RCD tripping Problem solved

[mojorebel]

This was an interesting problem and solution
I recently wired an community hall - the supply was a TPNE – one phase was being used for E7 heating. One phase was unused. And the third was to supply the lighting and power. I fitted a split board 12 way CU and ran two A2 radials for the sockets. The hall was fitted with around 30 wall mounted fluorescent lighting fittings via an 8 gang switchplate. I very wisely fitted a “lighting control” connection box (where all lighting cables terminated before going to the relevant switches and fittings) All circuits tested ok. But the RCD would trip occasionally when a light switch operated . The RCD would not always trip with the same switch . It could be any of the 8 – retested circuit – all ok, changed RCD – problem persisted . I took the lighting circuit off RCD protection (split board) . But the problem persisted. Fitted a separate CU for lights only with no RCD protection . Problem not solved. The RCD would still trip when light switches used (not every time , but now and again) I decided to put the lighting CU on its own phase – so utilised the unused phase. The problem still persisted. I eventually realised what the problem was ( Back emf on the neutral caused by inductive discharge lighting) . I solved the problem by fitted double pole light switches and switching both line and neutral conductors. First time in 40 yrs that I have come across this problem

 

[baazab]

that's sound good ,
but who did you get all the neutral down to the switches(grid)

 

[mojorebel]

that's sound good ,
but how did you get all the neutral down to the switches(grid)

I had taken all the cables to the fittings and from the switches to a large connection box- so it was an easy job to take neutrals down to switches ( I only had to change over a few connections inside the box) - but had to change the 8 gang to a 4 gang switch plate

 

[baazab]

that's good ,well planed

 

[Myles]

Interesting! Must have been frustrating too..could have stumped many a good man

How many circuits controlled the lights? Did you realise it was the fittings by changing all
the lights on one circuit or was it a guess/measurement/deduction by elimination?

If you kept switching a switch on off on off on off would it definitely go at some point?

what fittings were they?

Well done!

 

[Farmelectrics]

Ok well done but what made you think and how did you work out or find it was back emf I've always been very suspicious of this back feed on the neutral still struggle sometimes to work it out can anyone explain to me in idiot form lol

 

[Barry White]

Have to admit I would have struggled with that one well done for thinking it through.

Begs the question? What if the TPNE was feeding two flats and a Hall all on different phases? Same fault tripping the RCD in the imaginary flat?
Ps what was the earthing type?

 

[mojorebel]

Begs the question? What if the TPNE was feeding two flats and a Hall all on different phases? Same fault tripping the RCD in the imaginary flat? So true . I have used a similar scenario when trying to explain this problem to some of my students- It is like a light switch in one house tripping a neighbors RCD on the next street hahaha-

 

[mojorebel]

Ok well done but what made you think and how did you work out or find it was back emf I've always been very suspicious of this back feed on the neutral still struggle sometimes to work it out can anyone explain to me in idiot form lol

I arrived at the solution via inference and deduction . The neutral (and earth) were the only common factors. I know that inductors can cause back emf (Back EMF arises whenever current is removed from an inductive load). The inductive load resists the sudden change in current and produces a reverse voltage ) So I assumed that the back EMF was using the neutral and upsetting the local RCD.

 

[mojorebel]

Interesting! Must have been frustrating too..could have stumped many a good man

How many circuits controlled the lights? Did you realise it was the fittings by changing all
the lights on one circuit or was it a guess/measurement/deduction by elimination?

If you kept switching a switch on off on off on off would it definitely go at some point?

what fittings were they?

Well done!

I cant recall the fitting types (wall mounted, fluorescents) There were about 30 fittings controlled via 8 switches ( All terminations from fittings and switches went to a large connection box, which was fed from a single circuit from the Consumer Unit on a 10A MCB (if I recall correct ) Yes it was a guess/measurement/deduction by elimination after cyhanging all the switches and changing the RCD and fitting a separate CU etc. If I kept switching off a switch one of them would eventually trip the RCD. (usually after about 3 switches) As I said above. first time I have ever had this problem.

 

[mojorebel]

Ok well done but what made you think and how did you work out or find it was back emf I've always been very suspicious of this back feed on the neutral still struggle sometimes to work it out can anyone explain to me in idiot form lol

Back EMF arises whenever current is removed from an inductive load (such as a coil in a larger relay or solenoid). The inductive load resists the sudden change in current and produces a reverse voltage see similar issue here http://www.------.org/forums/forum/messageview.cfm?catid=205&threadid=25823&highlight_key=y)

 

[mojorebel]

Have to admit I would have struggled with that one well done for thinking it through.

Begs the question? What if the TPNE was feeding two flats and a Hall all on different phases? Same fault tripping the RCD in the imaginary flat?
Ps what was the earthing type?

I cant recall the earthing arrangements bit know for a fact that it was not a TT system

 

[th_walking_dude]

I have to say I was thinking back EMF when I was half way though reading you opening post.


Not really, that's a job well done and an interesting read, thanks for sharing.

 

[Philip Of Kent]

Were they WYLEX RCDs ?

 

[mojorebel]

Were they WYLEX RCDs ?

They were MK honeywell if I recall correct