Adding a Shower Circuit to an Older Board Without RCD Protection
A 1970s house in Bridgetown has a consumer unit with rewireable fuses and no residual current device. The client wants a new 8.5 kW instantaneous electric shower installed in a bathroom. The existing board has a spare way. The electrician proposes to run 10 mm² twin-and-earth from the spare way to a 45 A double-pole switch outside the bathroom door, then 10 mm² to the shower unit, protected by a 40 A fuse. The bathroom contains a metallic bath and a metallic shower mixer connected to copper pipework.
1. Identify the protection deficiencies in this proposal and state what must be added.
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Several deficiencies must be addressed before the circuit is acceptable.
- No RCD protection. A shower circuit in a bathroom, supplying equipment in a location where the user is likely to be in contact with earthed metalwork and water, requires 30 mA RCD protection. The existing board has none, so a 30 mA RCD must be provided - either by replacing the board, adding an RCBO for this circuit, or fitting a 30 mA RCD unit upstream of the shower circuit. The device must be double-pole or otherwise arranged so that both live conductors are disconnected.
- Overcurrent protection. An 8.5 kW shower at 230 V draws about 37 A. A 40 A fuse is marginally adequate for the load but must be checked against the cable's tabulated capacity for the installation method; 10 mm² clipped direct is typically rated around 52 A, so the cable is adequate, but the fuse rating must not exceed the cable capacity after any derating for grouping or thermal insulation.
- Supplementary bonding. The metallic bath and the metallic shower mixer, connected to copper pipework, are extraneous-conductive-parts within the bathroom. Supplementary bonding may be required between them and the protective conductors of the shower circuit, unless the requirements for omitting it are met - which depends on the main bonding being in place and the disconnection times being satisfied. This must be assessed on site, not assumed.
- Switch location. A double-pole switch outside the bathroom door is acceptable, but its suitability for the zone and the client's ability to operate it in an emergency must be confirmed.
2. The client asks whether the existing rewireable fuses can stay. Explain the testing and verification you would carry out before certifying the new work.
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The existing fuses can remain only if the installation as a whole is verified as safe and the new work is properly integrated. Certification of the new circuit requires more than testing the new cable.
Tests to perform:
- Continuity of protective conductors and main protective bonding, confirming the bonding conductor size and that connections are sound.
- Ring final circuit continuity if the board supplies ring circuits, since adding load may expose a broken ring.
- Insulation resistance of the new circuit and, where practical, of existing circuits, recording values.
- Polarity at the shower switch and at the shower unit.
- Earth-fault loop impedance at the shower unit, confirming the 40 A fuse will disconnect within the required time for the circuit; if it will not, the circuit design must change.
- RCD operation, including the operating time at rated residual current and at half rated current, for the newly fitted 30 mA device.
- Functional testing of the shower and the switch.
Documentation: a minor works certificate or an electrical installation certificate as appropriate, with a schedule of test results, and a note of any departures from the regulations with justification. The client should be told in writing that the rest of the installation has no RCD protection and that this is a recognised limitation, with a recommendation for upgrade.