Best practice guidance and advice from the experts at NICEIC.
RCD is a generic term that covers a range of residual current devices including RCBOs and RCCBs. Primarily an RCD is selected so that its residual operating current rating is appropriate for its intended purpose, whether that be for fault protection, additional protection or for protection against ﬁre. In addition, the following requirements of Chapter 53 of BS 7671: 2018 should be taken into account.
Compliance with standards
RCDs intended to be operated by ordinary persons should comply with the appropriate Standard listed in Regulation 5220.127.116.11. Such a device should be designed and installed so that adjustment or modiﬁcation of the rated residual operating current of the RCD can only be achieved by a deliberate act (Regulation 518.104.22.168). Whilst RCDs intended to be operated by instructed or skilled persons should comply with one of the Standards listed in Regulation 522.214.171.124.
Regulation 531.3.2 of BS 7671: 2018 requires measures to be taken to minimise the likelihood of unwanted tripping of RCDs within an installation, consideration being given to the following:
- Subdividing circuits such that any protective conductor current likely to occur during normal operation of the connected load will not cause unwanted tripping.
- Designing the installation so that the accumulation of protective currents and/or earth leakage currents downstream of the RCD does not exceed 30 % of the rated residual current of the device.
- Using short time-delayed RCDs subject to the applicable requirements of Chapter 41 being met.
- Co-ordinating between general type RCDs, selective type RCDs and time-delayed RCDs.
- Co-ordinating RCDs with surge protective devices (SPDs) according to Regulation 534.4.7.
Note: Where an RCD incorporates a time delay the device should be marked with the symbol shown.
One suitable device is an S-type RCD in accordance with BS EN 61008-1 or BS EN 61009-1.
Selectivity with respect to residual currents
In order to comply with the requirements of Regulation 314.1 to minimise inconvenience in the event of a fault, and to reduce the possibility of unwanted tripping of RCDs due to protective conductor (PE) currents, only the RCD nearest to, but upstream of, a source of protective current – whatever its cause – should operate.
Where RCDs are connected in series, as shown in Fig 2, selectivity will be achieved where:
- the upstream RCD is of selective type (Type S or time-delayed type with appropriate time delay setting), and
- the ratio between the rated residual operating current (I∆n) of the upstream RCD and the downstream RCD is at least 3:1.
Selectivity between upstream and downstream devices is not essential in all cases. Partial selectivity between devices is acceptable where this does not adversely aﬀect the safety of the installation (Regulation 536.3). For example, as shown in Fig 1 a dual RCD arrangement is typically used for domestic premises.
Where selectivity is required, veriﬁcation of the chosen design should be made by one of the methods listed in Regulation 5126.96.36.199. Typically, manufacturers will provide information specifying selectivity between RCDs.
For RCDs having an adjustable rated residual operating current and time delay, such as circuit-breakers incorporating residual current protection (CBRs), manufacturer’s instructions relating to selectivity should be taken into consideration.
Classiﬁcation of RCDs
If an RCD designed to operate with a sinusoidal current is connected to a circuit that draws a non-sinusoidal current it may not function satisfactorily. Considering, the growth of electronic devices, such as those used for power control and switching of led lighting, electric vehicle charging systems and micro-generator systems, even domestic installations are likely to have increasing numbers of circuits drawing non-sinusoidal currents with a DC component.
As a result, RCDs should not only have the appropriate residual current rating to provide fault or additional protection as required, it should also be an appropriate ‘type’ for the nature of the connected loads.
When used to describe the operating characteristics of an RCD, the term ‘type’ relates to the frequency and shape of current waveform for which tripping is assured. For these reasons, BS 7671 speciﬁes particular types of RCD for particular installations such as, for example, electric vehicle charging and PV supply installations.
RCDs are classiﬁed (AC, A, B or F) according to their suitability to function satisfactorily when carrying current of speciﬁed waveforms. Table 1 shows the symbols used to provide the user with a visual indication of the particular current waveforms (Regulation 531.3.3).
Guidance on electrical installation work, including the installation of RCDS and other electrical equipment and devices is contained in the NICEIC and ELECSA publication ‘Site Guide’.
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