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

 

[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

 

[snowhead]

What's the I.R from the Switch fuse end?

What's the route of the cable, does it pass thro' another flat?
Is there any work being done anywhere in the building?

 

[Engineer54]

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

There may be damage to the cable (or other components) that through blowing itself clear won't show any or little sign of the damage. For instance, a 16 mm conductor could have been reduced to 6mm or less by the discharge of energy clearing the fault.... Now what can you see potentially happening when that cable is loaded up for any period of time??

That's just one situation, and perhaps the easiest to describe and envisage, but there are also other reasons.

 

[R G]

When you say theres a separate earth , it could be the armour is not earthed at all i.e. plastic enclosures maybe and the clear reading to earth is not via the armour but the separate earth. have you tested to the armour ??


Another thought if it goes through someone else's flat , could be someone has been messing with it for a cheap leccy supply , you never know ...

 

[Guest77]

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

Crickey, really?

 

[Guest111]

this highlights the advantages of the good old Drummond test lamp,you made sure the load end was isolated and put the lamp across the fuse carrier,if the lamp lit you knew there was a fault and didn't put the fuse in.

 

[Guest77]

My suggestion, send a spark around, a few minutes with some basic test kit will identify the problem.

 

[nickblake]

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

Traffic wardens can be a hazard of the job but testing is a must , what you should do is disconnect both ends of the cable insulate the bare ends at the opposite end where your testing from and then test from one end this way your less likley to zap the curious customer could be mr DIY put a nail through the cable ,no sign of any smoking trainers any where

 

[7029 dave]

Fuses don't blow without reason, short circuit, or over load current.

 

[PEG]

Fuses don't blow without reason, short circuit, or over load current.

There is nothing in this life that happens without a reason,metaphysical discussions apart...unless your my 6 year old,in which case,it genuinely,"...just happened..." :smilewinkgrin:

 

[GLENNSPARK]

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.

yep....
i remember you telling me about that breeched paper-lead on the phone the other week...