Plug-in Solar and RCDs: What Is Recommended, What Is Required

Recommended: yes. Legally required: no. Both halves of that matter, and most of the argument happening about this comes from people holding one half and assuming the other is settled.

Electrical Safety First — the UK charity whose consumer guidance retailers are pointing buyers at — recommend that households have “at least a Type A bidirectional RCD protecting circuits intended for use with plug-in solar panels”, and that homes with “Type AC RCDs should not use plug-in solar systems without first upgrading”. That is real, published, credible safety advice, and dismissing it as a myth is wrong.

It is also not a legal requirement. A respondent to the government’s consultation suggested the plug should incorporate bi-directional residual current protection. The response was that the government “has not identified sufficient evidence to justify mandating a specific plug design incorporating bi-directional residual current protection”. No circuit RCD type is mandated in law for plug-in solar.

The phrase “Type A bidirectional” reads like a single property of a single device. It is two, and they are checked differently:

• The type marking — AC, A, F or B — which describes what kind of residual current the device can detect.
• Bidirectional capability — whether the device is built to work with energy flowing in both directions through it.

A device can be Type A and still be unidirectional. Because the two get run together in one phrase, the second check is the one that gets skipped — and it is the one ESF say most devices fail.

A plug-in solar device puts a small DC component onto the circuit. Different types of RCD and RCBO are designed, tested and certified for different kinds of residual current, and the type is usually marked with a symbol on the front of the device. ESF summarise them:

• Type AC — “Not designed or tested to function in the presence of any levels of DC residual current.” Unsuitable for the addition of plug-in solar.
• Type A — tolerates moderate DC residual current (above 6 mA). Suitable on the circuits it protects.
• Type B — tolerates varying levels and types of DC residual current. Suitable.
• Type F — tolerates varying levels of DC residual current (above 10 mA). Suitable.

So the recommendation is properly read as Type A or better, not Type A specifically. A common confusion worth heading off: the RCD or RCBO “type” is not the same as the B, C or D marking on a circuit breaker, which describes its tripping characteristic — a Type B MCB tells you nothing about residual current.

BS 7671 arrives at the same place independently. Regulation 531.3.3confines Type AC to fixed equipment where it is known that the load current contains no DC components. A grid-following inverter is a power electronic converter, so that positive knowledge is not available. That is the real technical basis for advising an upgrade, and it does not depend on anybody’s guidance.

The direction of travel goes further still. The government’s response notes that DC leakage reducing the effectiveness of older, mostly AC-type RCDs “is a problem not unique to plug-in solar devices”, and says that “in the longer term, we welcome the industry moving to type B or F RCD devices that don’t have this issue”.

This is the check that gets missed, and it is not the same question as the type.

Most UK homes were wired on the assumption that electricity flows one way: from the socket-outlet to an appliance. A plug-in solar device introduces a source into the circuit, so energy flows in both directions. ESF are blunt about what that means for protective devices: most are designed for unidirectional flow, and those “are NOT suitable for plug-in solar panels as they may fail to operate or ‘trip’ (in the event of a fault)”.

The practical difficulty is identification. ESF note that “few RCD/RCBOs can be easily identified as being bidirectionally capable by the consumer”. Some manufacturers use a suffix such as BD or Bi-dir on the model number, but many give no marking at all. Where there is nothing visible on the front of the device, the answer is to note the manufacturer and model number and ask the manufacturer, or have an electrician assess it. Never remove the consumer unit cover to look.

BS 7671 backs this up: Regulation 551.7.1(c) requires a protective device suitable for the purpose where energy flow is bidirectional, and 551.7.1(d) restricts connecting a source to the load side of an RCD. Regulation 551.7.2 was redrafted and split by A4:2026, and the product specification names it as the reference for replacing the overcurrent protective device of the circuit concerned — work that can only be done by a qualified electrician.

Four separate propositions get conflated in this discussion. They carry very different force, and keeping them apart is most of the skill:

• The device must not exceed 5 mA of smooth DC residual current (specification §5.7). This is a limit on the product, so it does not impair an upstream RCD. It says nothing about what is in your consumer unit.
• Consumers should be advised to check for modern residual current protection in good condition (specification §8.3.2). This names RCBOs, and is footnoted: installations using MCBs with residual current protection provided upstream “may also be acceptable, subject to verification through further testing now being undertaken by DESNZ”. Genuinely unresolved.
• Type AC is not appropriate here (BS 7671 531.3.3). A standard, not guidance — and the strongest basis for recommending a change.
• Energy flow becomes bidirectional (BS 7671 551.7.1). A suitable protective device is required where that is the case.

What none of them do is mandate a circuit RCD type for plug-in solar. If you are quoting for this work, recommend it as good practice with the reasoning above — that position holds up. Asserting a legal duty that a customer can look up and find does not exist does not.

Worth being accurate, because the honest version of this is more persuasive than the scary version. ESF state that if a home is protected by rewirable or cartridge fuses, these “will not be impaired through the addition of plug-in solar panels”. Adding a panel does not make an old fuseboard worse.

The point is a different one: an installation on fuses does not have the potentially life-saving protection that modern RCDs and RCBOs provide, whether or not anything is plugged into it. That is a conversation about the installation, and it was true before the solar arrived.

Where the consumer unit uses older fuse protection, is not clearly labelled, or is of unknown condition, the specification does say users should seek inspection and assessment by a qualified electrician before installing the product.

There is a clause in the specification that almost no coverage has picked up, and it matters more over time than anything else on this page.

Older residual current devices can be desensitised by equipment leaking small amounts of DC into the AC circuit — not just solar, but plug-in EV chargers, IT equipment and switch-mode power supplies, and it is the cumulative effect of several devices that does it. The specification therefore requires that the customer be instructed to test the RCD or RCBO periodically by pressing its test button while the plug-in solar unit is producing power.

And if it does not trip immediately, consumers “should be instructed to contact a competent professional electrician to discuss replacing the RCBO with a more modern unit”. ESF publish the same advice.

This came out of the consultation directly: because DC injection desensitising RCDs is not confined to plug-in solar, the government said consumers would be reminded to test their protective devices periodically, and the specification was amended accordingly.

For a householder, in order: identify what is protecting the circuit you intend to use; check the type marking on the front of the device without removing any covers; establish whether it is bidirectionally capable, which usually means asking the manufacturer with the model number to hand; and if either answer is wrong or unknown, get it assessed before you buy rather than after.

For an electrician, the proportionate remedy is usually a single RCBO change on the target circuit rather than a full board replacement — and it carries its own certification exactly as it always did. Replacing the overcurrent protective device of the circuit concerned is named in the specification as work that only a qualified electrician does, pointing at BS 7671 551.7.2.

A word on how to sell it. Nothing legally obliges a householder to involve you, and saying otherwise will be found out. The wider rules are on the plug-in solar hub page, and they point people at an electrician often enough without anyone needing to overstate it.