a fault for trainees to get their heads round

[telectrix]

this is a real life fault i came up against. without over-complicating the issue of the site, i'll try to explain just the electrical side. OK. 10mm SWA 3 core feed to a wiska box. no RCD protection. from the wiska box there are 2 6mm 3 core SWA branches.lets call them 1 and 2. with minimal loads on both branches i got L-N 217V. L-E 238V. now applying a 2.5kW load to branch 1, 175V L-N, still got 238V L-E. however, no volt drop apparent on branch 2 at the load end. so then i'm beginning to doubt my original thoughts of a high resistance N conductor in the 10mm. further investigation found the faults ( yes, more than one fault which gave rise to the problem). right. lets have your thoughts on this. TRAINEES ONLY FOR STARTERS.

 

[pennychew]

Good thread, on the way to work now but i'll have a look at it then and get back to you...

 

[Paul.M]

It is a really good head scratcher, Tel asked me earlier and this is a good fault to find. Have a go at it, throw your ideas up. Mentors PM Tel direct if you think you know?

 

[amckay]

Borrowed neutral somewhere? Possibly concealed

 

[telectrix]

Borrowed neutral somewhere? Possibly concealed

nothing borrowed or shared. . it's only a single circuit.

 

[pennychew]

You're mft is ferked? (just kidding)

 

[telectrix]

You're mft is ferked? (just kidding)

haha. never use mft for fault finding. analogue megger continuity/IR tester, circa 1979, and a multimeter was all i used here.

 

[happysteve]

with minimal loads on both branches i got L-N 217V. L-E 238V. now applying a 2.5kW load to branch 1, 175V L-N, still got 238V L-E. however, no volt drop apparent on branch 2 at the load end.

Do you mean (in the case where branch 1 is loaded) that (a) branch 2 L-N is 217V as before, or (b) branch 2 L-N is 238V ?

 

[Paul.M]

Steve trust you to have a go at this one. Interesting fault/s.

Tel is only talking about branch #1. Minimal load = 217V L-N, once a kettle is plugged in this drops to 175V???? L-E remains the same on branch #1.

This is Telectix thread, leave it to him to explain more when people have had their input and questions. I know the answer, he told me. Well he asked me and I just about worked it out, just. Tel likes to keep me on my toes lol

 

[telectrix]

Do you mean (in the case where branch 1 is loaded) that (a) branch 2 L-N is 217V as before, or (b) branch 2 L-N is 238V ?

load applied to branch 1 caused the L-N at the junction of the 10mm and both 6mm cables to drop to 175V. however, no loss of power was apparent at the load end of branch 2.

 

[telectrix]

well done to happysteve for getting it almost spot on. ( his answer was by pm, so the rest of you can still think about it without being influenced ).

 

[happysteve]

Thanks for the feedback via PM. And thanks for posting the question!

 

[Beavstro]

Realise this is a few months out of date...
but my guess would be that there is a neutral to earth fault either on whatever is attached to branch one and or loose connection somewhere...?

 

[telectrix]

the faults were basically.

1. high resistance in the N of the 10mm feed cable, causing massive volt drop in one branch.

2. the other branch had been incorrectly wired across L and E, so the volt drop L-N was not apparent.