. He was also prosecuted.

RCD Type for Electric Vehicles

If the manufacturer of the charging unit for EVs has specific requirements, then the requirements must always be followed. AS/NZS 3000 has requirements for New Zealand at Rule 7.9 and ‘informative’ Appendix – Appendix P. Electricians will be well advised to follow these requirements.

Type A RCDs can deal with a d.c. current component of up to 6 mA. Where allowed by the manufacturer and the EV charger has d.c. residual protection of up to 6 mA (what is called RCD-DD), a Type A RCD can be used to protect the final subcircuit at the board.

Some manufacturers will specify a Type B RCD. These are now available from some manufacturers as an RCBO (combined package of overcurrent and RCD protection) at a width of 2 poles.

Discrimination/Selectivity of RCDs

Whilst the discrimination discussion in the Wiring Rules is based around overcurrent protection, electricians are well advised to ensure reliability of supply by also including the coordination/selectivity/grading of RCD protection. IEC 60364.5.53 provides for the discrimination of RCDs in series by:

  1. Having the upstream RCD with a selective delay. This is called a Type IV RCD or sometimes called a Type S as they are generally labelled with an ‘S’ in a square; and
  2. Upstream RCD having a higher rated residual current than the downstream device.

Type IV RCDs have a minimum delay of 130ms at rated residual as per the respective equipment standard. This should still ensure disconnection within 300ms if the downstream device fails. One manufacturer supplies them in a Type A or Type B setup. Other manufacturers may supply the full range with a selective delay.

Testing and Tagging of RCDs

RCDs should be regularly tested as previously mentioned. The test frequency requirements of AS/NZS 3760 and AS/NZS 3012 (for construction and demolition sites) or other relevant standards are to be followed.

Tests include the push button and testing at the rated residual current with specialised test equipment. Additional tests can also be carried out.

Testing to AS/NZS 3000

Tests for compliance to the Wiring Rules are:

  • Testing that the poles actually switch
  • Operation of the RCD by push button tests or by using special test equipment.

Many don’t realise the switching of the poles needs to be tested. This is best done de-energised using an ohm meter.

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