So I have bought a new light fitting for our shower room (This is the bulb.)

On the instructions it says if fitting in a bathroom a 30mA RCD is required. We have a fairly old fuse box, so wired fuses no RCDs. I see the lighting circuit has a 5A fuse on it. Does this actually equate to a 5,000mA RCD?

Does this mean the light fitting is no good for our circuit? Would I need to wire up a whole new circuit just for this light? Why would fitting it in the bathroom affect the size of RCD required?

  • Can I presume you are in a 240V country? – ThreePhaseEel Dec 29 '15 at 15:36
  • Yes - United Kingdom – Mongus Pong Dec 29 '15 at 16:34

My understanding of UK law (I am not a lawyer) is that like-for-like replacement does not require you to bring an electrical installation up to current standards.

I'd interpret that to mean I can change an incandescent lightbulb to an LED bulb, of same or lower real wattage (not equivalent wattage), in the same fitting, without needing to worry about RCD protection.

I would make sure that the fitting is properly earthed (assuming it has metal parts) and that people using the bathroom cannot reach the light fitting.

I would also make sure that the fitting allows for adequate flow of air around the bulb as LED-based bulbs seem to need cooling much more than incandescent bulbs do.

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  • Thanks, I have fitted the light. Reading around about RCDs they do sound quite useful. We will probably update our fuse box shortly. – Mongus Pong Jan 4 '16 at 13:27

An RCD is different from a fuse. An RCD (residual current device) is known as a GFI (ground fault interrupter) or GFCI (ground fault current interrupting) device in the US.

In non-technical terms, if the RCD device sees current "missing" on a circuit - more leaving than returning - it shuts down power, based on the idea that that missing current may be starting a fire or shocking someone.

A 30ma RCD trips when 30ma of current is "missing." In some locations, a 30ma trip is the maximum allowable for personal protection, prevention of electrical shock injuries.

RCD / GFCI are especially important in bathrooms because people are in contact with water and pipes, which increases electrical hazards.

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  • I see. So would you use an RCD in place of a fuse? Would you generally keep your bathroom lights on a different circuit to the rest of the lights because of this? – Mongus Pong Dec 29 '15 at 16:42
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    In the US, if the panel isn't too old, you can usually buy a GFCI circuit breaker to replace the plain one in the panel. I believe similar devices are available for not-too-old panels in the UK. In the states, another option is to install a GFCI device somewhere in the circuit, wired to add GFI protection downstream. This can be a "deadface" GFCI, or a GFCI receptacle. I do not know if that's an option for you, hopefully someone better versed in the rules and products in the UK will chime in. – batsplatsterson Dec 29 '15 at 17:39
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    In the UK, an MCB in a CU can only be replaced by an RCBO, not with a plain RCD. The RCBO combines both functions - residual current detection (e.g. earth leakage) and, crucially, overcurrent detection to prevent fires starting in circuits e.g. due to too-many high-power appliances simultaneously operating on a ring circuit. If you buy a new CU it is likely to include one or two RCDs that each protect a group of MCBs (AFAIK) – RedGrittyBrick Jan 4 '16 at 15:11

Ok this is a very nice question to every electrician and Electrical personals but I would like to say that you can choose to ignore the rcd if only u have proper and quality earthing system to all your exposed metal workenter image description hererccb

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