Need some ideas - RCD tripping but everything tests fine....

[weevilward]

Hi all,
Customer reports sporadic nuisance tripping of RCD. I cannot reproduce it, or fault anything, except SOMETIMES on EFLI test it trips the RCD (megger mft 1553 'no trip' test).
I'm out of ideas here, can anyone help? This is what I've got:
- TT installation (small domestic);
- 30mA RCCB main switch incomer (Crabtree starbreaker);
- all circuits IR test 8MOhm to L&N-earth;
- RCD testing off load and on load all fine, ramp test 23mA off load 19mA with everything on.
- Ze about 160 ohms down the rod, drops to 12 ohms with all bonding connected.

 

[Pete999]

Borrowed Neutral on the Lighting circuit????

 

[telectrix]

intermittent fault caused by something like a fridge or other appliance that has a switching cycle. i.e. it switches on and off through a thermostat or timer.

 

[weevilward]

Borrowed neutral in lights - Nope. Even if there was, all 5 circuits are on the same RCD so it wouldn't show.

 

[weevilward]

intermittent fault caused by something like a fridge or other appliance that has a switching cycle.

I thought that but I've been here ages and cycled everything on / off. What is bugging me is the ELI test tripping it, never seen that before

 

[westward10]

Might need a leakage tester because combined leakage of the circuits maybe occasionally tipping it over the edge which is what your tester maybe doing.

 

[weevilward]

Might need a leakage tester because combined leakage of the circuits maybe occasionally tipping it over the edge which is what your tester maybe doing.

Ramp test with everything on and running 19mA - I've tested loads of RCDs on load like this and never has one tripped out.
I agree the most likely cause at the moment is combined leakage when something starts up, I just haven't been able to reproduce it.
She also says the RCD was 'tripped' after a power cut - I guess indicating that the startup leakage of everything together was >23mA when the power came back in again. But.... I cannot reproduce that either.

 

[weevilward]

Think I'm gonna have to just change the RCD for a new one today and see what happens... 'Tis 5 o'clock after all
I'm not convinced it's a faulty RCD though.
Please keep any ideas coming, I'll post if swapping RCD fixes it.
Cheers guys

 

[westward10]

Has she got any new appliances.

 

[weevilward]

Has she got any new appliances.

No, nothing new/changed in several months, the problem started about 3 weeks ago.

 

[buzzlightyear]

I depends on which circuit it is , any power to out side or lights you will a have to be Sherlock homes on that one .how big is the house .

 

[Ian1981]

Does she use an immersion heater perhaps through a booster/ timeclock?

 

[weevilward]

I depends on which circuit it is , any power to out side or lights you will a have to be Sherlock homes on that one .how big is the house .

Small 2 up 2 down with outhouse. Nothing major really, tested every, inspected everything.

 

[weevilward]

Does she use an immersion heater perhaps through a booster/ timeclock?

Good idea but no. CH and HW fully functioned - 3-way valve, boiler, pump, all sweet.

 

[buzzlightyear]

with rcd tripping do you know what circuit it is tripping the rcd !