finding correct wiring

[heating tech]

please can someone please tell me how to find a neutral wire please,so that if an incorrect wire has ever been used in a house i will be able to indentify by carrying out an accurate test to find the neutral by means of a multimeter etc.



i am a acs and oftec heating engineer.

thanks in advance

gary
______________________
moderator

moved from plumbing

 

[Spark123]

You can't just find a neutral wire and use it. You must find the correct neutral wire for the particular circuit. (Voice of experience, shared neutrals hurt )
For which application are you trying to find a neutral?

 

[ChrisR]

Scuse me, I'm a mere water wallah too, but he says he want to make sure he's found a neutral, not pinch it for something else.

In my amateur way I know what I'd do, but I might well be wrong for some installations so I'll shut up!

 

[heating tech]

there have too many instances where people use an earth wire as a switched live,i learnt along time ago never to assume that the colour of the cable is the correct one.

when i arrive on some sites the room stat has been removed and there are 4 wires sticking out of the wall,live neutral switched live and earth,i know how to find the permanent live no problems,it's the neutral wire i would very much like to learn the correct way of testing .

i would be extremley gratefull and learning something new.

many thanks

gary

 

[Spark123]

Sorry if I came accross a bit abrupt!! I have had a couple of shocks where a cables neutral tests dead until it is disconnected, and then all of a sudden 240v, ouch!!!

 

[heating tech]

not at all it sounded as though you had been on the reciving end where some cluts has wired things up incorrectly.

i'm just trying to still learn after 23 years in the trade,i still enjoy learning as i have always thought i cannot fix something unless i know how it works and how to test it.

many thanks

gary

 

[andy]

the neutral should be at the same potential as earth, and usually there will be a low resistance between neutral and earth. DO NOT TEST RESISTANCE BETWEEN LIVE AND EARTH. make sure voltage is 0 first

 

[ChrisR]

Ok I'll own up. I'd switch everything off and connect a phone teckie's warbler thing in the consumer unit and prod about on the mystery cables. Sometimes I wish it worked on pipes.

 

[Paul Barker]

Firstly at the rstat position, set meter at ac and above 250v place probe between phase and each other lead. the one with no voltage is switched live the other two should show the mains voltage, on of them is neutral one is cpc. You should only get a few volts between assumed neutral and assumed cpc. You chould get no voltage between assumed switched live and all other leeds.

To confirm find out which mcb it comes from, trun off via the mains switch that side or all of the consumer unit. Remove the phase and neutral from the neutral busbar and mcb, twist your phase and assumed neutral together at the rstat end, low ohms test at the consumer unit should confirm. Double check the cpc in the same way. Untwist leeds at rstat before reconnecting at consumer unit.

As you say no end of times the green and yellow wire is sleeved with a colour and used as something else, the sleeve could come off.

 

[heating tech]

thats really great thanks so much for taking the time to post a reply,it will be fun again when the colours change on the wiring.

but with your way connecting the neutral and live together and testing across them at the room stat.

i think nearly 8 times out of 10 someone has just installed 3 core instead of 4 core allowing the correct colour coding to be used

thanks again very kind of you all

all the best

gary

 

[andy]

,it will be fun again when the colours change on the wiring.

they have already changed

 

[Spark123]

Going down the same lines as Andrew, it may be better using neutral and earth as the live could be switched through the timer to the thermostat, hence 240v live is only there when the timer (and supply) are on;
Find the supply for the central heating and isolate.
Disconnect L,N,E wires for the central heating in fused spur / consumer unit.
Temp connect the central heating N & E wires together (Don't connect the N-E together on the supply side though).
Test between earth the other wires at the thermostat for a low resistance to find the neutral.
Hope this makes sense,