The MCB at the head end of my garage supply is tripping. It has been tripping intermittently for some months but now trips every time it's switched on..

The head end is fed directly from the meter into a small consumer unit containing a main switch and a 20A type B MCB. This is the one that trips. This connects to around 100m of 6mm SWA, partially buried, partially running above ground through some brambles (will get around to burying that one day!).. I have completely disconnected the SWA in the garage.

There was a resin-filled join in the SWA around a third of the way, so I cut that out as that was what I suspected of being the problem. If I connect just the first third of the SWA to the head end it doesn't trip. If I connect the rest it does trip, so the problem obviously exists in the final two thirds of the cable.

I've used a standard multimeter to test the resistance between L&N, L&E and N&E but all read open circuit (the meter shows a reading for an instant, then reads open circuit again). I don't have access to a Megger unfortunately.

I've visually inspected the parts of the cable I can get to, so now I'm stuck for ideas.

Any suggestions for how to proceed would be gratefully received!

On 13/02/14 16:22, Sooner or Later wrote:
The MCB at the head end of my garage supply is tripping. It has been
tripping intermittently for some months but now trips every time it's
switched on.

The head end is fed directly from the meter into a small consumer
unit containing a main switch and a 20A type B MCB. This is the one
that trips. This connects to around 100m of 6mm SWA, partially
buried, partially running above ground through some brambles (will
get around to burying that one day!). I have completely disconnected
the SWA in the garage.

There was a resin-filled join in the SWA around a third of the way,
so I cut that out as that was what I suspected of being the problem.
If I connect just the first third of the SWA to the head end it
doesn't trip. If I connect the rest it does trip, so the problem
obviously exists in the final two thirds of the cable.


so replace it.


I've used a standard multimeter to test the resistance between L&N,
L&E and N&E but all read open circuit (the meter shows a reading for
an instant, then reads open circuit again). I don't have access to a
Megger unfortunately.


that sounds like a major capacitative short to me.

maybe water and an electrolyte in the cable or something.

Doesn't natter what it is, it wont be 'fixable' so get some more SWA and
simply rewire the whole shebang using it.




I've visually inspected the parts of the cable I can get to, so now
I'm stuck for ideas.

Any suggestions for how to proceed would be gratefully received!



--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to
lead are elected by the least capable of producing, and where the
members of society least likely to sustain themselves or succeed, are
rewarded with goods and services paid for by the confiscated wealth of a
diminishing number of producers.

"Sooner or Later" wrote in message
...

The MCB at the head end of my garage supply is tripping. It has been
tripping intermittently for some months but now trips every time it's
switched on.

The head end is fed directly from the meter into a small consumer unit
containing a main switch and a 20A type B MCB. This is the one that trips.
This connects to around 100m of 6mm SWA, partially buried, partially
running above ground through some brambles (will get around to burying that
one day!). I have completely disconnected the SWA in the garage.

There was a resin-filled join in the SWA around a third of the way, so I
cut that out as that was what I suspected of being the problem. If I
connect just the first third of the SWA to the head end it doesn't trip. If
I connect the rest it does trip, so the problem obviously exists in the
final two thirds of the cable.

I've used a standard multimeter to test the resistance between L&N, L&E and
N&E but all read open circuit (the meter shows a reading for an instant,
then reads open circuit again). I don't have access to a Megger
unfortunately.

I've visually inspected the parts of the cable I can get to, so now I'm
stuck for ideas.

Any suggestions for how to proceed would be gratefully received!

The brief indication is the meter charging up the capacitance of the cable.
The cable is probably breaking down at a much higher voltage than the
(probably) 1.5v of the meter ohms circuit hence not showing a leakage
resistance problem.

Andrew

"Sooner or Later" wrote in message
...
The MCB at the head end of my garage supply is tripping. It has been
tripping intermittently for some months but now trips every time it's
switched on.

The head end is fed directly from the meter into a small consumer unit
containing a main switch and a 20A type B MCB. This is the one that trips.
This connects to around 100m of 6mm SWA, partially buried, partially running
above ground through some brambles (will get around to burying that one
day!). I have completely disconnected the SWA in the garage.

There was a resin-filled join in the SWA around a third of the way, so I cut
that out as that was what I suspected of being the problem. If I connect
just the first third of the SWA to the head end it doesn't trip. If I
connect the rest it does trip, so the problem obviously exists in the final
two thirds of the cable.

I've used a standard multimeter to test the resistance between L&N, L&E and
N&E but all read open circuit (the meter shows a reading for an instant,
then reads open circuit again). I don't have access to a Megger
unfortunately.

I've visually inspected the parts of the cable I can get to, so now I'm
stuck for ideas.

Any suggestions for how to proceed would be gratefully received!

Go to the far garage end of the cable and open the mains switch of the
consumer unit there and see if it still trips. the fault may be in the
garage.
Make sure the ends of the cable in the joint you have taken apart are clear
and undamaged
It's quite unusual for SWA cable to fail in the middle, it's usually a fault
in a joint, switch or some form of damage which is usually visible. In your
case someone might have "spiked/chopped" it whilst digging.

It is supposed to be buried 18" deep minimum to prevent this.

The Natural Philosopher wrote:
On 13/02/14 16:22, Sooner or Later wrote:
The MCB at the head end of my garage supply is tripping. It has been
tripping intermittently for some months but now trips every time it's
switched on.

The head end is fed directly from the meter into a small consumer
unit containing a main switch and a 20A type B MCB. This is the one
that trips. This connects to around 100m of 6mm SWA, partially
buried, partially running above ground through some brambles (will
get around to burying that one day!). I have completely disconnected
the SWA in the garage.

There was a resin-filled join in the SWA around a third of the way,
so I cut that out as that was what I suspected of being the problem.
If I connect just the first third of the SWA to the head end it
doesn't trip. If I connect the rest it does trip, so the problem
obviously exists in the final two thirds of the cable.


so replace it.


I've used a standard multimeter to test the resistance between L&N,
L&E and N&E but all read open circuit (the meter shows a reading for
an instant, then reads open circuit again). I don't have access to a
Megger unfortunately.


that sounds like a major capacitative short to me.

maybe water and an electrolyte in the cable or something.

Doesn't natter what it is, it wont be 'fixable' so get some more SWA and
simply rewire the whole shebang using it.




I've visually inspected the parts of the cable I can get to, so now
I'm stuck for ideas.

Any suggestions for how to proceed would be gratefully received!



Agreed.

On 13/02/2014 16:22, Sooner or Later wrote:
The MCB at the head end of my garage supply is tripping. It has been
tripping intermittently for some months but now trips every time it's
switched on.

The head end is fed directly from the meter into a small consumer
unit containing a main switch and a 20A type B MCB. This is the one
that trips. This connects to around 100m of 6mm SWA, partially
buried, partially running above ground through some brambles (will
get around to burying that one day!). I have completely disconnected
the SWA in the garage.

So its a cable fault then, or a faulty MCB. Try feeding it from any
other MCB as a test to eliminate the MCB.

There was a resin-filled join in the SWA around a third of the way,
so I cut that out as that was what I suspected of being the problem.
If I connect just the first third of the SWA to the head end it
doesn't trip. If I connect the rest it does trip, so the problem
obviously exists in the final two thirds of the cable.

sounds like it

I've used a standard multimeter to test the resistance between L&N,
L&E and N&E but all read open circuit (the meter shows a reading for
an instant, then reads open circuit again). I don't have access to a
Megger unfortunately.

I would have to be a fairly catastrophic insulation failure of some
description to sink the 100A+ necessary to open the MCB immediately.

I've visually inspected the parts of the cable I can get to, so now
I'm stuck for ideas.

Any suggestions for how to proceed would be gratefully received!

If the MCB checks out, replace the second section of cable.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 13/02/2014 17:34, harryagain wrote:

who can tell?

harry, is there a fighting chance you could install IE Quote Fix so that
the rest of us can work out where your reply is hidden in these posts?


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"Sooner or Later" wrote in message
...
The MCB at the head end of my garage supply is tripping. It has been
tripping intermittently for some months but now trips every time it's
switched on.

The head end is fed directly from the meter into a small consumer unit
containing a main switch and a 20A type B MCB. This is the one that trips.
This connects to around 100m of 6mm SWA, partially buried, partially running
above ground through some brambles (will get around to burying that one
day!). I have completely disconnected the SWA in the garage.

There was a resin-filled join in the SWA around a third of the way, so I cut
that out as that was what I suspected of being the problem. If I connect
just the first third of the SWA to the head end it doesn't trip. If I
connect the rest it does trip, so the problem obviously exists in the final
two thirds of the cable.

I've used a standard multimeter to test the resistance between L&N, L&E and
N&E but all read open circuit (the meter shows a reading for an instant,
then reads open circuit again). I don't have access to a Megger
unfortunately.

I've visually inspected the parts of the cable I can get to, so now I'm
stuck for ideas.

Any suggestions for how to proceed would be gratefully received!

There is a slight possibilty the MCB is faulty.
Try connecting to another one & see what happens.

On 13/02/2014 16:22, Sooner or Later wrote:
[...]

I don't have access to a Megger unfortunately.

Yes you do. Hire one, and a distance-to-fault indicator (TDR) too. Try
a local branch of Newey & Eyre, or similar large electrical wholesaler,
or a online hire operator.

Any suggestions for how to proceed would be gratefully received!

To check the MCB, disconnect the outgoing cable and load up the circuit
with a couple of electric fires, or similar, and ensure it doesn't trip.

Otherwise it does sound like a cable fault. TBH I'd just abandon the
present cable and lay a new one. Use 10 mm^2 at least - 6 is somewhat
undersized for that length of run, assuming the design load is the full
20 A In.

--
Andy