Using the correct Type of RCD reduces
the risk of Fire and Electrocution
The term “Type” when related to RCDs, refers to the type of residual current that the
RCD can be subjected to and operate correctly. 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.
E
lectrical loads are characterised by the current they draw from the supply, and the eff ects on the shape of the
supply waveform i.e. linear or non-linear load - see examples below. Linear loads produce sinusoidal residual
currents. 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 defi ned by the internal design and components included in the equipment
and the location of the earth fault (generic examples are given below). 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 manufactures installation and operating instructions relating to RCDs.
It is an off ence 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 – see Reg 120.3
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 signifi cantly 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 fi res
resulting from residual currents tracking across contaminated
surfaces: See on-line video “How RCCBs Prevent Fires”
WWW.doepke.co.uk
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 fi re.
This article provides a synopsis of Doepke Technical Publication 16, covering RCD principles and selection for UK installations,
which can be downloaded from: http://www.doepke.co.uk/download/Techpub-16.
Chaz Andrews – Technical Manager, Doepke UK Ltd
Issue 31
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