MCB trip (not RCD) during loop test

[timhoward]

I just finished a garden room supply. The customer already had a CPN garage consumer unit that the last sparks left there, brand new in box, and with some misgivings I agreed to use it.
It came to testing.

Zs@DB of 0.3 ohms in garden room.
CPN RCD main switch tripped x1 at 21ms
The lighting circuit on a B6 had infinite IR values.

Semi-interestingly, a 'no-trip' Loop test at the light tripped the B6 breaker (and not the RCD). Both a Megger and a Metrel tester did this.
I got a measured Zs by shorting R1 and R2 at the fitting and testing bus bar to outgoing live at the CU. (0.40 ohms.)

I'm musing whether the MCB is just unusually twitchy, or it's actually faulty. Or maybe they all do it - I'm not used to CPN gear!
Any thoughts?

 

[nicebutdim]

I see quite a lot of CPN gear in site cabins. Generally never encounter any issues, except one occasion when two RCBOs were faulty and wouldn't hold.

 

[Pretty Mouth]

I've never come across this fault, but a faulty breaker seems most likely to me