RCD tripped when cutting cable with breaker off

[BraileTrail]

Hi,
As we decorate each room I swap the old light switches, pendants &
sockets for modern ones. I was replacing a dual gang socket yesterday
and had switched off the circuit breaker for that ring and tested that
the circuit was dead. I had to cut about 75mm of spare cable off to
get the new socket to fit in to the shallow depth backbox and in doing
so the RCD tripped in the consumer unit. I used a normal pair of
electricians side cutters to cut the 2.5mm T&E as I always do.

Is the tripping of the RCD indicative of a problem with the wiring or
is it just because I momentarily bridged the 3 conductors as I cut the
cable? I've checked the wiring visually as far as I can and used one
of those plug-in-tester things and everything came up green. We don't
generally have nuisance trips.

Thanks,
BraileTrail

[Andy Burns]

BraileTrail wrote:

> I was replacing a dual gang socket yesterday
> and had switched off the circuit breaker for that ring and tested that
> the circuit was dead. I had to cut about 75mm of spare cable off to
> get the new socket to fit in to the shallow depth backbox and in doing
> so the RCD tripped in the consumer unit.

Switching the breaker off for the circuit you're working on only
isolates the live, so when you cut the cable you briefly short the
neutral and earth.

The RCD works by making sure that (within limits) the amount of current
supplied via its live returns via its neutral, your short allows some of
the current from circuits *other* than the one you're working on to
return via earth instead of live, so it trips.

[Andy Burns]

Andy Burns wrote:

> The RCD works by making sure that (within limits) the amount of current
> supplied via its live returns via its neutral, your short allows some of
> the current from circuits *other* than the one you're working on to
> return via earth instead of live, so it trips.

^^^^^
neutral

[Mike Clarke]

On Tue, 10 Apr 2012 01:24:30 -0700 (PDT)
BraileTrail <brail...@gmail.com> wrote:

> Is the tripping of the RCD indicative of a problem with the wiring or
> is it just because I momentarily bridged the 3 conductors as I cut the
> cable?

This can be quite common, it's due to earth and neutral being shorted
together.

Switching off the circuit breaker (or removing the circuit fuse) only
interrupts the line conductor. The neutrals for all the circuits in
your house are connected together in your consumer unit so when you
short earth and neutral together on your circuit you are providing an
additional return path for the other circuits via your earth conductor.
This results in the RCD seeing less return current in the neutral than
supply in the live if anything is in use on the other circuits, causing
it to trip.

--
Mike Clarke

[BraileTrail]

Thanks for the quick and reassuring replies. I guess I've just been
lucky and it's never happened before.

Thanks,
BraileTrail

[Mike Clarke]

On Tue, 10 Apr 2012 02:56:31 -0700 (PDT)
BraileTrail <brail...@gmail.com> wrote:

> Thanks for the quick and reassuring replies. I guess I've just been
> lucky and it's never happened before.

You can reduce the chance of this happening if you snip in one
conductor at a time and open up the "wound" as you go by bending the
cable so that the jaws of your cutter don't touch the exposed ends.

--
Mike Clarke

[The Natural Philosopher]

an earth neutral short will trip an RCD.

Guaranteed when cutting a 'dead ' cable that's still connected at the CU.



--
To people who know nothing, anything is possible.
To people who know too much, it is a sad fact
that they know how little is really possible -
and how hard it is to achieve it.

[Jim K]

On Tue, 10 Apr 2012 13:11:13 +0100, The Natural Philosopher
<t...@invalid.invalid> wrote:

> BraileTrail wrote:
>> Hi,
>> As we decorate each room I swap the old light switches, pendants &
>> sockets for modern ones. I was replacing a dual gang socket yesterday
>> and had switched off the circuit breaker for that ring and tested that
>> the circuit was dead. I had to cut about 75mm of spare cable off to
>> get the new socket to fit in to the shallow depth backbox and in doing
>> so the RCD tripped in the consumer unit. I used a normal pair of
>> electricians side cutters to cut the 2.5mm T&E as I always do.
>> Is the tripping of the RCD indicative of a problem with the wiring or
>> is it just because I momentarily bridged the 3 conductors as I cut the
>> cable? I've checked the wiring visually as far as I can and used one
>> of those plug-in-tester things and everything came up green. We don't
>> generally have nuisance trips.
>> Thanks,
>> BraileTrail
>
> an earth neutral short will trip an RCD.
>
> Guaranteed when cutting a 'dead ' cable that's still connected at the CU.

yup as pointed out 3 hours ago....

Jim K

[tony sayer]

In article <op.wcjtv...@sbseee.lan>, Jim K <jk98...@gmail.com>
scribeth thus

Provided that there is still some current flowing from the Live incomer
thru the RCD to trip it. Thats to say there must be some current greater
than its rated trip current flowing back on the neutral then when you
short the earth and neutral together that sets up the imbalance, but if
there is no current flowing IN on the live then where is the trip
current going to flow in order to cause it to trip?....
--
Tony Sayer

[Dave Liquorice]

On Tue, 10 Apr 2012 14:01:17 +0100, tony sayer wrote:


> Thats to say there must be some current greater than its rated trip
> current flowing back on the neutral then when you short the earth and
> neutral together that sets up the imbalance,

30mA is 7W... One Sky box in standby takes more than that. B-)

--
Cheers
Dave.

[tony sayer]

In article <nyyfbegfubjuvyypb...@srv1.howhill.co.uk>, Dave
Liquorice <allsortsn...@howhill.co.uk> scribeth thus

Isn't that the output of a large power station on a nation-wide
basis;-?...
--
Tony Sayer

[The Natural Philosopher]

na...

20 million households so 140 million watts.

About the average output of all the wind turbines on a light breeze
day, but nothining like the billion or wo watts of a real power staion.

But it does suggest a useful 'reneable' measure 'could power up to 10%
of the countries sky boxes' etc etc.

[Jim K]

nah it's Hoorah Harray's current peak output...

Jim K

[The Medway Handyman]

On 10/04/2012 10:19, Mike Clarke wrote:
> On Tue, 10 Apr 2012 01:24:30 -0700 (PDT)
> BraileTrail<brail...@gmail.com> wrote:
>
>> Is the tripping of the RCD indicative of a problem with the wiring or
>> is it just because I momentarily bridged the 3 conductors as I cut the
>> cable?
>
> This can be quite common, it's due to earth and neutral being shorted
> together.

Hence the reason I check to see if anyone is using a computer before I
do anything.


--
Dave - The Medway Handyman www.medwayhandyman.co.uk

[Roger Mills]

On 10/04/2012 14:01, tony sayer wrote:

>
> Provided that there is still some current flowing from the Live incomer
> thru the RCD to trip it. Thats to say there must be some current greater
> than its rated trip current flowing back on the neutral then when you
> short the earth and neutral together that sets up the imbalance, but if
> there is no current flowing IN on the live then where is the trip
> current going to flow in order to cause it to trip?....

Do you mean current flowing in other circuits served by the same RCD,
even though the circuit in question is switched off? Maybe.

I always assumed it was current flowing between neutral and earth
because they're not at exactly the same potential. There would then be
current flowing in the N but none in the L - creating an imbalance.
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

[John Williamson]

Roger Mills wrote:
> On 10/04/2012 14:01, tony sayer wrote:
>
>>
>> Provided that there is still some current flowing from the Live incomer
>> thru the RCD to trip it. Thats to say there must be some current greater
>> than its rated trip current flowing back on the neutral then when you
>> short the earth and neutral together that sets up the imbalance, but if
>> there is no current flowing IN on the live then where is the trip
>> current going to flow in order to cause it to trip?....
>
> Do you mean current flowing in other circuits served by the same RCD,
> even though the circuit in question is switched off? Maybe.
>
> I always assumed it was current flowing between neutral and earth
> because they're not at exactly the same potential. There would then be
> current flowing in the N but none in the L - creating an imbalance.

Is the correct answer. But only when the neutral isn't at earth
potential, which only normally happens if the earth and neutral aren't
bonded together at the consumer unit.

--
Tciao for Now!

John.

[ARWadsworth]

Preferably the bonding should be done at the suppliers cut out and not at
the CU.
http://wiki.diyfaq.org.uk/index.php?title=Earthing_Types
--
Adam

[Graham.]

What percentage of domestic installations are PME?

I wouldn't like to think people are strapping E&N together when it's
inappropriate.


--
Graham.
%Profound_observation%

[tony sayer]

In article <9ujhtg...@mid.individual.net>, Roger Mills
<watt....@gmail.com> scribeth thus

>On 10/04/2012 14:01, tony sayer wrote:
>
>>
>> Provided that there is still some current flowing from the Live incomer
>> thru the RCD to trip it. Thats to say there must be some current greater
>> than its rated trip current flowing back on the neutral then when you
>> short the earth and neutral together that sets up the imbalance, but if
>> there is no current flowing IN on the live then where is the trip
>> current going to flow in order to cause it to trip?....
>
>Do you mean current flowing in other circuits served by the same RCD,
>even though the circuit in question is switched off? Maybe.

No. Take a PME system like what we have here. Incoming cable is a
co-axial sort, live central conductor outer screen the neutral return.
At the main fuse there is a connector block where the Neutral on the
outer SWA screen is connected to the Earth block. So the Neutral and
Earth are bonded together there.

Now go just the other side of the RCD nothing connected anywhere all
sockets unplugged and lights switched off. Now connect the neutral to
earth the consumer side of the RCD and it won't trip as there is no
current flowing.

Now switch on a load and the current will of course flow through the RCD
on the live wire then back to the supply via the Neutral and in this
instance the earth. So in effect there is some current being shunted
past the RCD and some flowing thru the RCD on the neutral line. Of
course theres more going out on the live than coming back in on the
neutral as its being bypassed on the earth.

When sufficient is flowing then once the current in-balance is high
enough then the trip will trip and break the circuit.

And as stated above nothing flowing and the earth neutral short still
there unless something is connected to the supply the consumer side of
the RCD then no tripping...

>
>I always assumed it was current flowing between neutral and earth
>because they're not at exactly the same potential. There would then be
>current flowing in the N but none in the L - creating an imbalance.

It's the difference of current amounts flowing in Both lines of the RCD.

Doesn't matter if say theres 40 odd amps flowing in both of them. Once
theres a difference of more than 15 milliamps or so thats exceeded then
it will trip..


--
Tony Sayer

[tony sayer]

In article <7f79o79lhloih0fj8...@4ax.com>, Graham.
<m...@privacy.net> scribeth thus

Dunno .. but I would think that in most all Urban, Suburban, and places
supplied with underground cables all of them perhaps...

'Tho I'm willing to be proved wrong;!..

>
>I wouldn't like to think people are strapping E&N together when it's
>inappropriate.
>
>

--

Tony Sayer

[John Rumm]

In new installs, PME is the most common system IME. However many older
urban installs are TN-S. So I would not be surprised if in terms of
total installed numbers, TN-S still wins.


--
Cheers,

John.

/=================================================================\
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\=================================================================/

[ARWadsworth]

More than you would believe when you find the NE shorts behind the sockets
on a non RCD protected circuit:-)

> I wouldn't like to think people are strapping E&N together when it's
> inappropriate.

They do see my reply above:-)

--
Adam