rcbo only resets when other mcb is off

[copey84]

Hi there, i have recently wired a lighting circuit and a radial circuit for two socket outlets. Run the feeds for both circuits using a 5 core 2.5mm high tough cable from the distribution board to a jointbox. From the j/b i took my feed for lights and feed for sockets, used same earth for both circuits. Lighting circuit is on a 10a mcb and radial for sockets is on a 20a rcbo.
Completed wiring everything worked fine until i hit test button on rcbo and found it would not reset. Only when i switched off the 10a mcb for lighting circuit would the rcbo reset, found this very strange! Have tried different rcbo, but same thing happens, checked all wiring everything looks ok. I know using the same earth for both circuits is not ideal but can't see how it could effect the test button. Really not sure about this one any advice would be welcome.

 

[copey84]

Both mcb and rcbo are fitted in a 3 phase d/b which is protected by a 300ma s type 3 phase rcd, so neutrals are not independent. How could this be effecting rcbo through test switch?

 

[VoltzElectrical]

it could be accumulated earth leakage that your new circuits have taken the rcd over the edge. Had this before.

 

[Sav]

Where does 21 Ohms come from? I've only heard of 200Ohms for TT system being "acceptable". Not come across the 21 ohms figure before?

Sorry D80 ment to say max 200 Ohms, but NIC say max 100 Ohms.
Apologies for my error !!!

Sav

 

[telectrix]

niceic may say 100 ohms, but a higher authority (engineer54) will tell you that that figure is rubbish.

 

[Engineer54]

No, not a higher authority, ...Just simple run of the mill commonsense!! ...lol!!

 

[telectrix]

so, more common sense than niceic = higher authority. Q.E.D.

 

[Engineer54]

Depends really, if you are willing or not to totally rely on an RCD never failing!! I certainly wouldn't at this particular time!! lol!!

 

[mikehopkins]

Depends really, if you are willing or not to totally rely on an RCD never failing!! I certainly wouldn't at this particular time!! lol!!

This may leave a lot of TT systems a bit hazardous, and there is a lot of TT systems about!!
it sounds like the whole contracting world has had arguments with nic engineers about ee values!!

 

[barnzi]

How long is this cable?

 

[copey84]

The 5core 2.5mm high tuff is 25m long

 

[Engineer54]

This may leave a lot of TT systems a bit hazardous, and there is a lot of TT systems about!!
it sounds like the whole contracting world has had arguments with nic engineers about ee values!!

Well at least the Nic's recommended 100 ohm Ra is a 100 ohm lower than what is recommended in BS7671!! lol!! Both are basically useless in real terms, and both totally relying on a single RCD as the main protection against earth faults...

 

[barnzi]

Could well be capacitance between the cores. The leakage at 50Hz will be slight, but it just might be enough to cause difficulties when resetting the RCD.

Being capacitive leakage, it won't show on an insulation test unless your tester can do AC. Most are DC only.

 

[Knobhead]

Over a distance of 5 metres, are you being serious?

Have a look on Google for cable capacitance, we’re talking a short distance not half way across the Gobi desert.

 

[barnzi]

OP said it was 25 metres, not 5. Assuming 50pF/m, that's 1.25nF. That's going to look like roughly 2.5MOhm to 50Hz, which would give 0.02mA... Okay, yeah. Not a lot! XD