From Newsgroup: uk.d-i-y
On 18/09/2026 13:52, Sparky wrote:
John Rumm wrote:
On 13/09/2026 20:18, Sparky wrote:
A while ago the RCBO in my EVSE (AKA "EV charger") started
occasionally tripping, both with and without the car connected. The
EVSE is fed from a 40A MCB in the CU so I replaced the RCBO with an
RCCB (one that includes protection against DC "blinding" FWIW).
Yesterday, I found that the MCB had tripped - everything was fine
once it was reset, and so it continues. Coincidentally, there was a
brief power cut during the night.
I can't get my head around what might be happening.
The house supply is over-head 2-wire TN-C-S with an additional earth
rod.
Any observations?
For clarification, what it the upstream 40A MCB in the main CU that
tripped?
Sorry, just spotted my typo - the question was supposed to be "*was 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
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:
https://wiki.diyfaq.org.uk/img_auth.php/d/d4/Curve-MCBTypeB.png
Would likely trip on a sustained load of 90 in <= 100 seconds.
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
10kW+ ?). So that leaves high inrush current tripping the MCB's fault
current trip mechanism, or a faulty MCB.
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)"
40A over current, 30mA imbalance current, made by contactum, Max
breaking capacity 6KA, RCD type A (like normal type AC but will also
detect pulsed DC leakage currents), with a B type response to Fault
current (i.e. will trip on >= 5x 40A)
https://www.tlc-direct.co.uk/Products/CPCBR0640AB.html
--
Cheers,
John.
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