An interesting fault - a bit of a Quiz

[Phoenix]

We do some work for a farming estate that has a few cottages dotted about, today's job concerns one of (two I think) their properties along the 'main road' through the village. I say main road, theres quite frequent traffic, but we are definatly 'in the sticks' The job card said the immersion heater kept needing to be be reset, and then stopped working altogether, and the tennent had been recieving shocks off the outside tap initially and then also off the inside plumbing, which were described as something like "not really really bad, but most definately painfull" (I paraphrase) and was wondering if the two were related.... as did I, I was expecting to find a split immersion element and a dodgy TT system.

Alas no..... it was PME (and not a dodgy conversion). Earth loop at the board about 0.25ohms. Ok then switch off main switch and do a proper Ze.... Main earth flashes to the earth bar as I pull it away... odd... definatly something untoward here. Out comes he wandering lead to a temporary spike in the garden, nothing between that and the CU earth bar... at least thats normal, now test the suppliers PME earth.... 193v between that and the physical earth. Time to call the supply authority out, fault engineer turns up, confirms my findings, sends for a lines team, who go up the pole in the field behind the house and check the 16kva "pole pig" sp tx, the same physical ground to neutral earth readings are present there, But disappear when the LV fuse is pulled. Fault enginneer goes to check another property fed from the same tx (which luckily is owned by the same landlord) and comments that the polarity was reversed there... thats odd I think, I did an EICR there last Winter and I wouldn't have missed that!, Did start to think "was it reversed at the cutout, and then swapped back at meter to be presented right at the board" , anyway supply authority chap asks me to just have a check as they arn't really supposed to open up the customer's board. This property is on TT, polarity was correct, but what caused the confusion is that a voltstick lit on the neutral of the concentric supply, think there was about 130v between neutral and TT(earth) and only 94 between live and TT earth. On the previous visit to the property the supply chap had disocnnected the supply from the cutout, and he had done the same in the property where the shocks were experienced (PME earth also disconnected on that one).

Anyone want to hazard a guess what the cause ended up being?

(Nothing to do with the immersion heater, I'll give you that one, that was a dodgy stat, 5 and a half ohms across it when it was closed, and when I pulled the old one apart in the van later on, the contacts had been burning up)

 

[ProDave]

Open circuit PEN conductor and it was only the cross bonding earth actually creating a neutral path.

And once again the good old DNO relying on JUST a volt stick to diagnose technical issues.

 

[Phoenix]

The PEN was not open. It was still 190 old volts at ground when tested at the earth/neutral on the transformer joint box just below the tx.

Properties were separate detached buildings (these were actually opposite sides of road, and probably about 100m apart)

The DNOs over reliance on volt sticks is slightly off, as is the fact that when at the TX they were about to declare everything alright on that because they'd tested Live to the earth running down to the rod below and got 230v and 0.15 loop , I prompted them to check further, brought over the temporary rod and wander lead "just humour me... and check between neutral and true ground over here" sure enough it was 190v like in the house

 

[binky]

anything odd is always neutral fault in my experience.

 

[ProDave]

So there was a problem with their local earth? Wire going underground but broken from it's pigtail? (they tend to use a coil of bare copper wire buried in one of the trenches rather than a solid rod. Usually 2 in fact a separate HV and LV earth going off in opposite directions in the trench)

 

[Sharpend]

Where’s Tony? I’m sure he’d have some input on this? 

 

[Tony S]

I would suspect there’s more than one fault.

I had a similar problem with my own house. TT OH 3Ø feeding twelve houses, I was at the remote end on the red phase. I forget the N→E voltage but there was up to 40A flowing from the water to the gas pipe and earth rod.

The OH line crew found a burnt line tap on the neutral which on its own would have caused problems with varying voltages, this wasn’t happening. By now we had the entire road disconnected and could test for earth faults. Somewhere in one of the twelve houses there was a N→E fault. (This explained why we weren’t getting voltage shifts.) We narrowed it down to one house, with that isolated the supply to the rest of the road was restored. It took about half an hour to find a socket front screw had bitten in to the neutral so their supply was restored. It had been like that for years and yet there was no discolouring of the screw?

Eleven guys from the electricity board and me, five hours to sort it out.

 

[Sidney]

Had similar on a TT system where the overhead cables had been rubbing together over the years and finally shorted out causing both line and neutral to be at the same potential. The incoming line conductor was supported by a catenary which was also the neutral, only two properties running of the TX with the other property being on a different phase.

 

[Phoenix]

The time has come to give the answer. It was actually a L-E fault in an another property (an old metal framed farm building) that was not being disconnected by a protective device . This was a TT installation and was of probably 50s/60s vintage with and old cast iron fuseboard (could have even been DP - did not look closely) with an upfront VOELCB which was either defective, or not connected properly, or simply rendered ineffetcive due to the steel framework creating a secondary earth path. There was however a newer digital meter and the meter tails had been changed in the last 20 years!

Anyway, cutting to the chase, we walked in here, I commented on the fact that the meter was flashing to show consumption, but there did not seem to be anything on at all. It was at that point that the DNO chap saw a bit of a flash, looking closer there was a conduit arcing away on the ceiling, I'm guessing VIR singles had rubbed through, had been faulting to the conduit and had over time blown a hole through the side of it so that the arcing could be seen.

I'm guessing that with only a small transformer, and only two from what we saw electrodes on the supply side, and the barn probably being a better earth, that the fault path through the ground, back to the transformer, was creating a volt drop across the rods on the supply side i.e. between the physical ground and the neutral side of the tx

Quick vid here taken while the supply authority bloke ran to his car to get his PPE and tools to pull the supply fuse. Following that, they took both L/N tails out the head and sealed it back up, think its the only time I've seen an installation so bad that it had to be cut off by the supply board on safety grounds!

 

[Geoff1946]

There must have been nails in the fuseboard to melt the conduit and still not blow!

 

[Tony S]

They took both L/N tails out the head and sealed it back up, think its the only time I've seen an installation so bad that it had to be cut off by the supply board on safety grounds!

The fault I described earlier was tracked to an unsafe installation. The lecy boards engineer turned in to Hitler’s twin brother and did as you said. He disconnected and sealed the supply, he then issued a formal danger notice to the householder.