“Unidirectional breakers” aren’t a thing for AC circuits.
If the circuit will be supplying power too (e.g. battery storage, an EV and EVSE that supports powering the house from the EV, etc) then you need a bidirectional RCBO.
People with no differential fault protection need not worry about any of this, they'll just be killed when it goes badly wrong.
Source: Am a UK electrician
Example: https://assets.cef.co.uk/downloads/pdg/wylex_nhxs1b32_datash...
EDIT: To say nothing of people with unidirectional electricity meters; plugging these into those setups will get them prosecuted for electricity theft. All SMETS 2 smart meters are bidirectional; you'd best check your meter if it isn't one of those.
Pre-smart non-prepayment electronic meters (for those with old meters, still submitting manual readings, and paying by direct debit) will be fine. Most of these meters, and all smart meters, are inherently bidirectional, because they maintain 4 counts (energy imported and energy exported, in kWh and kVARh) and your energy provider will do all the necessary math to figure out what to actually bill you for (residential customers are not billed for kVARh usage).
The UK government in 2011 announced plans to have 50 million smart meters installed by the end of 2020. In typical overpromise underdeliver government fashion, they didn't even achieve half of that; by then, only 23.6 million had been installed, and of those, 4.5 million had stopped working because they were initially (and stupidly) designed to be tied to a specific energy provider and the customer had changed provider. This even affected me.
Nevertheless they'd still accurately track energy consumption and export even if they'd lost their reporting capability, so you have nothing to fear here. This situation has been rectified at the redesign stage with provider-independent SMETS 2 meters, and all SMETS 1 meters still in service have been hotpatched to bring them into line (restoring their smart functionality regardless of provider).
Even today (well, as of last September), this number is only 40 million, with only 36.7 million of them actually working as designed (reporting readings automatically).
This leaves up to 16 million properties with a meter that may stop working and expose you to a theft investigation when you obtain generation capacity that even momentarily exceeds your usage (for example if you have a dual RCD board and one of the RCDs trips, taking out half of the circuits in your home, but not the one the inverter is plugged into).
Realistically the true figure is probably around a quarter of that; prepayment meters were very popular among the renting population of the time, and those who wanted to track their energy usage carefully and only pay for it with cash as and when needed, and sometimes people had these meters forced upon them by suppliers after the customer had demonstrated poor payment history, but they were far from the norm.
Average home owner buying plug-in solar at a supermarket isn't going to know or care about any of this. They'll just plug it in, and it will work, until one day maybe it doesn't and their supplier opens a theft investigation.
So if you spent a week with the meter connected normally, then you swapped the input and output cables around for a week, the meter would be back at zero. Free electricity!
They used anti-tamper seals to make it more detectable, but there are ways around that sort of thing.
However in the case of a UK home, where you may have a single ring circuit connecting all the sockets on the whole floor, what's in the breaker panel isn't going to protect you with plug-in solar anyway. Better hope what you are plugging in meets UK standards and isn't just some Chinese rubbish that claims it does.
After some research the main reason seems to be two-fold:
Answer #1: Many UK RCDs/RCBOs are actually single-pole devices and don't disconnect the neutral. In the simplest case, this means pressing the test button might burn out the test resistor when backfed. I don't imagine this to be a problem in practice, since grid-tie inverters shut down very quickly if the grid disappears under them, especially plug-in inverters. RCDs/RCBOs elsewhere are virtually always disconnecting the neutral, so don't care about this.
Answer #2: It looks like some/many one-module wide UK RCBOs _do have_ electronics in them, even if type A, because they're actively driving the trip solenoid of the MCB part, and if you sketch this out and do it in a very cheap way it's easy to see how you could burn that out if backfed (i.e. powering the trip solenoid during a fault is assumed to disconnect in a very short amount of time, but if backfed for longer than the disconnect time that might be enough to toast the solenoid or the driver).
Notably neither of these has anything to do with the direction of power flow.
This is not correct; all type AC and type A RCDs used in British consumer units disconnect the neutral as well. Some RCBOs do not disconnect the neutral and this is a problem in some circumstances. The datasheet I linked for Wylex NHXS1 RCBOs explains that these ones do disconnect the neutral.
> Answer #2: It looks like some/many one-module wide UK RCBOs _do have_ electronics in them [...] but if backfed for longer than the disconnect time that might be enough to toast the solenoid or the driver
This is correct. For an example of this construction in an RCBO, see [1]. This illustrates that if the supply is connected to the "To Load" part of the schematic (toward the end of the video), as it would be if the supply is a solar PV inverter with battery storage, then it can continue powering the electronics and be shunted out by the thyristor after it has supposed to have tripped, very quickly burning itself out.
Bidirectional RCBOs are not designed in this manner. They have more complicated circuitry that makes them more expensive to manufacture, but are absolutely required in situations like this if you don't want your protective devices to burn and/or explode when they operate.
> Notably neither of these has anything to do with the direction of power flow.
Yes it does, because if the power is flowing backwards to how they designed it, that is backfeeding it, keeping its circuitry powered after it should have been disconnected.
But that's rather the point here that consumers are the ones who are going to be plugging in these devices, with no appreciation for their circuits and safety devices. The only code that matters is the last version of it adhered to when their home was last wired. In extremes, that can be 40 years or more.
The conventional practice in the US is still to use GFCI receptacles rather than breakers.