rcd tripping

[electric8]

Doing a cu change and ring circuit is tripping the rcd. was working ok before on a non rcd breaker
any ideas what might be causing this.

 

[electric8]

Thanks for the advice it needs more investigation tomorow

 

[electric8]

yes that would explain the ir readings but not the rcd triping

 

[PC Electrics]

still dont understand if ir test shows dead short L-N why would this not trip the breaker

I do and I've already given you a clue. I'll have to come back with the answer later though.

yes it will be reported to the client and rectified once i have permision

You need "permission" to remove a danger when you're doing a CU change? It's a requirement of doing a CU change that the circuits are safe to be connected to the new unit. Your circuits are not safe by dint of a borrowed neutral. This issue could have been 'fixed' in a trice by simply putting all the lights on one MCB. Not ideal, but safe and would have taken you no time at all whilst wiring up the new unit.

 

[tony mc]

I would fully document on the EIC what you have done ie putting lights on one mcb and the same RCD so the customer can not hide the issue if the work is not carried out in the future.

 

[PC Electrics]

still dont understand if ir test shows dead short L-N why would this not trip the breaker

I did ask what units you were measuring in, but you didn't answer. From the above I take it you are using your meter in IR mode and are measuring in Mohm at 500v.

Your meter (depending on model) will probably read Mohm to 2DP, eg 7.82.
When the measured value of IR drops below 0.01Mohm the meter can only display "0.00"
So, even though your meter is showing zero (0.00), the actual IR could be anywhere between zero and 0.009Mohm (0 to 99.9kohm).
What you should do next is put your meter into ohms range and measure the actual resistance of the 'short'.

An example:
A fault develops between L & N with a resistance of 240ohm.
We know from v=IR that the current this fault will draw is 1A.
Way too low to trip an MCB or fuse.
But your IR meter will show 'zero'
Use your low ohm range to measure 240ohm.

Depending upon the magnitude of the 'fault' you measure, this might give a clue as to what might still be connected.
If the 240ohm fault is L-E, it still won't trip an MCB, but will trip an RCD.

Does that help?

 

[electric8]

thanks for that will put circuits on same mcb . I meant i needed permission to disturb decoration as i plan to run a new neutral to landing light.

 

[electric8]

using a megger mft 1552 dont think it has another range

 

[PC Electrics]

using a megger mft 1552 dont think it has another range

I use a 1552 too. I suggest you read the instructions. Or even, just look at the selector dials !!

 

[tony mc]

Try this you really need to know your test equipment..


Megger MFT1552 & MFT1553 Demo Part 1