Chaz Andrews, Doepke UK Technical Manager, explains why it’s important to select the correct Type of RCD – Amendment 3 BS7671 – and identifies some of the basic terms.
The term “Type” when related to RCDs, refers to the type of residual current that the RCD can be subjected to and operate correctly, to meet circuit protection requirements.
Selecting the correct Type of RCD and sensitivity requires an understanding of the existing Regulations, RCD characteristics, the installation design/layout/risks and the characteristics of the loads.
RCD Generic term for a fault breaking device operated by residual current (I∆n) e.g. RCCB, RCBO
RCCBs (EN61008) do not include short circuit and overcurrent protection, which must be provided by external fuses and or MCBs. RCCBs are manufactured in ratings <125A with a wide variation in sensitivity and Types.
RCBOs (EN61009) include short circuit and overcurrent protection, but are normally limited by their characteristics to final circuit protection applications < 50A.
CBRs (EN947-2 Apx. B) MCCB with an in-built RCD protection module for applications >100A.
Note: RCMs (Residual Current Monitors) cannot be used in place of RCDs.
Earth Leakage Current (Protective Conductor Current): This is the current that should flow to earth via the protective (earth) conductor under normal operation. In the event of the loss of the PE conductor, a person touching uninsulated parts of the equipment would be subject to the leakage current to earth.
Residual Current: Term used to distinguish between the earth leakage current present in a healthy circuit and current that flows in the event of an insulation fault to earth.
Rated residual operating current I∆n: RCD sensitivity I∆n is defined in mA.
RCDs with an I∆n ≤ 30mA: Can be applied as an “additional protection measure” See 410 and 415.
RCDs with an I∆n ≥ 100mA: Can be applied for automatic disconnection of the supply/fire protection only.
Protection against the risk of fire: Wiring systems protected by an RCD, I∆n must be ≤ 300mA.
In the UK, Time delay or Selective RCDs “S” must only be applied in ratings 100mA and above.
Types of RCD
Electrical loads are characterised by the current they draw from the supply, and the effects on the shape of the supply waveform i.e. linear or non-linear load – see examples below.
Li near loads connected to 50Hz supply produce sinusoidal residual currents at 50Hz. Non-linear loads produce complex leakage and residual currents with various characteristics including High frequency AC, Pulsed DC and Smooth DC components.
Selecting the correct Type of RCD
The residual current under fault conditions will be defined by the internal design and components included in the equipment and the location of the earth fault. Consequently the manufacture must specify the Type of RCD to be used with their equipment. If this information is not available, the Designer/Constructor to meet BS7671, would have to use an RCD that covers all of the possible scenarios i.e. Regulation 132.8: “The protective devices shall operate at values of current, voltage and time which are suitably related to the characteristics of the circuits and to the possibilities of danger.” The Inspector cannot sign-off the installation if it does not meet BS7671.
Check the equipment manufacturer’s installation and operating instructions relating to RCDs. It is an offence under various UK Laws, to install equipment without the appropriate manufacturer’s instructions.
Regulation 114.1 explains the relationship between BS7671, Codes of Practice and statutory Regulations, relating to installation design/performance and equipment used in or connected to the installation.
Using the correct Type of RCD is critical to achieving the required disconnection times. The initial shock current will be limited by the resistance of the shock path through body + any additional resistance in the fault path. Dry and unbroken skin is a poor conductor of electricity, however if the skin blisters or is broken, the resistance path will drop significantly and the shock current will rise accordingly. Reducing the duration of shock current reduces the risk of irreversible injury.
RCD operating time is critical, with regard to reducing the damage caused by the initial shock current. Using the incorrect Type of 30mA RCD could delay the operation of the RCD or prevent it tripping, resulting in an increased risk of electrocution.
RCDs also help to reduce the risk associated with fires resulting from residual currents tracking across contaminated surfaces.
RCDs must operate reliably when other means of protection have failed. Designing, Installing/ Signing-off an installation which is incorrectly protected, could increase the risk of serious injury from electrocution and or fire.