[formerly_proven]

It would be really interesting to know what's so special about these UK units that they can be "damaged" by being fed from the "wrong" side (as per some other article), considering that the only place where these behave like that is an island north of France.

[fy20]

These are not just circuit breakers/MCBs, they are RCBOs which combine an MCB + RCD in a single unit. RCDs traditionally only measure - and protect - current flow is one direction, so if you are using them for solar you need a bi-directional unit for full protection. The device will not be damaged, it just won't protect you.

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.

[formerly_proven]

Outside the UK, neither RCDs nor RCBOs (type A/AC) are generally distinguished by bidirectionality (all search results about this being .co.uk), since the RCD part of these devices is just a current transformer driving a trip solenoid; there is nothing in it that's powered by the line, nor something which could sense net power flow direction. The situation is different for AFDDs or type B RCDs, since those have active, powered electronics in them which need to be fed from the line side.

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.

[aaronmdjones]

> Answer #1: Many UK RCDs/RCBOs are actually single-pole devices and don't disconnect the neutral.

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.

[1] https://www.youtube.com/watch?v=8kWIITspYvk