Am I missing something here

[Pondy]

Ok I get called to a job where there is no power to the flat. Intake is in a small basement cupboard, 25mm swa protected by a Bs88 80a fuse in a switch fuse. Test for continuity and the fuse has blown. Fit a new one and I'm sure you can guess the outcome..... New fuse blows.
Go up the the tenants cu and ir at 250v the incoming swa tails, live to neutral, live to earth etc and get a reading of around 150mohms on each. Now This job was last thing tonight and my brain is frazzled, what am I missing, the cu is a 16th edition skeleton board, around 15 years old, no one else is affected in the flats, oh and the new fuse blew with the main switch off

 

[Octopus]

Sounds like the tails from the switch fuse to the CU have a problem!

 

[Darkwood]

Do you make a habit of changing fuses without testing first... Id have been inclined to at least switch D.B. off and test the supply cable before replacing a blown fuse... then confirmed the circuits before energising if supply cable tested fine.

A bit arris about face your methods Pondy but I'm sure you'll do the correct sequence next time

 

[davesparks]

Since you tested after replacing the fuse and not before I would suggest that the fault has blown itself apart when the new fuse was put in.
It is always sensible to tests before replacing a blown fuse, especially when larger fuses are concerned.

 

[PEG]

On seeing a fuse of that size blown,more than a cursory glance would be necessary before jamming another round up the breach and shooting off in to the dark. :stooge_curly:

 

[Pondy]

Agreed a spot of testing wouldn't of gone amiss, but I was parked in the road in a permit area and had one eye out for the traffic warden, the cu main switch was off darkwood,surely insolation resistance of the tails from the main switch back would of found an issue

 

[Knobhead]

What’s wrong with the “bang” test? Some people are just too fussy.

 

[davesparks]

The you should have parked more sensibly! What if the arc at the fault when you applied power had started a fire? Would you stand up in court and use your poor parking as an excuse for burning down a house?

Insulation resistance testing could have found a fault before you put the fuse in, but if the fault blew itself apart when the fuse was put in it won't now will it?

 

[davesparks]

What’s wrong with the “bang” test? Some people are just too fussy.

Nothing wrong with the bang test, if you know how to apply it!

 

[Adam W]

When you say '150mohms' presumably you actually mean 150MΩ?
There's a big difference between milliohms and Megohms.

 

[Pondy]

Adam, sorry yes I do mean mega ohms, dave sparks it wasn't poor parking and I wasn't making excuses, all the pay and display was full and there nearest was quarter of a mile away, and a permit space in the road outside the job was empty.
I don't understand quite what you mean, surely the fault is still there and as such should be traceable

 

[bigdavs83]

I think what he's getting at is that with the power being put back on it's literally blown the fault clear. However what will be glaringly obvious now is something not working but with your main switch off it would point to a supply issue

 

[Knobhead]

You can blow a fault clear. But it’s usually due to damp. The heat of the flashover dries the fault out.

I spent nearly a month looking a UG cable fault. Every time the fuses blew it would test perfect afterwards. Finally found it by putting copper links in place of the 450A fuses. The ground erupted and a tree fell over, gave the game away really. The tree roots had disturbed a through joint and damp was getting in.

 

[Darkwood]

What’s wrong with the “bang” test? Some people are just too fussy.

Only worth it if you have spare fuses or reset devices and a risky nature

What kind of tree was it Tony ..possible an amPEAR tree


Already getting my coat:89:

 

[davesparks]

Adam, sorry yes I do mean mega ohms, dave sparks it wasn't poor parking and I wasn't making excuses, all the pay and display was full and there nearest was quarter of a mile away, and a permit space in the road outside the job was empty.
I don't understand quite what you mean, surely the fault is still there and as such should be traceable

For example if there was a short in an SWA cable where say a screw had penetrated it and made a connection between the armour and live then a large current will have flowed through the screw before the fuse operated. This fault current often manifests itself as a loud, bright discharge of energy which can dislodge the screw, melt through it or melt through the copper of the cable. Any of these things happening will remove the short circuit but obviously leave the cable significantly damaged.

 

[dingledong]

Works a treat with pyro when I was a lad

 

[Pondy]

That part I don't get, is surely if I've meggered from the cu live and neutral. Live to earth etc that proves that there is no fault with the swa as the insolation resistance is fine, but the fuse is still blowing

 

[Richard Burns]

Did you test to the armour as well (assuming this is not used as the earth!) and is the armour properly terminated at both ends.
It may be possible to have a line to armour fault and the armour not earthed at the CU end only the supply end.

EDIT: or perhaps an internal short inside the switch fuse before the switch
Is there a landlords CU that actually has the fault on it that is connected to the same supply cable?

 

[Engineer54]

That part I don't get, is surely if I've meggered from the cu live and neutral. Live to earth etc that proves that there is no fault with the swa as the insolation resistance is fine, but the fuse is still blowing

This is why you ''NEVER'' replace a blown fuse (of unknown cause) ''BEFORE'' conducting tests to confirm that no fault is present. Your statement above about everything is now fine could well be wishful thinking on your part, significant damage to the cable or other components could well exist, just waiting to cause even more problems later!!

 

[Pondy]

E54, yes I know I should of done, Richard thanks for your input, I know the armour is not used as an earth, it has a separate earth, just can't see how there is a problem still and its not showing up