The Questions of Residual Current Devices

The terms RCD, RCCB, RCBO and MRCD are often used although not always understood. Residual current devices are categorized into four key product areas.

 

 

A device that combines the protective elements of a MCB and a RCCB is called a Residual Current Circuit Breaker with Overcurrent, or an RCBO.  These devices come in various product orientations depending upon the requirements of number of poles/phases being protected, module spaces, as well as electrical characteristics.

 

This is a form of RCCB that can be assembled with a MCB by the installer to become a RCBO.  This arrangement allows special characteristics of a MCB to be married to an RCCB in one assembly.  The RCD Block is covered by the standard AS/NZS 61009 Annex G allowing for a residual current unit designed for assembly on site.  A requirement of the standard is that it shall be possible to assembly the residual current unit and the MCB only once. If the unit is disassembled there shall be permanent visible damage of the disassembly.  This means the MCB characteristics cannot be altered easily once a design choice was made unless the complete unit is replaced.

 

A Modular Residual Current Device (MRCD) is a device that can detect a residual current usually via an externally mounted toroid and provides a trip signal contact that can operate a circuit breaker via a shunt trip or similar.  These devices usually provide the facility to adjust the sensitivity of the earth leakage current as well as a time delay for operation of the relay.  (Note: it is not allowed in the standards to provide extended time delays on 30mA protection).  An amendment to the standard AS/NZS 60947 Annex M requires modular MRCD’s to comply with a total operating time of the MRCD and the associated circuit breaking device, known as the combination time.  Before the Annex M clause a MRCD complied with an operating time in the standard without consideration of the operating time of the associated circuit breaker.  At present MRCD’s are available that comply with either the operating time with or without the associated breaker and it is essential to ensure the correct MRCD is selected according to the requirements of the installation or specification.  The manufacturer of the MRCD should have tested the combination of the MRCD and circuit breaker to ensure compliance with Annex M.

 

Residual current devices are also categorized according to their functionality.   Depending upon their design an RCD can operate either dependent of line voltage or independent of line voltage.  This basically means the RCD can sense and operate on the presence of an earth fault with or without its appropriate supply voltage connected.

RCD behaviour in the presence of non-sinusoidal earth faults

Residual current devices are categorized according to the wave form of the earth leakage currents they are sensitive to. 

 

Type AC – suitable for alternating currents only

Type A – suitable for alternating currents and/or pulsating currents with DC components

Type B – suitable for alternating currents and/or pulsating currents with DC components and continuous fault currents

 

The Type B device is sometimes referred to as a universal type. Please note that although the Type B device can sense a continuous (DC) earth fault waveforms it is not designed to work with DC supplies. 

 

Type B devices can also sense high frequency earth faults which makes this type the recommended model for VSD applications. Type B devices also provide a high degree of resistance to nuisance tripping.

 

When protecting a UPS should we use a Type AC or a Type A RCD?  The electronics of a UPS can generate a pulsating current waveform that could lead to an earth fault of similar pulsating current waveform.  For this reason a Type A RCD would be the best option as this can detect these earth fault waveforms.

 

All residual current devices are very reliable and high precision as they are a safety device.  Like any electrical protective device they will tend to spend most of their life in the ON position which does not allow the internal mechanism to “exercise”.  For this reason all manufacturers recommend that RCD’s are operated on a regular basis by pressing the test button on the front of the device and allowing it to trip and then be reset.  This action proves the RCD is in working order as well as providing movement for the mechanical components inside the device to ensure they have a free action.  

 

Some RCBO devices have an extra connection required in addition to the usual power connections of active and neutral.  This extra wire is provided on RCD’s that have a flying lead as the neutral supply connection and is usually in the form of a small gauge cable called a potential earth.  The purpose of the potential earth is as a safe guard against the loss of the neutral connection on voltage dependent RCD’s.  As stated above voltage dependent RCD’s require the supply voltage in order to operate and a loss of neutral will prevent the operation of the RCD under a fault condition even though an active could be supplied to the load.  The potential earth connection ensures the safe operation of the RCD or RCBO even without the neutral.

 

The safe trip time of an RCD is defined in the appropriate product standards, both IEC standards and AS/NZS standards.

 

This table reflects the requirements of AS/NZS 61008 (RCCB) and AS/NZS 61009 (RCBO) for maximum operating times.  The Type 1 device relates to RCD’s with a sensitivity of 10mA.

 

An RCCB does not have overcurrent protection therefore this device cannot protect against short circuit fault currents.  In the manufacturers technical data a recommended upstream protection by fuse or circuit breaker should be listed to allow the appropriate back-up protection levels to be achieved. 

 

Ian Richardson

ABB Australia