Serious service cable fault???

[apprentice87]

Hi all, I am only a newbie and do not pretend to know it all, and I am rather worried that i might have a bit of a problem here, and so some advice is needed!!

Firstly; I have a 70 odd year old house with a single phase supply [TNS] House is on Line 2. Ze is .27 with PEFC 863A [Line to neutral is almost the same with supply impedance .28 and PSSC 827A

Secondly; I have a shed behind the house with a 3 phase PME supply. This supply is connected to the main only about 5 feet away from the house supply. This supply was installed only about 9 months ago.

I did some testing on shed today, here are the results;

Ze;

L1 = .30

L2 = .79

L3 = .32

Line to neutral currents are;

L1 = 810A

L2 = 313A

L3 = 853A

Phase to phase currents are;

L1 - L3 = 1.91KA

L1 - L2 = 458A

L2 - L3 = 469A

I can only think that the much lower currents [both to earth and neutral] can only mean that there is a faulty joint in L2 where the service cable joins the main??

If L2 to neutral in the house has an impedance of .28 and L2 to neutral in the shed has an impedance of about .76 then that is a difference of .48 of an ohm. So for a current of 80A down that phase, there would be a volt drop of 38.4 volts. Volts squared divided by resistance gives power. So this gives 3000 odd watts.

Something is going to get very hot, very quickly...

Am i right in all this, and what should i do??

john...

 

[Andy™]

sounds like a high resistance joint somewhere. contact DNO and they should sort it

 

[Badger]

am i reading this right that you have 3 phase in your garden shed!!!

 

[Andy™]

am i reading this right that you have 3 phase in your garden shed!!!

well, Admin does have 11Kv to power his motorised shed workshop

 

[apprentice87]

Yep, certainly have!!!! How do you post photos on here, i will show you the mess they made of installing the service cable [which happily is in a duct] First they turned up with the wrong size cable [too small] so i made them go and get some more. The second length was too short by 3 feet so they have the main "torpedo" and then a smaller one about 18" away.....

Everything was working ok, but i was just having a play with my new Megger 1730 when i discovered the problem....

john..

---------- Post Auto-Merged at 21:34 ---------- Previous post was made at 21:32 ----------

Shed is perhaps not the right word though, i have not finished building it yet [i have a smaller on too] The big shed is 30 feet by 20 feet though!!

john..

 

[Evans Electric]

I'm puzzled by the phase to phase " Current" TBH

 

[Andy™]

I'm puzzled by the phase to phase " Current" TBH

PSCC

 

[apprentice87]

Hi again, Yes PSCC between phases is what i meant. How are they so low?? Notice how it is when you "pair" L2 with another phase that the current is so low. Phase to neutral in the house, [about 10 Metre service cable no idea what size though, as house built 70 odd years ago] .28 ohms and 827A

In shed the same L2, but this time service cable 35mm csa 25 meters long, .76 ohm and only 313A

Definitely a fault on L2 I think.

What do you suppose the DNO will say??? Their contract states that the Ze will be .35 max, but it is .79 on the one phase.... Remember, this was only installed last october..

john..

 

[Andy™]

phone DNO. tell them you have a much higher reading on L2 and think there is a fault somewhere

call to a job the other day - office & workshop sockets not working. turned out to be a cable fault and most of estate lost L2

 

[Evans Electric]

Is the house switched off when you read the Ze at the shed.

 

[apprentice87]

Hi Deke, No, do you think it will matter?? Perhaps i should try again with the house off. But i suppose about a third of the houses in the street are on L2 too. I will try though. I am a bit worried that L2 will disappear and damage my motors and welders. I saw a compressor when i was a kid that lost a phase, it was still running but got that hot steam was coming off the motors, [it used to live outside]

Do you all think the DNO will care?? I expect they will if they have a burnout in their joint and it cuts the whole street off!!

john..

 

[Evans Electric]

I'm unsure to be honest, other houses in the street will be spread across the other phases.

I was trying to visualise the test circuit on a TPN supply with a SP house supply tapped off the yellow phase and if it would affect the Ze readings . All I can think of is the joint where they have taken the SP feed from is affecting your reading in the shed so, ipso facto ; it must have dodgey earth continuity . So switching off would make no difference.

DNO will probably not be interested TBH . I'm suprised they guaranteed the Ze , thats usually an average that they try to achieve but never guarantee an earth connection for domestic premises . Praps they regard your "Shed" as commercial.

 

[Andy™]

anyone on L2 (after the fault) with have a higher than normal Ze

 

[apprentice87]

Hi Again, when i had a written quote for the supply it said in there things like what Ze would be and PFC so i suppose this is part of the contract between me and them... Still it cannot be right that the Ze for one phase is more than double what it should be.

I am more worried about the volt drop across the fault and what it will do too my motors and the like. Think of the I2R losses at the fault, it will be red hot. I should connect my biggest load across this phase and fry it on purpose, they will be interested when it takes out the entire street!!!!

john..

 

[NozSpark]

They will be interested if it just supplies your house,,, call them up and they will come out and fix it

 

[apprentice87]

Hi Noz, thanks for that. I just emailed them now, Just have to wait and see what happens now..

Here is what i sent; Hope it does not sound like the ravings of an idiot!! [i am very new to all this!!]

Hi Rachel, [This is my contact there]

Can you please forward this to whoever deals with faults..

About last October I had a new three phase supply connected to the workshop at the rear of the above address.

Sadly, I think one of the joints where the service cable joins the LV main in the street has developed a fault.

This supply is PME so max Ze should be about .35, however;

L1 is .30

L3 is .32

But L2 is .79

Much worse than this, the existing house service cable [also on L2] joins the LV main only about 1.5 meters away from the

 

[steptoe]

they will deffo get a shock that someone has even noticed IMO.!

I think they will be interested and try and sort it for you.

 

[apprentice87]

Hi Steps, I have to be fair, when they installed their cable they were more than helpful. Cable they brought was too short though, so they extended it by three feet to reach the main!!! Two resin joints within 18" of each other now though!! Not a good idea!!! Still, just as well i ran the entire thing in a duct.... I understand that they are not allowed to have two joints on a main within a metre of each other. I expect they will have to replace my entire service cable and cut 20 feet out of their LV main now though!! If i knew how to post a piccy i would post one of the present arrangement!!

john..

 

[SLIPSHOD & SLAPDASH]

http://www.talk.electricianforum.co.uk/your-pictures/627-how-do-i-post-pictures-forum.html

 

[apprentice87]

Hi again, I have got some pictures of the supply cable to main joint. As i said before, first they turned up with a cable that was the wrong size, then one that was not long enough so they went back to the yard to get another 4 foot of cable and used two joints next to each other!! There is a minimum bending radius for this cable too, that got ignored where they had to do a "U" turn with it. I hope it did not damage the neutral as it is a concentric cable, and i could well live [literally] without the neutral disappearing!!

Here are links to photos, i hope they work!!

http://i1178.photobucket.com/albums/x373/resistance87/2010_1015Image0038.jpg

http://i1178.photobucket.com/albums/x373/resistance87/2010_1015Image0009.jpg

http://i1178.photobucket.com/albums/x373/resistance87/2010_1015Image0039.jpg

No idea what will happen now..

john..