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18th Edition Mythbuster

Guide to the New 18th Edition Wiring Regulations

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www.cudis.co.uk<br />

Unwanted Tripping (531.3.2)<br />

RCDs shall be selected and the circuits subdivided in such a way to avoid unwanted tripping by protective<br />

conductor currents and/or earth leakage currents. The accumulation of such currents shall be not more than<br />

30% of the rated residual operating current e,g, 30% of the RCD rating of 30mA. Another option is to have more<br />

individual RCD/RCBO circuits or even use a RCD/RCBO for every circuit and Cudis have recently introduced a<br />

range of RCBO loaded boards to cater to meet this need.<br />

Current Ratings (536.4.202)<br />

The rated current of a switch or RCD shall be based upon one of the following:<br />

- Sum of final circuit current demand after any applicable load diversity factors or<br />

- As above together with allowances for diversity between final circuits or<br />

- The sum of the OCPD’s rated current multiplied by a diversity factor<br />

Cudis recommend a diversity factor of 60% for it’s OCPDs and also recommend that designers conduct proper<br />

electrical design assessments rather than standardising on 100A RCDs in consumer units, as currently suggested<br />

by one OCPD supplier, as this may result in inappropriate device selection.<br />

Type of RCD (531.3.3)<br />

Differing types of RCDs behave differently in the presence of DC components or different<br />

frequencies. The appropriate RCD should be selected for the specific application:<br />

Cudis have decided to retain their high integrity dual RCD boards with Type AC RCDs but<br />

have available Type A RCDs and Type A miniature RCBOs which the installer can buy, and<br />

separately change, for less common applications.<br />

RCDs are marked with a different symbol according to their type and the most common types<br />

are shown below.<br />

Type AC:<br />

RCD can detect & respond to sinusoidal residual AC currents. May be used for general<br />

purposes and suitable for the majority of applications. Other RCD types, described below, use<br />

more expensive electronic tripping technology and are used in special applications.<br />

Type A:<br />

RCD can detect & respond to sinusoidal residual AC currents and residual pulsating DC<br />

currents up to 6mA.<br />

Suitable for electric vehicle charging.<br />

(722.531.2.101: each charging point shall be protected by it’s own RCD if at least Type A having<br />

a rated residual current not exceeding 30mA).<br />

Type F:<br />

RCD similar to Type A but for composite residual currents and pulsating DC currents up to<br />

10mA.<br />

Suitable for equipment with frequency controlled speed drives.<br />

Type B:<br />

RCD can detect & respond to sinusoidal residual AC currents and pulsating DC currents and<br />

smooth DC currents up to 20kHz.<br />

Suitable for electric vehicle charging, photo voltaic and wind generators.<br />

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