RCDs – to ‘B’ or not to ‘B’?

RCD’s – to ‘B’ or not to ‘B’?

RCD requirements and tripping RCDs still remains a fairly hot topic on our incoming Segen Tech Support phonelines in recent times, so we thought we’d take a more in depth look at the whole subject of UK RCD requirements and specification when installing solar PV, battery storage and EV chargers.

Do you really need an RCD?

The easiest way to stop an RCD tripping is to not install one in the first place. RCCBs, and more recently RCBOs have become so commonplace nowadays that the decision to install an RCCB or RCBO is almost automatic. However, there are specific circumstances that mean that you don’t need an RCD at all – and if you don’t have one, then it can’t trip!

The most straightforward example is where the main fuseboard and the PV inverter is installed in the same vicinity (both within a garage or plant room). When the supply cabling for your inverter is completely visible from the origin all the way to the inverter, without disappearing into the fabric of the building then you can clearly argue that there is no real need for an RCD. You’d like to think that the risk of someone penetrating that cable which is clipped direct to the surface or within surface trunking is pretty slim.

Indeed, section 8.8 of the IET COP for Grid Connected Solar PV confirms that there is “no fundamental requirement to fit an RCD to the circuit that feeds the inverter(s). It goes on to say that wherever possible, it is actually recommended designing a system so that inverters are not fed from an RCD.

(NB – disregard this point for EV chargers. BS7671 & the IET COP states that a 30mA is needed in all cases).

So, you’ve decided on an RCD for your specific install – which type do you choose?

Back when I was a trainee electrician all those years ago, different RCD variants weren’t really something that was taught – an RCD was essentially just an RCD. We now know of course that this basic flavour of RCD (or RCCB) is nowadays referred to as an ‘AC’ type, where there are also Type A, Type B and Type F to consider in modern electrical installations for specific scenarios.

The UK has lagged behind Europe with regards to RCD recommendations for many years. European Cenelec recommendations have essentially outlawed the basic AC type RCD for years, and now the UK regs have finally caught up in the latest iteration of BS7671.

531.3.3 of the Wiring Regs has a good overview of the different RCD types, and importantly this section of the regs introduces a statement to say that an AC type RCD is only suitable for equipment with no DC loads. This means that it’s most definitely not suitable for any of the renewable technologies you will be installing, which will most definitely have DC componentry within. In essence, the AC type is now only fit for purpose to serve purely resistive load circuits, as DC within the circuit can potentially ‘blind’ the Type AC RCD so it doesn’t trip when needed or can alternatively cause nuisance tripping.

So, you’ve decided that your installation needs an RCD for additional protection. We know we can’t use a Type AC device, so we offer our attention to the Type A and Type B variants. The Regs tells us that a Type A is suitable up to 6mA of pulsed DC current. In most cases, we will find that a Type A RCD is suitable for many of the renewable technology devices that we install, however you should always check with manufacturer’s documentation or directly with the manufacturer if you have any doubts.

Also refer to the specific RCD selection tables in the relevant IET COP books.

Double Pole or Switched Neutral RCDs.

The important regs within BS7671 that we’re looking at here are 551.7.1 for the likes of PV and battery storage, relating to generating sets, and 722.531.3.1 for EV chargers.

These regs tell us that RCDs serving such equipment should be such that the protective devices disconnect all live conductors, which includes the neutral conductor. In most cases, installing the likes of an RCCB will typically tick this box (check your specific device for confirmation though), but that’s not always the case when installing an RCBO. If you look carefully at the diagram printed on the side of your RCBO you should be able to determine if your RCBO switches the neutral or not. The diagram will either show just the Line conductor switched (more common) or will show both Line & Neutral switched.

It’s worth pointing out that a switched neutral device does not necessarily strictly mean ‘Double Pole’. A double pole device works in a specific way offering overcurrent protection on both the live & neutral side. Often RCBOs are described as being 1P+N or SP+N, meaning single pole with a switched neutral – whilst they are compliant in switching both conductors, they will only have overcurrent protection on the live conductor.

Bidirectional devices

This is an interesting subject. Whilst I can’t personally find any specific reference in BS7671 or the IET COP for Solar PV, there is definitely a reference to bidirectional devices in the IET COP for Electrical Storage Systems, recommending that the device takes into consideration forward and reverse current flow.

In practice a unidirectional device will have the terminals labelled specifically Supply & Load (or In/Out or similar), whereas a bidirectional device will not. A bidirectional device will also ensure that the RCD test resistor button works regardless of the current flow direction, which is not necessarily the case for a unidirectional device.

For further reading on the use of unidirectional and bidirectional RCDs with PV, battery storage and EV chargers, I would recommend downloading & reading the Beama Technical Bulletin from August 2023 on this subject.

In summary, there is plenty to think about with RCDs when installing renewable technologies, but the biggest consideration is the very first point mentioned in this article… Does your installation really need one?

– By Steve Donovan, Head of Technical UK & Ireland.

Glossary of terms
• RCD – Residual Current Device
• RCCB – Residual Current Circuit Breaker
• RCBO – Residual Current Breaker with Over-Current

Don’t forget – we have a team of extremely qualified and experienced Technical Support engineers available to support you for your pre-sale and post-sale Segen queries. We are available from 9am to 5pm by calling 0330 9000 141, then selecting option 2.

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