RCD description
An RCD ‘Residual Current Device’ is an electrical device that monitors the current flowing through either an individual circuit such as with an RCBO ‘residual current operated circuit-breaker with integral overcurrent protection’ or when supplying multiple circuits usually within domestic installations using an RCCB ‘residual current operated circuit breaker without integral overcurrent protection’.
The RCDs purpose is to monitor the current in a circuit to detect any that may be leaking to earth either through Line to Earth or Neutral to Earth and if necessary, disconnect the circuit from the electrical supply.
The RCD monitors this by wrapping the Line and Neutral conductors around a ferrous coil within the RCD device called a toroid and because within a normal electrical circuit the line current flowing out and neutral current flowing in are equal and opposite (line current + the neutral current = 0) no voltage is induced within the coil and the device need not disconnect the supply
However, if within a circuit there was either the line or neutral conductor leakage to earth it would create an imbalance between the sum of the line and neutral passing over the coil that would induce a voltage in the tripping coil which then activates the RCD at a pre-determined level (i.e. 30 mA) and disconnects the electrical supply from the affected circuit.
RCD Types
Not to be confused with the different types available for circuit breakers such as Type B, C or Ds for BS 69898.
Due to some equipment having DC leakage current under normal operation or during fault conditions RCDs also have different types and should be used in different instances…
Type AC:
Type AC RCDs were the standard RCD at one time and can still be found in many installations especially in 17th Edition split load distribution boards.
A Type AC RCD will respond to any AC earth fault current, but they have a problem as they can easily be “blinded” by DC fault currents.
This means that if an appliance produces a DC fault current a Type AC RCD may fail to respond in the required disconnection time, if at all.
Whilst this wasn’t such a problem in years gone by, with the introduction of so many electronic devices into households that are capable of producing DC residual / fault current has meant that the wiring regulations have had to address this.
The 18th edition AMD2 have introduced some requirements surrounding the use of type AC RCDs.
These are to be used ONLY…
- For fixed equipment and NOT socket outlets of any kind
- Where any type of DC fault currents are not anticipated
Examples of suitable circuits…
- electric showers
- oven
- hob
- immersion heater, and
- tungsten lighting.
This has made the use of Type AC RCDs all but redundant.
Type A:
A much newer addition to the different types of RCDs is a Type A RCD which are used for alternating sinusoidal residual current and for residual pulsating direct current up to 6 mA.
Meaning that whilst also reacting to AC fault currents as per the original Type AC RCD, Type A RCDs are also able to react to pulsating DC fault currents that are superimposed onto an AC fault current up to 6mA.
Type A RCDs are installed to protect circuits where equipment with electronic components are installed such as…
- inverters
- Class 1 IT equipment
- power supplies for Class II equipment
- lighting equipment including dimmers and LED drivers,
- induction hobs, and
- electric vehicle charging equipment with smooth residual DC current less than 6 mA.
This protects against the “blinding” problem from any possible pulsating DC fault currents caused by electronic equipment or semiconductors.
*Note: Type A RCDs do not detect smooth DC. Therefore, they cannot be used if there is a risk of the smooth DC component exceeding 6mA.
Type F
Type F RCDs are a more recent introduction to the family of RCDs and are designed to provide further protection than that offered by Type A RCDs and although currently quite rare, it is expected that Type F RCDs will become more frequently used in the near future.
They are particularly for use where single phase variable speed drives are used as the potential waveform of fault current encountered in this instance could be a composite of different frequencies e.g., 10 Hertz + 50 Hertz + 1000 Hertz.
As more and more appliances begin to use variable speed drives that produce mixed frequency leakage currents that cannot be reliably detected by Type A or Type AC RCDs., fitting type F RCDs will be a necessity to ensure that the differing fault current characteristics do not blind the RCD.
The following modern appliances are examples of items which use variable speed drives:
- Washing Machines
- Dishwashers
- Tumble driers
- Air conditioning compressors
- Some class 1 power tools
Type F RCDs are designed to withstand enhanced disturbance characteristics such that they do not trip on surge current. They are capable of operating and safely tripping even with 10mA of pure DC superimposed onto a sinusoidal or pulsed DC differential current.
Type B:
Type B RCDs have been about for a bit longer than type F, and cover AC, pulsating DC, multi frequency DC and smooth DC fault currents so are suitable for Type AC, Type A and Type F RCD applications.
Most people will be familiar with these from EV charging points, they provide ‘cover all’ protection against a wide variety of different fault currents which may be encountered with modern electronic devices (typically using 3 phase rectifiers).
Examples of instances where a type B RCD may be required are:
- EV charging points (this protection is now often built into the EV charger itself)
- Certain Solar PV installations
- Certain medical equipment
The use of type B RCDs is stipulated in certain instances within the section 7 (special locations) part of the wiring regulations. The use of Type B RCDs has grown in recent years because they provide comprehensive protection against all types of Residual Fault Currents.
Type B RCDs are used where DC residual currents could occur, for example, three phase rectified supplies, but also with microgenerators or SSEGs (Small Scale Electricity Generators) such as solar panels and wind generators. IEC 62109-1 specifies requirements for inverters used in photovoltaic systems that states clearly that only Type B RCDs should be used in such systems because Type A or Type AC RCDs cannot provide appropriate protection.
Type B RCDs have numerous applications and are suitable for many industries including, electric vehicle charging, photovoltaic systems (solar panels), mines, tunnels, shipping, and any installation where there is a risk of DC current flowing. It is the technology that brings us closest to total protection for all known residual fault currents.
The standard Type B RCD will detect residual current faults comprising of pure AC up to 1000 hertz, rectified AC, pure DC and pulsating DC.
Type S
The Type S RCD is totally different in that the ‘type’ of the RCD does not dictate the different fault current types which can be detected by that particular RCD.
This is not a new introduction into the RCD regulations, indeed type S time delayed devices have been about for nearly as long as original type AC devices.
Indeed, you can have a Type S – Type AC RCD or a Type S – Type A RCD (confusing, I know!). Although I cannot think of any particular RCDs on the market that are Type S and cover either Type F or Type B fault current, it is not to say that these do not exist, or will be brought to market soon.
The inclusion of Type S into the naming of the RCD denotes that the RCD in question is of a time delayed tripping type.
These RCDs are mainly used in TT earth installations to provide fault protection to circuits which have a particularly high impedance, without interfering with selectivity (previously called discrimination before the 18th edition) with RCDs which may be used for additional protection.
An example of this would be an up-front S Type 100mA RCD on a TT installation. This has a time delay built in (sometimes adjustable) which means that downstream 30mA RCDs/RCBOs have chance to trip before the up-front device does (which would remove power for the whole board/installation). This ensures that selectivity is maintained with RCDs fitted in series.
You can easily identify a time delayed RCD by the logo on the device. This is adjacent to the logo displaying the ‘type’ of the RCD and is a letter S in a box
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