RCD Socket or RCD Plug: What Each One Actually Protects
Updated: Oct 9
An RCD socket is a wall socket with a residual current device built in, so whatever is plugged into it is protected. An RCD plug, or plug-in RCD adaptor, sits between one appliance or extension lead and any ordinary socket, so it protects only that lead. Both are useful, but neither protects the fixed wiring in your walls the way an RCD in the consumer unit does.
What an RCD does, in one paragraph
An RCD constantly compares the current going out on the line conductor with the current coming back on the neutral. If some of it is leaking to earth, possibly through a person, the two no longer match and the RCD cuts the power. Electrical Safety First says an RCD can disconnect in as little as 40 milliseconds, which is fast enough to reduce the severity of a shock. For additional protection, BS 7671 uses RCDs with a 30 mA rated residual operating current, so that is the rating to look for on any socket or plug-in unit.
Electrical Safety First names three main types: fixed RCDs in the consumer unit, socket-outlet RCDs, and portable RCDs. This post is about the last two.
RCD socket: pros and cons
An RCD socket replaces an ordinary wall socket and has test and reset buttons on the faceplate.
It is always there. Nobody has to remember to use it, which suits a garage, a workshop or the socket nearest the back door.
It protects anything plugged in, including extension leads run from it.
Fitting one can be a like-for-like swap. Replacing an existing socket with an RCD socket in the same place is not notifiable under Part P. The wiring is the same as any socket, as our guide on how to wire a plug socket explains, though the body is deeper and may need a deeper back box.
The limits are just as clear. It protects only what is plugged into it, not the cable behind it, not the other sockets on the circuit, and not your lights. For a room or a whole house, fixed protection is the better answer, which is where our socket and power installation work often ends up: adding RCD sockets where they make sense and pointing out where the circuit itself needs protecting.
RCD plug: pros and cons
An RCD plug is either a plug with an RCD built into it, fitted to the appliance's flex, or a plug-in adaptor that goes between the wall socket and the appliance or extension lead.
It goes where you go. Take it to a relative's house, the allotment or a job.
It is the simplest fix for a house without RCDs. Electrical Safety First's advice is that if you have no RCD in your consumer unit, or it is not working, you should use plug-in RCDs for all the sockets in your home.
It is the standard advice for garden tools. Electrical Safety First says to use an RCD with all outdoor electrical equipment, and a plug-in one if your fusebox does not have one.
The downsides: it only works if someone plugs it in, it protects one lead at a time, and quality varies. Electrical Safety First's product recall list has included a plug-in RCD adaptor, so buy from a reputable seller and check for recalls.
Which one to use when
Older consumer unit with no RCD: plug-in RCDs now, and plan for fixed protection.
Mower, hedge trimmer, pressure washer: a plug-in RCD every time, unless the socket is already RCD protected.
Garage or workshop socket: an RCD socket is a sensible permanent choice.
New outdoor socket: fixed RCD protection is required, either at the board or built into the socket. A plug-in unit is an extra, not the protection.
Why neither replaces an RCD protected circuit
BS 7671 requires 30 mA RCD protection for socket outlets rated up to 32 A in new work. Other regulations in it require RCD protection for cables concealed in walls less than 50 mm from the surface (unless they have an earthed metal covering or mechanical protection), and for lighting circuits in homes. An RCD socket or plug does nothing for any of those. If a nail goes through a cable in the wall, or a light fitting develops a fault, only an RCD at the consumer unit can see it.
That is why, on an older board, the long-term fix is a consumer unit upgrade with RCD or RCBO protection on every circuit. RCD sockets and plugs then become extra protection at the points of highest risk, not the only protection.
Testing them
Every RCD has a test button, and it should trip when pressed. IET guidance now suggests testing fixed RCDs every six months (the label used to say quarterly), and a handy reminder is to do it when the clocks change. For portable units, follow the maker's instructions. If an RCD does not trip when tested, stop relying on it: replace a plug-in unit, and have an electrician check a fixed RCD or RCD socket.
SM Electrical fits RCD sockets, adds RCD protection and upgrades consumer units for homes and businesses across Shifnal, Telford, Wolverhampton and the West Midlands, including anyone looking for an electrician in Bridgnorth.
RCD socket and plug questions, answered
Is an RCD socket the same as an RCBO?
No. An RCBO lives in the consumer unit and combines RCD protection with overcurrent protection for a whole circuit. An RCD socket protects only what is plugged into it.
Can I plug an extension lead into an RCD plug?
Yes, if the adaptor and lead are rated for the load. Everything on the lead is then protected, though it is still one lead, and the rule about never plugging one extension lead into another still applies.
Why does my RCD socket keep tripping?
Usually an appliance with a leakage fault, damp in an outdoor lead or plug, or a failing RCD. Unplug everything, reset it, and plug items back in one at a time. If it trips with nothing plugged in, the socket or the wiring needs checking. When you look inside any accessory, knowing the live wire colours helps, but a tripping RCD is a job for an electrician.
Comments