Odd EVSE MCB behaviour

Water ingress is the first thing to look for.

For clarification, what it the upstream 40A MCB in the main CU that tripped?

Downstream is 6mm T&E to an Emlite ECA2 meter, then 6mmT&E to the EVSE where it connects directly to the RCCB (was RCBO). From the RCCB it's connected to both a contactor (and then to the car) and to a MCB (and then, via a small contactor, to a 13A socket). Upstream it's just an isolator, to smart meter and 100A fuse

Sorry, just spotted my typo - the question was supposed to be "*was it* the upstream 40A MCB in the main CU that tripped"

ok I read that as "yes"...

a normal MCB only has two trip mechanisms - a thermal one to detect sustained overload. Here it will trip after some delay that is dependant on the magnitude of the over current. So for example a MCB with the Type B characteristic:

There is also a magnetic trip mechanism that will respond quickly (0.1 sec or less) on "fault currents" (i.e. you have just "nailed through the cable" rather than "plugged in too many things at once"). The threshold for that will be dependant on the Type rating. A Type B it will be 5x the nominal current, so >= 200A.

It seems unlikely that you are seeing a sustained overload charging the car (I am guessing that most domestic chargers don't charge at over

Power cuts can sometimes trigger RCD trips, but are less likely IME to trip a MCB, unless the circuit is loaded with lots of high inrush loads that automatically reconnect (like big transformers, or other inductive loads)

Has it done anything unexpected since that initial trip?

BTW, I suspect that "RCCB" is not the right term here... (that is short for "Residual Current Circuit Breaker", aka RCD.

The RCBO (Residual current Circuit Breaker with Overload protection) is basically a RCD and MCB combined into a single device. That unfortunately complicates the nomenclature, since the MCB part of it can have trip characteristics of Type B, C & D, and the RCD element can have characteristics of AC, A, F, & B. So a RCBO could have fault current threshold of Type B, and a RCD type of B - but the Bs both mean something different!

Usually they are listed with a long descriptive title to capture all the bits:

"40A 30mA RCBO - Contactum 6kA (Type A) B Curve (Compact)"

Thanks, but I'm familiar with MCB and RCBO characteristics and nomenclature.

The oddity here is that the original RCBO would trip (occasionally) with and without the car connected, that's why I replaced it. Now that the RCBO has been replaced by an RCCB/RCD (the manufacturer calls it an RCCB, that's why I've used the term) the RCCB has been fine but the upstream 40A MCB has tripped once (I can't now recall whether the car was connected or not). The inside of the EVSE is dry and there's no sign of arcing anywhere downstream of the MCB. I'm a bit stumped so a period of "watchful waiting" is called for, but I don't need the meter to be in circuit now so may remove that when I find the appropriate tuit.