2 way switch fault

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Good evening everyone,

I had a call to go and sort out relatives lighting problem which I have rectified but trying to figure out the theory side of things (hope you can help).

Installation details

2 gang dimmer in hallway

1 gang switch on landing

Wired in the old way (well not the way I was taught at college in 2006) switch feed to common from hall light then in singles L1 to L1 and L2 to L2 upto landing light switch.

Then common used as switch live to landing light.

Fault

Relative switched landing light from landing light switch and bulb blew and circuit breaker tripped. This was a couple of weeks ago. However you can still turn that switch on and off and the dimmer with no further tripping until you replace the lamp.

I found live to earth fault (switch wire to backbox on landing) and sorted that.

But cant understand why the fault only happens when a new lamp was installed and turned on.

Everything seems to be ok now.

 

extension15

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Surely where there is a 'potential' for a fault to occur (though not immediately), given time, insulation/arcing etc will occur at some point.

Potential faults can lie dormant for years (before occuring)..

:)

 
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Only suggestion that comes to mind is that the switching is incorrect polarity; i.e. switched neutral not live.

If this is the case, you would not have a live condition on the cable unless a bulb was in circuit.

Strongly suggest doing a polarity check on the whole property. Dangerous situation to have m8.

 

extension15

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Potential faults can lie dormant for years (before occuring)..

:)
A property I visited last year (PIR), had a new carpet fitted some 7 years ago..

4 years after fitting, that room went up in flames, the carpet fitters had nailed a cable, and yes it took that long to eventually burn.

(It was April 1st, by the way (when it caught fire)

; \

 
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You need to explain this further. Is there one fitting with more than one light bulb, or more than one fitting ?

Does this light work now with out you turning the circuit breaker on ?

One of the most common found faults like this is when the light bulb is being changed, the holder has twisted round and eventually (sometimes after many years) the wire snaps . Sometimes shorting the live to earth, sometimes a bit stuck in the lampholder shorts to earth. This only shows up once a good light bulb is inserted to make the return short.

 
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One side point to this issue......

nothing to do with the fault...

But....

Perm live from downstairs...

strappers to upstairs....

switched live upstairs to bulb...

Wheres the neutral come from???

Upstairs or Downstairs?

Borrowed neutral maybe here me thinks?? ; \ :eek:

 

extension15

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You need to explain this further. Is there one fitting with more than one light bulb, or more than one fitting ?Does this light work now with out you turning the circuit breaker on ?

One of the most common found faults like this is when the light bulb is being changed, the holder has twisted round and eventually (sometimes after many years) the wire snaps . Sometimes shorting the live to earth, sometimes a bit stuck in the lampholder shorts to earth. This only shows up once a good light bulb is inserted to make the return short.
B)

 
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Do your relatives do any DIY at all?

is it possible someone had a quick look in the light switch before you got to site??

Was the switch wire to back-box fault a squashed wire or bare conductor exposed through nick in insulation / cut back too short??

possible squashed wire scenario

e.g.

bulb blows...

put in new bulb... (cheepo bulb, new bulb faulty)

still not working...

opens up switch to look for proble...

put switch back & trap or squash wire!

?????

Just another possible angle to consider m8? ?:| ? :| ?:| ? :|

 
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Thanks for all the replies:)

I Will need to go back then and check for switched or borrowed neutral.

as this is an ex council property everything was surface mount and my relative has changed everything to flush.

Problem with landing switch was another T&E cable ran up behind backbox so he couldnt chop out enough to recess the new steel backbox.

This causing K/o box to protrude 3mm from wall, reducing clearance behind switch and switch wire pinched.

Also he didnt line up box correctly with cable run so he cut new holes in back box leaving sharp edges. (wished I 'd took a picture to show you).

Thanks everyone I will let you know outcome when I get chance to return.

 

Admin1

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Also he didnt line up box correctly with cable run so he cut new holes in back box leaving sharp edges. (wished I 'd took a picture to show you).

It's not too late, If you are going back. ]:)

 

steptoe

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Only suggestion that comes to mind is that the switching is incorrect polarity; i.e. switched neutral not live.If this is the case, you would not have a live condition on the cable unless a bulb was in circuit.

Strongly suggest doing a polarity check on the whole property. Dangerous situation to have m8.
strong possibility, ex council and all that.

also the diy bit, changed fittings and misplaced sw wire, are the sw wires in single red or plain old black?

 

steptoe

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Flippin` `eck steppie. Try coming in like yourself next time; you scared the heck outta me then!Just leavin`. Not that you`re scrap pongs or anything, but...............................
Im on the softly softly approach these days,

apparently i was scaring the little kids away,,,, :| :| :|

 
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The switches are wired in red singles Steptoe.

Basically 2 gang dimmer in hall and perm live for hall light is linked from common to common for landing light, then 2 red singles taken to landing switch then another single red for switch wire to landing light.

Hope this makes sense!

cheers BCL

 

steptoe

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The switches are wired in red singles Steptoe.Basically 2 gang dimmer in hall and perm live for hall light is linked from common to common for landing light, then 2 red singles taken to landing switch then another single red for switch wire to landing light.

Hope this makes sense!

cheers BCL
yep, deffo sounds like you could be picking a N from upstairs, as has been said, then when good bulb/lamp fitted a backfeed occurs through the lamp and down the switched live to the upstairs light sw.

could be a bad/poor connection somewhere on the upstairs N back to the CU

 
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