RCBOs, bidirectional current and plug-in solar in the UK
If you have already read can you plug solar panels into a normal socket in the UK?, this is the technical reason electricians keep slowing the conversation down.
Plug-in solar is not just about whether a plug fits. Once generation can feed into an installation, protective devices have to be suitable for the way energy may actually flow. That is exactly why RCBO selection and Amendment 4 matter.
Short answer: when generation is connected in parallel with the installation, you should not casually assume a protective device is suitable just because it physically fits the board or looks like a standard domestic RCBO.
In practice: if a device is marked line and load, or arrows indicate a required direction of power flow, treat that seriously. If suitability for bidirectional use is not clearly supported, do not guess.
This is one of those topics where a small assumption can create a big misunderstanding.
Most people picture a domestic circuit in a simple way: supply at one end, load at the other, current flowing one way. That mental model works for a lot of normal household equipment.
But plug-in solar, PV systems, hybrid inverters and battery systems can change the picture. Once a source is feeding into the installation, energy flow may no longer fit the simplest supply-to-load assumption.
That does not mean every installation is automatically unsafe. It means the installer has to stop assuming and start checking.
Why this matters more now
Amendment 4 is a useful wake-up call because it reflects the way installations are changing.
Homes are no longer just places that consume electricity. More of them now generate, store and manage it as well. That includes solar PV, batteries and hybrid arrangements.
Once that happens, protective device suitability matters in a more specific way than it did when the whole installation was treated mainly as a one-way system.
Simple version: the more an installation starts behaving like a small energy system rather than a passive set of loads, the less sensible it is to make lazy assumptions about protective devices.
What “unidirectional” and “bidirectional” mean here
In practical installer language, a unidirectional protective device is one that needs the supply and load sides to be connected the right way round. A bidirectional device is intended to tolerate power flow in either direction without damage.
That distinction matters because some compact RCBOs use electronics as part of the residual current function. Those devices are often not the same thing as a traditional assumption-friendly, either-way component.
This is where people can go wrong. They see “RCBO” and assume all RCBOs behave in the same way. They do not.
Diagram 1: Normal one-way thinking
Diagram 2: Generation changes the picture
What Amendment 4 changes in practical terms
The headline point is not that every RCBO in every existing board suddenly becomes wrong overnight.
The more useful takeaway is that Amendment 4 reflects modern installations more honestly. Public IET material around the amendment makes it clear that bidirectional energy flow now needs more explicit attention when selecting protective devices and arranging generation or storage.
That is especially relevant for:
- solar PV connected in parallel with the public supply,
- hybrid inverter systems,
- battery storage systems,
- prosumer-style installations where the property both imports and exports energy.
In plain English, the amendment is helping push the industry away from old lazy habits and towards a simpler question: is this protective device actually suitable for the way this installation may behave?
What this means for RCBOs in the real world
This is the part that matters on site.
Many installers are used to compact single-module RCBOs in domestic boards. Those devices save space and are familiar. But familiar does not always mean interchangeable.
If a device is marked to distinguish supply and load, or marked with arrows, it is telling you something important: the distinction matters.
That does not automatically mean the product is bad. It means you do not get to ignore its intended connection arrangement.
The safest working assumption is this:
if direction matters on the device markings or the manufacturer data, direction matters in the installation. Do not “assume it will probably be fine” because the board closes and the circuit energises.
Why “unless specifically stated, assume unidirectional” is a sensible site rule
There is a reason this rule of thumb has become more common.
In a normal domestic conversation, the danger is not usually that someone spends too much time checking manufacturer data. The danger is the opposite: people assume all RCBOs are effectively the same.
So while the precise answer always depends on the actual device, the practical installer mindset is sound:
- if the device is clearly marked line and load, respect that,
- if arrows indicate a direction of power flow, respect that,
- if suitability for bidirectional energy flow is not clear, do not invent it,
- if the manufacturer supports bidirectional use, keep the evidence and follow the instructions.
That is a much better approach than guessing based on appearance alone.
How this links back to plug-in solar
This is exactly why I keep saying plug-in solar should not be treated like just another appliance purchase.
Once a small inverter is feeding power into the installation, the conversation is no longer just about sockets and wattage. It is also about protection, connection method, product suitability and whether the arrangement makes electrical sense.
This does not mean every plug-in or microgeneration setup is automatically wrong. It means the installer must understand what the equipment is doing to the installation.
That is also why your plug-in solar decision should sit alongside the wider questions covered here: is plug-in solar legal in the UK?, does it need G98 or DNO notification?, and what should you watch out for before buying?
What about RCD type and PV-specific issues?
Bidirectional suitability is one issue. RCD type is another.
In PV and inverter-related work, DC residual current and the possibility of blinding can also matter. That is why product instructions and the broader PV requirements still need checking properly rather than folding everything into one vague “just fit an RCBO” answer.
In other words, getting the direction question right does not remove the need to get the rest of the protection question right as well.
Simple takeaway: “bidirectional” does not mean “problem solved”. It only answers one part of a wider protection and design question.
A simple way to check an RCBO before you assume anything
- Look for line and load markings.
- Look for arrows showing direction of power flow.
- Read the product data sheet, not just the front label.
- Check whether the manufacturer explicitly supports bidirectional energy flow.
- Check whether any inverter, storage or microgeneration arrangement changes the expected connection conditions.
- Do not treat “it seems to work” as proof that the device is suitable.
Diagram 3: What to check on the device
Diagram 4: Better decision path
What this does not mean
It does not mean every existing domestic board must be ripped out tomorrow.
It does not mean every RCBO is unidirectional.
It does not mean bidirectional suitability is the only thing that matters.
And it does not mean the answer can be reduced to a social-media rule with no reference to the actual device or the manufacturer’s information.
What it does mean is that the industry is moving in a healthier direction: fewer assumptions, more attention to actual product suitability, and more realism about how modern installations behave.
The bottom line
If plug-in solar and modern domestic generation are going to be discussed honestly, RCBOs and bidirectional current cannot be treated as an obscure side issue.
They are part of the reason competent installation work still matters.
Amendment 4 does not create this issue from nowhere. It helps bring it into clearer focus.
Type AC vs Type A RCDs: what changes with solar
Type AC vs Type A (simple visual)
Another area that often comes up when people start looking into plug-in solar is the difference between Type AC and Type A RCDs.
In older domestic installations, Type AC RCDs were very common. These are designed to respond to pure alternating residual currents.
The issue is that many modern electrical devices — including inverters used in solar systems — can introduce DC components into the system.
Type AC devices are not designed to detect all forms of residual current where DC components may be present. In some situations, DC can affect how the device operates.
That is why many electricians will now say that Type AC RCDs are not suitable in installations where modern electronic equipment, including solar inverters, is involved.
Simple takeaway:
Type A RCDs are designed to handle a wider range of residual current conditions than Type AC. In modern installations, they are generally considered the more appropriate choice where electronic equipment or inverters are present.
This does not automatically mean an installation is unsafe just because it contains a Type AC device. But it does mean that assumptions should not be made about suitability without checking the actual setup.
As with RCBO selection, the correct approach is to look at the specific installation, the equipment being connected, and the manufacturer’s guidance rather than relying on old habits or assumptions.
The simplest honest summary is this:
where energy flow may be bidirectional, protective device suitability should be checked, not assumed. If an RCBO is directional, treat it as directional. If bidirectional suitability is not clear, do not guess. That is a far safer mindset than pretending every RCBO is automatically interchangeable.
This page is intended as practical guidance, not a substitute for the actual standard, product data or manufacturer instructions. For design and installation decisions, check the current edition of BS 7671 and the specific device documentation being used.