Selecting RCDs

Factors such as the rated voltage, load current, residual current and number of poles should be fairly obvious, so I will deal with some of the other discretionary factors.

1. AC, A or B Type
In order to choose an RCD, the user or installer should first of all decide on the type of protection required. As an absolute minimum requirement, all RCDs will provide protection against AC residual currents. If protection against pulsating DC (rectified AC) fault currents is also required, the user should choose a Type A device.

One problem that arises here is how the user or installer is to know where pulsating DC residual currents could arise. The answer is that any circuit where the mains supply is likely to be rectified will have the ability to produce pulsating DC residual currents. This applies to power tools, motor speed controllers, etc. In some countries, Type AC RCDs are not permitted, therefore if you are making a panel for an overseas customer it will be important to check their national rules to see if Type AC devices are acceptable. Whilst Type AC RCDs are permitted in Ireland and the UK, the Wiring Rules in each country do warn installers to take into account the likely effects of electronic equipment on the installation and to select an appropriate RCD.

Type B RCDs are used where DC residual currents could occur, e.g. three phase rectified supplies, but also with microgenerators or SSEGs (small scale electricity generators) such as solar panels, wind generators, etc. IEC 62109-1 specifies requirements for inverters used in photovoltaic systems (solar panels) and states clearly that only Type B RCDs should be used in such systems because Type A or Type AC RCDs cannot provide appropriate protection. Although the use of Type B RCDs has grown due to the requirements of photovoltaic systems and other special applications, their use has also grown because they can provide protection against any type of earth fault current and they future proof an installation.

2. Discrimination in terms of (IΔΝ) and Trip Times.
An installation can comprise of a main circuit and several sub-circuits. If the installation is protected by a single RCD on the main circuit, that device will trip in response to a rated residual current on any part of the installation including the sub-circuits, thereby removing power from the entire installation. It is common practice to fit an RCD on the main circuit with additional RCDs being fitted on some or all of the sub-circuits. Such practice is referred to as discrimination. A rated residual current on any sub-circuit will trip the local RCD only. However, in the event of a major earth fault current or in the event of failure of a downstream RCD to trip, the main RCD will trip. As a rule, downstream RCDs will have a trip current and a trip time less than that of the upstream RCD, i.e., downstream RCDs will be more sensitive and faster responding than the upstream RCD. Downstream RCDs should never have trip currents or trip times greater than those of the upstream RCD.

3. Response to Abnormal Supply Conditions
If low supply voltage conditions are not of concern, use an ML type. If low supply voltage conditions are a concern, use an EL type.

4. Solid or Opening Neutral.
Some RCDs are fitted with a solid neutral conductor which means that the neutral will not be disconnected from the load when the RCD trips. These devices are only available in RCBO form, not in RCCB form. These devices are sometimes referred to as single pole (SP) RCBOs. This term is not strictly correct because it is possible to have a three pole RCBO which also has a solid neutral. However, it is fair to say that the vast majority of RCBOs with solid neutral are SP types. SP RCBOs can come in single module (18mm) or two module (36mm) widths because their design is usually based on an existing MCB. Confusion sometimes arises because a two module width RCBO may give the impression that it is a two pole device. This is not a problem where disconnection of the neutral is not required. However, where disconnection of the neutral is required, (for example on IT and TT systems), it is vitally important to ensure that a device with a solid neutral is not used. The main advantages of single module SP RCBOs are that they can be used to replace a single pole MCB thereby adding RCD protection to a circuit with overcurrent protection, and they take up the minimum amount of space for an RCD. The use of SP RCBOs has grown substantially in Ireland and the UK in recent years due to revisions in the Wiring Rules in both countries.