bulbs blowing - voltage on 2way switch wires

[a-z electrics]

Hi All,
Wonder if anyone could throw some light on this one (forgive the pun)-

Was called out to sort problem with landing light which blows the bulb when switched on.. mcbs not tripping and lighting is not on an rcd protection.
visited and looked at switches, both 2-way 2 gang coupled with the hall switches (which work fine) - nothing obvious there altho I noticed each wire had a voltage indication using a probe. Also no earthing at the switches - it is an old house with some of the cabling in rubber sheath.
Looked at the ceiling rose - feed in, another loop out both in pvc and the switch wire in the rubber. closer inspection reveraled the wire insulation on the switch wires had gone brittle and cracked off, with bare wires for about 50mm. aha, problem found thinks I and promptly repaired the wiring and fitted a new rose and 2-way landing switch for good measure - however, I was still getting a "live" indication on the switch wires; upon measuring it was 22 volts to earth with no discernable difference with switching .
Client was quite happy for me to disconnect the 2 way wiring and just use the landing switch for the landing light which all works well..
Question 1 - why was bulb blowing? my understanding is that overvoltage will blow the bulb, but 230 + 22v would this be a possibility?
Question 2 - where was the 22 v coming from on the 2-way wiring, both ends of each wire was disconnected!!
The Zs measured 18ohms at the rose but couldnt determine what earthing type it was as the earth wire came up separately from under the floor, presume either tns or tt. also the whole lighting seemed to be on the one circuit, even tho there were 2 other 6A mcbs.

 

[bigbaddave]

With the age of the cable id be confirming IR values before doing anything.. Id then check for external earth before doing ZS. ZS could be a fault internally on the earth which if it it not connected externally wont show.

What was IR?

22V on two way sounds like induced induction by the way..

 

[Thefailsafe2012]

Just as an FYI, I have read various other posts that have said digital meters can give out duff readings so the 22v might be false, sorry mate can't offer you any more than that

 

[i=p/u]

A leaky breaker, 22v could be from a battery somewhere

 

[shocking_eg]

some light fittings can hold a charge for a while remember

 

[guest119]

Couple of suggestions.
Have you got a good main incoming earth and lost it for that circuit and bringing in 22v from house through the wall, semi detached?(on a different phase)
230 volts will increase with a slack connection across an inductive load, bulbs ( errrrrr, lamps :wink5:. Bulbs fir t gaden)
Bell transformer in the circuit?

Boydy

 

[a-z electrics]

some light fittings can hold a charge for a while remember

What could be blowing the bulb tho?

 

[shocking_eg]

Do some IR tests mate

Reckon the cable might have a fault on it

 

[a-z electrics]

Couple of suggestions.
Have you got a good main incoming earth and lost it for that circuit and bringing in 22v from house through the wall, semi detached?(on a different phase)
230 volts will increase with a slack connection across an inductive load, bulbs ( errrrrr, lamps :wink5:. Bulbs fir t gaden)
Bell transformer in the circuit?

Could have been a bell in the circuit I suppose but I was getting this reading with both ends of the 2way switch wire disconnected, between hall and landing. isnt a lamp a resistive load?

 

[guest119]

Did you measure the voltage at the rose lamp connections with the switch on and lamp removed, or blown? 230?

18 ohms is high for a domestic lighting circuit zs.(way above the bs60898 max zs for a 6a mcb)

Can you confirn neutral is good? Long lead from db neutral bar and check continuity from this to rose neutral.

Reason for checking neutral is that the appearing earth wire could be coming from a joint box which is wet and passing voltage to it, especially considering the recent weather.

If you have 22v coming from the earth wire to a poor neutral neutral it could find its way to the switchwire via the lamp filaments.

 

[Sintra]

I would go for an induced voltage. Is the 2 way wired in the conventional or conversion method? Is it a TN or TT system?

 

[markc123]

Most unhelpful reply of the day is about to be awarded to me...

but aren't bulbs what you bury in the ground? :smug2:

 

[Sintra]

Most unhelpful reply of the day is about to be awarding to me...

but aren't bulbs what you bury in the ground? :smug2:

Where did you get that info mate, from post #6 by any chance

 

[markc123]

awww bloomin hell boydy!!!

I just saw the title of the thread and didn't bother reading the whole of it. Now i look a complete (insert own word of choice here)

- - - Updated - - -

Lession learnt Sintra, will read carefully before trying to act clever in future

 

[a-z electrics]

ok, ok.. lamps!
2-way wiring, not sure which one you would call it but theres 3 wires between the 2 switches. the interesting thing is the landing end is rubber sheath, the hall end is pvc, so must be connected somewhere..
also didnt check completely the type of earthing (as client didnt want me to pursue the investigation with the 2-way) but it wasnt tncs, possibly tns but also possibly tt as the origin of the earth cable wasnt accessible, forgot to check for stake outside by the end..
is an induced voltage of 22v normal in these situations? sorry but relatively new at this..

 

[a-z electrics]

As I say, the measured 22v was between the ends of 2 of the disconnected 2 way switch wires and earth (ceiling rose earth, which was giving 230v to phase, 0v to neutral as one would expect) - in other words measured at the ends of (in theory) a piece of wire!!
altho as I also alluded to, there must be some sort of connection between the 2 ends as the landing end is rubber sheath and the hall end pvc, so altho not impossible, there should be no need for earths or neutrals around the sw wires at all?
Theres no earth wires at all in the switch boxes.
I did strongly recommend a rewire due to the age of the lighting wiring which the customer agreed was probably a good idea.. I left it with her..

 

[Rockingit]

22v will almost certainly be induced voltage - have seen it as high as 50v and able to trigger an energy saver! Caused by the run of the strappers in close proximity to themselves and others. Fit a snubber to get rid of it.

As for what is making the lamp blow - either a really naff connection somewhere that's causing arcing (probably on the neutral) or cheap lamp to start with! Lamps will operate happily in excess of 245v if all the other boxes are ticked.

 

[a-z electrics]

22v will almost certainly be induced voltage - have seen it as high as 50v and able to trigger an energy saver! Caused by the run of the strappers in close proximity to themselves and others. Fit a snubber to get rid of it.

As for what is making the lamp blow - either a really naff connection somewhere that's causing arcing (probably on the neutral) or cheap lamp to start with! Lamps will operate happily in excess of 245v if all the other boxes are ticked.

50v! that is surprising - had no idea this could be so high..
I've come across the term "snubber" before but could you elucidate please? (showing my youthful ignorance now..)
many thanks for your useful input.

 

[Rockingit]

50v! that is surprising - had no idea this could be so high..
I've come across the term "snubber" before but could you elucidate please? (showing my youthful ignorance now..)
many thanks for your useful input.

Easiest to take a look here Snubber - Wikipedia, the free encyclopedia

 

[bigbaddave]

With the age of the cable id be confirming IR values before doing anything.. Id then check for external earth before doing ZS. ZS could be a fault internally on the earth which if it it not connected externally wont show.

What was IR?

22V on two way sounds like induced induction by the way..

Cough Cough..#2

 

[Silly Sausage]

22V on two way sounds like induced induction by the way..

or even capacitive coupling...

 

[guest119]

or even capacitive coupling...

Do you mean a flux capacitor?

Boydy

 

[Silly Sausage]

Do you mean a flux capacitor?

Boydy

Only when I'm on-board the USS Enterprise.

 

[guest119]

Aye, this forum is full of space kadets.:tongue3:

Boydy

 

[Rob Smith 643]

Have had same/similar event today. While doing pre-install tests on a lighting circuit picked up 50v between cpc and switched live...at two-way switching. Bungalow with almost pointless two-way switching. 2gang 2way at each end of short corridor/passageway operating luminaires about 3 to 4 metres apart. Hoping snubbers wii sort this out.