RCDs, DC leakage and plug-in solar

What an RCD does

An RCD compares the current flowing out along the live wire with the current coming back along the neutral. If some of it is missing, it is leaking somewhere, possibly through a person, and the RCD switches the circuit off. An RCD used for additional protection against electric shock is expected to disconnect within 40 milliseconds.

In a modern consumer unit you will usually find either one or two RCDs protecting groups of circuits, each circuit with its own miniature circuit breaker (MCB), or an RCBO on every circuit. An RCBO combines an RCD and a circuit breaker in one device. Both arrangements have a switch for each circuit, which is how you tell them apart from older rewireable fuses.

RCD types: AC, A, F and B

RCDs come in types that describe what kind of leakage they can detect:

• Type AC detects alternating leakage current only. It is the older design and is still found in many homes.
• Type A also detects pulsating DC, the kind produced by many household electronics, and copes with a small amount of smooth DC.
• Type F adds protection for the mixed-frequency currents some variable-speed appliances produce.
• Type B also detects smooth DC.

The type is marked on the device, usually as a letter or a small symbol. If you cannot tell which you have, an electrician can.

Bidirectional RCDs: which way the power flows

Type is only half the question. The other half is direction. Most protective devices are designed on the assumption that power arrives at the line terminals and leaves at the load terminals. Plug-in solar breaks that assumption, because the kit pushes power back into the socket circuit.

A device built for that is called bidirectional. The IET’s Wiring Matters puts it simply: “a protective device that does not have markings to indicate line and load terminals is a bidirectional device, where power flow in either direction will not cause damage”, and “unidirectional protective devices should not be used for such power sources as the power flow could be in either direction”. Applying a supply to the load side of a unidirectional device can damage its internal electronics, and in the worst case disable the residual current protection without any outward sign that anything has changed.

The safety charity Electrical Safety First has gone further than the regulations require. Its published advice is that “given unresolved concerns about the potential impact of plug-in solar panels on certain RCDs, we recommend households have at least a Type A bidirectional RCD protecting circuits intended for use with plug-in solar panels”, and that “until these concerns are addressed, homes with Type AC RCDs should not use plug-in solar systems without first upgrading to a minimum Type A bidirectional RCD on those circuits”. It also says a kit should never be connected through an extension lead, cable reel, multiway adaptor, travel adaptor or plug-in RCD adaptor, and that households should have their installation assessed by a competent electrician registered with a competent person scheme before buying or using a kit.

This is advice from a charity, not a condition in the regulations: nothing in the rules requires a particular RCD in your home, and the duty they place is on the kit instead. But it is advice worth acting on, because it is cheap to check and expensive to get wrong.

How to tell what you have. Consumer units are rarely labelled “bidirectional”. Note the manufacturer and the model code printed on the RCD or RCBO, then look that model up in the manufacturer’s own datasheet: terminal markings for line and load, or a datasheet that names a supply side, indicate a unidirectional device. Treat that as a starting point and have it confirmed by a registered electrician, who can also tell you what replacing it would involve. Replacing a consumer unit is not a small job and can cost more than the kit.

Why DC leakage matters

Inverters, chargers and power supplies can leak a small amount of smooth DC current into your wiring. Enough of it can make an RCD less sensitive, so it may not trip when it should. Older type AC devices are the most affected. The government’s consultation response points out that this is not unique to plug-in solar, because most modern switch-mode power supplies leak small amounts of DC as well, and says it welcomes the industry moving to type B or F devices in the longer term.

For comparison, the response notes that the standard for electric vehicle chargers keeps DC leakage below 6 mA for use with type A RCDs.

What the rules require of a compliant kit

The government’s Interim Product Specification is the rulebook every compliant kit must meet. On RCDs it says the device must be designed so that it “does not adversely affect the operation of upstream residual current devices”, and that any smooth DC residual current into the mains must not exceed 5 mA in normal operation and relevant fault conditions, tested at maximum output. That 5 mA limit was added after the consultation.

The specification also requires manufacturers to tell you how to identify the protective devices in your consumer unit, how to test your RCD with the kit running, and when to call an electrician. The government says it is running further tests on older type AC devices and will review the 5 mA limit once the results are in.

A kit meets these requirements only if that exact model is listed as compliant on the ENA Type Test Register. See every kit the register lists.

What engineers are still worried about

The Institution of Engineering and Technology (IET) told New Scientist in September 2026 that its concerns about RCDs were not resolved before plug-in solar became legal. It says most RCDs in UK homes were not designed for circuits where power can flow in both directions, that the government’s tests used kits bought abroad and new RCDs, and it advises an electrical inspection before you commit.

The Department for Energy Security and Net Zero told the magazine that an independent contractor tested plug-in solar devices on UK house wiring and found them safe, and that tests with older protective devices found no evidence their protection was affected. Read both sides of the argument.

How to check your consumer unit

1. Look without opening anything. A row of switches, with RCBOs on each circuit or circuit breakers behind one or two RCDs, is a modern set-up. Fuse carriers without switches are an older design.
2. Check the labels and find which sockets are on the circuit you want to use: switch one breaker off and see which sockets lose power. Sockets that go off together are on the same circuit.
3. Use a socket circuit only, not a lighting circuit or a spur that feeds fixed equipment such as a cooker or boiler, and plug straight into the wall socket, never an extension lead or adaptor.
4. Test the RCD with the kit running. With the kit producing power, press the test button on the RCD or RCBO that protects that circuit. It should trip at once. Switch it back on afterwards.
5. Stop and call an electrician if it does not trip straight away, or if your consumer unit is old, unlabelled or of unknown condition.

These checks follow the consumer guidance the government’s specification tells manufacturers to give. They do not replace an inspection by a qualified electrician.

When to call an electrician

• You are not sure how to identify your circuits safely.
• Your consumer unit does not have modern protective devices, is not clearly labelled, or you do not know its condition.
• Your RCD does not trip straight away when you test it.
• Your kit has more than 960 W of panels: the specification says owners of these should consider a professional assessment of their electrical installation first.
• Anything would need changing in your fixed wiring. That is work for a qualified electrician only.

See do you need an electrician for plug-in solar.