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Which RCD do I install?

The use of an RCD – Residual current device is to provide additional or supplementary protection to electrical installations. It’s been around since the early nineties; so, this article is for those wanting know which one to choose.

RCD’s have proven their worth in providing additional protection to the humble circuit breaker or fuse, especially for the protection of a person from electric shocks. Remember, the use of RCD’s is intended only to augment other measures of basic protection; i.e. fuse or circuit breaker. RCD’s are not to form the only protection provided on final sub-circuits.

Over the years the requirement to install an RCD, has progressed to a point where the wiring rules has made it mandatory on nearly every circuit. The preferred location of the RCD is within the switchboard at which the final sub-circuit originates. Currently the mandatory requirement is only in domestic electrical installations. RCDS shall be located within the switchboard at which the final sub-circuit originates.

Note, the mandatory requirement to fit an RCD at the switchboard at which the final sub-circuit originates is a requirement of the 2018 edition of the wiring rules for the non-residential and non-domestic installations. Some exceptions do apply refer clause 2.6 of the 2018 Wiring Rules.

As the requirement to install RCDs is becoming common knowledge, more and more questions are being asked as to what TYPE of RCD should be installed. Electricians are discovering that not all RCD’s are suitable for every application. There are numerous “types” of RCD’s on the market, with most manufacturers producing a full range using the following types of classifications;

Common types of RCDs and their appropriate applications:

Type I RCD’s

  • Designed to operate when residual sinusoidal alternating currents are detected of the magnitude not exceeding 10mA.

Application

  • Used extensively in the medical electrical installations to protect medical electrical equipment and patients.

Type II RCD’s

  • Designed to operate when residual sinusoidal alternation currents are detected of the magnitude exceeding 10mA but not exceeding 30mA.

Application

  • Most commonly available general use type RCD’s used extensively in the domestic, commercial and industrial electrical installations to protect power and lighting circuits.

 

Type III RCD’s

  • Designed to operate when residual sinusoidal alternating currents are detected of the magnitude exceeding 30mA but not exceeding 300mA. The type III RCD does not have selective tripping time delay characteristics.

Application

  • A common general use RCD used extensively in the protection of property and not personal protection. Can be installed upstream of the type II RCD’s to give discrimination, without the need to use more expensive type IV RCD.

 

         Type IV RCD’s also known as Type S RCD’s (Signifying selectivity)

  • Designed with a built-in time delay. They are a specially designed RCD for which tripping is designed to occur after a predetermined operating time delay, Corresponding to a given value of residual current.
  • The “type S” is designed to be installed upstream of RCD’s with the same value or lower values of residual current tripping. The delay designed into the S type RCD allows for the  downstream RCD to operate before the upstream Type S, avoiding unwanted nuisance tripping. Also, useful for circuits subject to harmonics for high inrush currents.

Type AC RCD’s

  • Designed to operate when residual sinusoidal alternating currents are detected. Ideal for circuits containing AC operated equipment they have a general-purpose application

Type A RCD’s

  • Designed to operate on both sinusoidal alternating current and residual pulsating direct currents. Ideal for use with circuits containing both AC and DC operated equipment.

Type B RCD’s

  • Designed to operate on sinusoidal alternating current, alternating current up to 1000Hz, pulsating direct current superimposed on a smooth direct current, residual pulsating direct currents that may result from rectifying equipment. Ideal for use with circuits containing both AC and DC operated equipment.
  • The “type B” RCD’s are usually application specific recommended to be used with solar inverters, data centres and can be used with electric vehicle recharging equipment.

Type F RCD’s (Signifying frequency)

  • The “type F” RCD’s are designed to provide protection against residual current faults Comprising of composite frequencies. Used where multi-frequency components exist in such equipment, as motor control systems [variable speed drives VSDs], inverters and the like where an AC waveform is created electronically from a DC supply. An additional advantage is that they offer a higher level of immunity to surge currents, therefore are less prone to nuisance tripping from surges, lightning etc.

Now that you have selected the correct type of RCD for your application, you now need to decide which format to use.

The format can be an RCD or RCBO – combination of both RCD function and circuit breaker function. A RCBO is also known as a RCCB – residual current circuit breakers.

Next is to determine the number of “poles”. i.e. 1 – pole single –phase circuits, 2-poles where both active and neutral are required to be switched [e.g. construction sites], 3-poles used on three-phase circuits without a neutral conductor [E.g. motors], 4-poles  for either three-phase distribution with a neutral or three single-phase circuits.

The full-load current rating is either the maximum demand of the circuits being protected by the RCD or the highest rated circuit breaker of the circuits being protected, whichever is greater – reference clause 2.6 of the wiring rules.

Selecting the required RCD to suite your particular installation can be quite complex. For support, guidance or further specific information, contact manufactures/ supplier or the technical team at NECA VIC.

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