Consumer unit switch tripping despite RCD
 

Radial spur off of main consumer unit in garage. The spur feeds
exterior lighting via a dusk till dawn unit - 3 x 40 watt halogen lamps
and 1 x 150 watt halogen security lamp. It was cheaper for me to buy
an RCD plug than a separate RCD and fuse box so I simply plug the
exterior lighting in to the radial spur using the RCD plug.

When I first plugged in the circuit the consumer unit fuse tripped, but
the RCD didn't cut out. I'm not sure I understand why, can anyone
help? I thought the RCD would cut out before anything else and
therefore protecting the main consumer unit.

In attempt to find out the cause of the fuse trip on the consumer unit
I checked every single connection on the dusk till dawn, junction
boxes, and lamps, and they all looked fine. So I flicked the switch on
the fuse and this time I heard a pop outside and again the fuse
tripped. One of the lamps had popped - identifiable from a black ring
on the brick around the lamp. Again I checked the connections in the
junction box leading to that lamp and again also inside the lamp. They
all are fine, but I noticed more carbon around the manufactured
attachments where the cable runs in to the lamp unit. So I'm figuring
the lamp is faulty. I'll be replacing this lamp unit and of course I
do not leave the circuit on. But that said, strangely, I flicked the
trip switch on the fuse in the consumer unit back to on and the circuit
works fine now - not sure why either!

So my questions are -

1. why didn't/doesn't the RCD trip before the fuse on the consumer unit
2. any ideas what the likely cause of the problem is with the lamp -
just out of interest? Like I say I'll just get a replacement from B&Q
anyway but just curious.

The RCD only protects against a fault to earth
What you had was a short circuit in a faulty lamp, this would not produce a
fault to earth and therefore not operate the RCD. This fault would product a
current sufficient to operate the fuse in the CU.
Hope this helps

--
Ellis Greensitt
Supplychain(Doncaster) Ltd.

www.supplychain.org.uk
01302 761205 : Tel
01302 761206 : Fax
"Dundonald" wrote in message
ups.com...
Radial spur off of main consumer unit in garage. The spur feeds
exterior lighting via a dusk till dawn unit - 3 x 40 watt halogen lamps
and 1 x 150 watt halogen security lamp. It was cheaper for me to buy
an RCD plug than a separate RCD and fuse box so I simply plug the
exterior lighting in to the radial spur using the RCD plug.

When I first plugged in the circuit the consumer unit fuse tripped, but
the RCD didn't cut out. I'm not sure I understand why, can anyone
help? I thought the RCD would cut out before anything else and
therefore protecting the main consumer unit.

In attempt to find out the cause of the fuse trip on the consumer unit
I checked every single connection on the dusk till dawn, junction
boxes, and lamps, and they all looked fine. So I flicked the switch on
the fuse and this time I heard a pop outside and again the fuse
tripped. One of the lamps had popped - identifiable from a black ring
on the brick around the lamp. Again I checked the connections in the
junction box leading to that lamp and again also inside the lamp. They
all are fine, but I noticed more carbon around the manufactured
attachments where the cable runs in to the lamp unit. So I'm figuring
the lamp is faulty. I'll be replacing this lamp unit and of course I
do not leave the circuit on. But that said, strangely, I flicked the
trip switch on the fuse in the consumer unit back to on and the circuit
works fine now - not sure why either!

So my questions are -

1. why didn't/doesn't the RCD trip before the fuse on the consumer unit
2. any ideas what the likely cause of the problem is with the lamp -
just out of interest? Like I say I'll just get a replacement from B&Q
anyway but just curious.

On 5 Jun 2005 15:43:29 -0700, Dundonald wrote:

1. why didn't/doesn't the RCD trip before the fuse on the consumer
unit

An RCD does not provide any overload protection. It only trips if
there is an inbalance in the current flowing in the live an neutral
wires connected through it. If you have live/neutral short an RCD
won't see that as a fault condition as 6kA if going and 6kA is coming
back.

--
Cheers
Dave. pam is missing e-mail

In message , Ellis Greensitt
writes
The RCD only protects against a fault to earth
What you had was a short circuit in a faulty lamp, this would not produce a
fault to earth and therefore not operate the RCD. This fault would product a
current sufficient to operate the fuse in the CU.
Hope this helps


Look ellis this is how it works

__________________________________________


[Post you are replying to with irrelevant bits snipped]

[Your contribution]

Sig sep

Your sig - to which you can put a link to your site

___________________________________________


Hope this helps and hope you're able to take this simple concept onboard



--
geoff

Look ellis this is how it works

__________________________________________


[Post you are replying to with irrelevant bits snipped]

[Your contribution]

Sig sep

Your sig - to which you can put a link to your site

___________________________________________


Hope this helps and hope you're able to take this simple concept onboard



--
geoff

Sorry Geoff, I had all the bits but in the wrong order.
Outlook express puts the quoted portion at the end by default. Im new to
this, thanks for putting me right.

--
Ellis Greensitt
Supplychain(Doncaster) Ltd.

www.supplychain.org.uk
01302 761205 : Tel
01302 761206 : Fax

Dundonald wrote:


1. why didn't/doesn't the RCD trip before the fuse on the consumer unit

As Ellis said, an RCD only detects faults to earth, most obviously L to
E. If your wiring or lamp had an L-to-N short, that'll make the MCB pop,
but the RCD will be happy as Larry, since just as much current - in this
case, lots and oodles - is flowing out down the L as is coming back up
the N.
2. any ideas what the likely cause of the problem is with the lamp -
just out of interest? Like I say I'll just get a replacement from B&Q
anyway but just curious.

As mentioned above, a fault making the L shorted to N. Either a fault in
the lamp, or - more likely, I'm afraid - you wired it up wrong (or ended
up with bare L and N conductors touching when you sqaushed them back in
place). The blackening of the brick, and the popping of the MCB, says
you had quite a lot of electricity flowing for a short time. When big
fault currents like this flow, they can 'clear' the fault by burning
away the metal bits wot are touching where they shouldn't be - so
explaining your 'well now it doesn't trip the MCB any more!' observation.

HTH - Stefek

Ellis Greensitt wrote:

Sorry Geoff, I had all the bits but in the wrong order.
Outlook express puts the quoted portion at the end by default. Im new to
this, thanks for putting me right.

Use this niffty little plug in for OE

http://home.in.tum.de/~jain/software/oe-quotefix/


--
Mark
Please replace invalid and invalid with gmx and net to reply

"Stefek Zaba" wrote in message
...
Dundonald wrote:


1. why didn't/doesn't the RCD trip before the fuse on the consumer unit

As Ellis said, an RCD only detects faults to earth, most obviously L to E.
If your wiring or lamp had an L-to-N short, that'll make the MCB pop, but
the RCD will be happy as Larry, since just as much current - in this case,
lots and oodles - is flowing out down the L as is coming back up the N.
2. any ideas what the likely cause of the problem is with the lamp -
just out of interest? Like I say I'll just get a replacement from B&Q
anyway but just curious.

As mentioned above, a fault making the L shorted to N. Either a fault in
the lamp, or - more likely, I'm afraid - you wired it up wrong (or ended
up with bare L and N conductors touching when you sqaushed them back in
place). The blackening of the brick, and the popping of the MCB, says you
had quite a lot of electricity flowing for a short time. When big fault
currents like this flow, they can 'clear' the fault by burning away the
metal bits wot are touching where they shouldn't be - so explaining your
'well now it doesn't trip the MCB any more!' observation.

HTH - Stefek

it does however raise the worrying question as to what would have happened
to any insulation in the immediate vicinity of this self-righting
extraneous-metal-bit-vapourising fault, though.

--
Richard Sampson

mail me at
richard at olifant d-ot co do-t uk

Theres nowt wrong with top posting or interspersed posting. Top is
partcilarly good for a quick comment reply.

The insistence on bottom posting is a convention from days now gone.

Its side Ellis Greensitt wrote:
posting
you've Look ellis this is how it works
got to
watch for __________________________________________
!

[Post you are replying to with irrelevant bits snipped]

In article . com,
writes:
Theres nowt wrong with top posting or interspersed posting. Top is
partcilarly good for a quick comment reply.

Actually top posting, bottom posting, no trimming of quoted
text, broken or overlarge signature block, etc remain as useful
as they always have -- for quickly identifying the postings that
aren't worth bothering to read when you're strapped for time.

Those who write good concise informative responses tend to be
those who know how to communicate effectively and understand the
purpose of the quoted text, and thus provide their responses
interleaved in appropriately trimmed quoted text. The ability to
identify such responses with just a fraction of a second glance
is extremely valuable when you want to only read through the
best responses.

The insistence on bottom posting is a convention from days now gone.

There never was one. Top posters who didn't understand why they
shouldn't top post often incorrectly assumed they were being
asked to bottom post instead.

--
Andrew Gabriel