Alternating current.

I’m not sure it would help a broken PEN conductor anyway , for solar they should be and other small scale generation systems.

it should be decided on a case by case basis 

They are prone to be more costly, but double pole are only really necessary for certain situations such as Reg 551.6.1. Would it help with PEN faults in the street? During such faults is it not your exposed and extraneous earthed parts with potential rise relative to true earth? They are indeed more fitting for protecting circuits that have sources of generation or storage, such as solar PV, battery storage, or car to grid systems. As discussed before on previous posts theses sources can generate reverse currents to flow back to the supply, which may escape the detection of a uni-directional RCBO. It is possible that, with the way things are going, it might become a necessity to fit bi-directional RCBOs if it becomes a requirement to generate electricity at home to support the national grid. After all it is now a requirement to install an EV charger with new builds (Part S)

Agreed.

The RCBO have electronics which expect the current to flow the correct way

The current flows in both directions all the time in any event. As far as I can tell, whether it's in sync with the voltage or 180 degrees out of phase (plus or minus a bit), isn't likely to worry most mechanisms. There can be an issue where the voltage on the "load" terminals doesn't disappear when the device opens - e.g. where the test circuit is simply connected across the load terminals if someone holds in the T button after the device trips then the test current continues to flow and the resistor and/or solenoid and can burn out, possibly permanently damaging the device. I'd be interested if anyone know of any other failure scenarios ...

   - Andy.

Personally, I much prefer DP switching - especially where there's another RCD upstream (fairly common for TT). It's embarrassing to have "isolated" a circuit by its MCB/RCBO, cut through a cable or touch a N to a back box and then have the whole building black out.

   - Andy.

AJJewsbury said:

cut through a cable

It took me a while to figure that one out. I was only allowed to isolate the circuit which I was working on, but the joiners, etc. were getting grumpy every time that their saws stopped.