Installing new mains Smoke Detector - Earth test

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Hi,

I'm replacing 3 smoke alarms in my property, mains ones. The documentation for the new alarm says the I might have up to 4 cables coming into the alarm housing: Live, Neutral, Earth and Link. It says the 'Link' cable if installed is normally black.

I only have three cables going into the existing detector, brown (assuming live), blue (assuming neutral). The third is white! Logic would dictate that this is the Link cable, as you'd expect any earth connection to be properly marked and the old detector was all plastic so probably didn't need earthing.

If I disconnect the white cables from all three alarms, is there a quick multimeter test I can do to see if the white cables are connected to Earth or not?

Thanks!
 
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There's no point. You do not have enough conductors at your alarms, so they must be rewired.
 
Enough for what?! The new alarm does not require an earth - so provided the the white cable is in fact the link cable, it's a perfect fit for the new alarm.
 
Brown blue and white is not a standard colour combination in UK cable.
 
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Enough for what?! The new alarm does not require an earth - so provided the the white cable is in fact the link cable, it's a perfect fit for the new alarm.
If a fixed cable has mains voltage on one or more conductors then in almost every case it should have a protective Earth conductor in the cable. At some time in the future a new detector may require an Earth connection
 
Enough for what?! The new alarm does not require an earth - so provided the the white cable is in fact the link cable, it's a perfect fit for the new alarm.

RF is right, you do not have enough conductors. You need a Live wire ,a Neutral wire, a Interlink wire and Earth/CPC wire.

Standard 3 core + earth cable has the the colours of the cores of Brown, Black, Grey, and bare copper for the CPC.
Twin and Earth has the the colours of the cores of Brown, Blue, and bare copper for the cpc.

I only have three cables going into the existing detector, brown (assuming live), blue (assuming neutral). The third is white!

Are you sure the blue wire is not actually a black wire that has been sleeved blue and the white wire is actual a light grey wire. Have you checked that the earth/CPC has not been wrongly cut off or that it is not tucked away in the cable.

Regards: Elliott
 
I have seen this colour combo before (not smoke installations) where the installer has used some alarm cable sheathing for the cpc.

Is it possible the installer has sleeved the cpc this way and used it as the link wire?

Could do with pictures really!
 
Regulation 411.3.1.1 ...a CPC shall be run to and terminated at every point in fixed wiring....
 
Some interconnected (extra) low voltage alarms with a panel and battery backup use 2c+e to connect between alarms and have the (functional) earth sleeved white, it's not inconceivable that a mains voltage alarm could have a similar system.... to the OP - why? and what make/model of alarms are you replacing? were they interconnected before (if one alarm goes off they all do)?

Sounds more likely though that it needs rewired in 3c&e
 
Check the white wire. Is it a white sleeve loosely slipped over a bare copper wire?

If it is a bare copper wire then it would appear you can't fit the new alarms.

If it is an actual insulated wire (seems doubtful as blue, brown, white cable doesn't seem to me recognised cable colours) then you may be ok.

There's enough old lighting circuits out there without an earth that are deemed ok as long as class 2 double insulated or plastic fittings are used. Admittedly it seems your cables may be quite new.

However, if that white wire is in fact an earth wire, which seems likely, then you may be able to fit RADIOLINK alarms.
 
THE OBVIOUS THING TO CONSIDER IS; HOW WERE THEY WIRED BEFORE?

This may give a clue.
 
Check the white wire. Is it a white sleeve loosely slipped over a bare copper wire?.
Sounds quite likely given the color combination brown/blue/white. So far as I'm aware, the only time any standard U.K. T&E-style cable has contained a white core was with the red/white/blue used prior to circa 1965.

A picture would help.
 
Some smoke alarms were/are supplied with a socket connected to 3 wires, which were blue, brown and white. The idea being that the alarm has pins on the back which connect to the socket, and the wires would be connected to fixed wiring within the ceiling.

Other than that, not clear how such a wire colour combination could be obtained.
 
Some smoke alarms were/are supplied with a socket connected to 3 wires, which were blue, brown and white. The idea being that the alarm has pins on the back which connect to the socket, and the wires would be connected to fixed wiring within the ceiling.
Good point - I was thinking about the fixed cabling, but perhaps that is what is being seen here.
 
Some smoke alarms were/are supplied with a socket connected to 3 wires, which were blue, brown and white. The idea being that the alarm has pins on the back which connect to the socket, and the wires would be connected to fixed wiring within the ceiling.

Other than that, not clear how such a wire colour combination could be obtained.
It would appear you are correct! There were no pins, but I pulled the blue/brown/white cable out even further and look what followed. 2c+e coming in, and 3c+e going out with the extra conductor acting as the link. Seem like its good to go for a straight replacement.
 

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