Ah, the havoc caused by xmas lights!

[acvc]

Happy New Year to all you folks out there. I hope it's a successful one for you all and many thanks for all your help and good advice last year


I was called to a guys's tripping ring circuit today. 32A ring final with BS3871Type 2/Old Wylex BS 3036 board with adaptors/TNCS/Main earth present and 6mm/ Water+Gas main bonding present and 6mm.

No recent building or DIY. The bloke plugged in his xmas lights and 5 minutes later the tripping began (could be just a coincidence). From then on, with definitely nothing plugged in, the MCB will not reset. There is some moisture in a couple of sockets on the ring. I put the conductors in these boxes into connector blocks and taped them up. Then went on to test.

Continuity 0.60 on R1 and Rn end to ends.
Continuity 28 ohms and fluctuating on R2 end to end.
Crossover R1 and Rn 0.30 at all points.
Crossover R1 and R2 variable from lowest 0.25 to 1.72 highest.
Ins Res 0.00Mohm between each three conductors.

Then .... I split the ring middlish and ....

for quick location of short I went straight to INS RES and still says 0.00Mohm on BOTH legs, between all three conductors!
AND neither of the (now separated) legs of the ring trip the mcb.

Put a 16A BS60898 on to activate one leg, and highest Zs on the leg is 0.72.

I am 99% certain that I identified the appropriate ends of each conductor in the board.

Now I'm very very confused. My thoughts are:
1. There's a parallel earth path present AND
2. (Not sure about this but) As two separate radials the Zs on each isn't low enough to activate the MCB. But when joined as a ring, the Zs is lowered (halved) as the earths are now effectively in parallel.

Anyone come across this fault scenario before? Thanks chaps.

 

[slocm3105]

Its kinda hard to understand what you mean by your two points at the end. But have you only split the ring down once in the middle? Becuase a fault is not always only on one leg of cable. I have have many circumstances where i have only left 2 sockets working as 2 radials before now due to things such as rodent damage and the penetration of water. Try splitting the circuit down further on each leg the narrow down the fault more precisely.

Matt

 

[acvc]

I've just edited this posting ....

I should have written in point two that the Zs isn't low enough, not high enough. Sorry chaps.

I will return to further split down the ring on both legs. Silly assumption to make that it would be a single fault on one leg only. Time wasn't on my side this afternoon.

I still don't understand how the mcb will trip when protecting a ring, but not when protecting the same ring split into two legs ........

Cheers

 

[acvc]

OK I'll conclude this one sided posting!!

I still can't explain the IR readings of zero megaohms on both legs AND the mcb not activating.

I'm also still wondering about the question of whether or not the the r2 of a ring will be less than the the sum of the R2s of that ring split into two radials (due to the parallel nature of the ring versus the series nature of the radial). Anyway .......

The good news is that I eventully found a junction box submerged in a puddle of kitchen sink waste water under the floor of the kitchen, and had a lovely afternoon wet and stinking of waste under a floor. Reterminated and dried out, the ring now tests well and behaves.

Cheers

acvc

 

[cbw]

Blooming plumbers!