Why isnt the RCD tripping?

[HappyHippyDad]

I have just completed an EICR.
The install is TT, Ze is 308 ohms.
Zs on all circuits is <1.5 ohm due to bonding.
There is an up front 30mA RCD which trips with the push button but does not trip when tested either at a socket, light or actually in the RCD itself.
I know the RCD is faulty as I had a spare RCBO which I used and tripped as it should.

The lighting circuit had N-E IR readings of 0.27ohms. All other circuits were >2M ohms.

My question is why did the RCBO not trip instantly when I powered up this circuit through the RCBO?

 

[HappyHippyDad]

Always good to give an update!

Identified the problem circuit, broke it down in a couple of hours to between 2 points.

It was between a wall switch for a wall light (containing PL and N) and a ceiling pendant. Approx 50cm (DIRECTLY) above the switch was the wall light, with a screw that looked suspiciously directly in line with the PL+N cable. Removed the screw and the fault disappeared! Always feels good doesn't it when you find it, especially when it's a very visible problem that you can show the customer.

Oh yes, and the RCD trips as it should do now when tested. Actually, I better rephrase that otherwise I'll get a few damns from DaveS . The RCD always tested fine when tested correctly with all outgoing cables disconnected. It now even tests with them connected .



A big thankyou to all those who replied, especially @Lucien Nunes and @davesparks who as usual gave very useful advice.

 

[Grant1987]

Always good to give an update!

Identified the problem circuit, broke it down in a couple of hours to between 2 points.

It was between a wall switch for a wall light (containing PL and N) and a ceiling pendant. Approx 50cm (DIRECTLY) above the switch was the wall light, with a screw that looked suspiciously directly in line with the PL+N cable. Removed the screw and the fault disappeared! Always feels good doesn't it when you find it, especially when it's a very visible problem that you can show the customer.

Oh yes, and the RCD trips as it should do now when tested. Actually, I better rephrase that otherwise I'll get a few damns from DaveS . The RCD always tested fine when tested correctly with all outgoing cables disconnected. It now even tests with them connected .

View attachment 51925

A big thankyou to all those who replied, especially @Lucien Nunes and @davesparks who as usual gave very useful advice.

Just read all of this topic and this is one of the main reasons why I joined this forum to learn from the more experienced and intelligent sparkys had to read the posts a few times to understand but now I do another tick off the box ?