Discuss FAULT FINDING in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Eire79

-
Reaction score
9
ok, iv a domestic house with a shed 50 mtrs away supplied via 3x10 SWA on a B32A MCB. fault has been occurring the last few days which is the MCB in house tripping.
I arrive and open both boards and remove both PE and N conductors from terminals followed by IR testing and was getting very poor results between all conductors, then just out of interest i powered back up with phase only and main switch in shed isolated plus PE and N still removed .
I was recording voltage of 230v from the disconnected PE to the MET at the house and 180v between the PE and N conductor supposedly dead legs therefore i was expecting a trip again when i connected back up all conductors but no , nothing happened.
I am pretty confidant that the SWA is damaged but why is it not tripping constantly, is it a high resistance fault thereby not enough fault current flowing.
I also carried out fault loop at the house and shed which were .15 ohms and .33ohms.
Looking forward to your opinion's.
 
If you just used a multimeter or similar you could easily see that sort of voltage just from the cable capacitance.

When you say very poor IR what values did you see for L - N&E and from N - E? (presumably cable only, with the shed disconnected)

Did you also IR the shed to see if something at fault there? Water or small creatures getting in to socket boxes, etc, springs to mind...
 
If you think the SWA is damaged, what are the IR test results (measured at 500V) for the cable, disconnected both ends: L to N, L to E, N to E, and then each in turn of L, N and E to the armour (I've assumed this is a 3-core cable).
 
If you think the SWA is damaged, what are the IR test results (measured at 500V) for the cable, disconnected both ends: L to N, L to E, N to E, and then each in turn of L, N and E to the armour (I've assumed this is a 3-core cable).
IR results were in the region of 30 to 50 M ohms, that was on everything at 500v and even at 250v the results were poor. I think your correct with cable damage or maybe a joint underground not correctly terminated. either way a new cable is required now.
 
30-50M is not great, but it is not really an indication of "very poor" either. Has it been very dry since the last fault case?

What were the IR results on the shed circuits?
 
If you just used a multimeter or similar you could easily see that sort of voltage just from the cable capacitance.

When you say very poor IR what values did you see for L - N&E and from N - E? (presumably cable only, with the shed disconnected)

Did you also IR the shed to see if something at fault there? Water or small creatures getting in to socket boxes, etc, springs to mind...
No I used the multifunction to check voltage.IR was 30 to 50 M ohms with both ends disconnected. I understand what you mean with capacitance, I came across a fault once in a industrial premises where a spark installed a metalclad RCD socket that would not trip, when I checked voltage to earth it was 230 until a load was plugged in resulting in the 230v to earth disappearing.
That's why I keep telling lads to test everything.
[automerge]1591636370[/automerge]
Unless there is an intermittent fault the insulation test is failing to break down then in my eyes 30-50 Mohm is not an issue.
Sorry all I mean 3 to 5 M
[automerge]1591636547[/automerge]
Sorry all its 3 to 5 M ohms , although its been very dry lately so I'm baffled as to tripping issue , id say something if it was raining
 
Last edited:
so it's not an earth leakage fault. it's an overload or a short circuit. could even be a duff MCB.
It doesn't often happen, but I replaced a faulty MCB recently: it had been poorly-terminated to the bus bar, which had over time caused thermal damage (visible discolouring), which led to occasional tripping. Even the physical switch mechanism had been affected though, and the bus bar terminal was very tight and gritty to unscrew.
 
Update, mcb still tripping so got meters out again and this time 980 ohms between phase and earth so cable damaged . Bit of a awkward job ahead now getting a new cable out.
 

Reply to FAULT FINDING in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock