Room thermostat wiring

I have to fit a room thermostat into a building where there is already an old three-wire connection for a (mechanical) thermostat, but no existing thermostat. So it is not immediately obvious which wire is which.
There is no earth connection. Using a multimeter, what is the simplest and safest way of identifying each wire?
Max
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With all the electricy off at the MCBs, but on at the main switch, find which wire is less than a couple of ohms to neutral in any handy socket. (Possibly one of the others will only be few tens or hundreds of ohms if there is something inductive connected, but ignore this.) That wire is probably neutral. With the electricity on check which wire is live. That is the live wire. The third wire is switched live.
Alternatively, look at the other end and note the colour code.
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Roger Hayter

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On Sunday, 15 October 2017 09:44:49 UTC+1, Maxwell Boltzmann wrote:

Live is easy to identify, it's 240v to the nearest earth. Neutral & switched live: with power off N has near zero resistance to neutral, swL will have much more R.
NT
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snipped-for-privacy@gmail.com posted

Thanks to you and Roger. One of the problems is that there *is* no handy socket nearby to which I can measure the voltage/resistance to earth and live; but no doubt I can get round that.
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Max
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posted

Yep, just run an extension cord to any socket you like.
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Neutral, not earth for the resistance measurement. They may not be sufficiently similar if you don't have the right sort of house earthing.
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Roger Hayter

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On 15/10/2017 14:25, Maxwell Boltzmann wrote:

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Cheers,

John.
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On Sunday, 15 October 2017 14:26:15 UTC+1, Maxwell Boltzmann wrote:

socket, metal light fitting, radiator, water tap or drain, most of those ar e earthed, but not all. The other option to identify live is to measure the 3 voltages between the 3 wires, live to N and live to swL will both give 2 40v, N to swL should give 0v. (Y plans do complicate this.)
Obviously safe practice is required.
NT
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Live to swL can give you Ov either because it is open circuit or because (as you say) some other control has connected ti to live. If N to L and N to swL are both 240v then further measures are necessary. Probably, as you say, something to do with motorised valves and their switches.
If N to swL is Ov, which is which, if live to either gives 240V? I think your scheme only works if you know exactly what is connected at the other end and even then ambiguities are left.
Another possibility if you use a high impedance meter and swL is open circuit is that N to swL will give you 5 or 10V AC. What do you conclude from this?
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Roger Hayter

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On Monday, 16 October 2017 00:10:25 UTC+1, Roger Hayter wrote:

if so the system is broken

is it too hard to switch the programmer to off?

but they aren't

I'm pretty sure I addressed that question

3 readings are required to reach any conclusions, not 1. Enjoy trolling.
NT
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On 15/10/2017 09:40, Maxwell Boltzmann wrote:

Finding the other end of the cable and finding out what is connected to what.
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Michael Chare

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On 15/10/2017 23:49, Michael Chare wrote:

That's called stating the bleeding obvious:-).
Although I would want to know why there is no earth connection.
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ARW wrote:

It's been 'promoted' to switched live for maximum surprise?
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On 16/10/2017 22:02, Andy Burns wrote:

vbg
The further question also needs to be asked.
Does the boiler actually work without the missing roomstat?
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Aren't most thermostats made of plastic with no accessible metal parts? The one I'm proposing to install certainly is.
Incidentally, the wiring diagram printed inside the new thermostat doesn't show a switched live connection but a switched neutral.
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Jack

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There are advantages in having an earth wire available and I believe it is considered good practice to install one.

Any chance of hosting a scan somewhere? That seems a bit surprising for something on sale in the UK.
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Roger Hayter

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No doubt, but lots of appliances are supplied without an earth. I was rather horrified the other day to find that my 3kW convection heater (the kind you mount on the wall) had no earth wire, even though it's made entirely of metal. It's one of the leading brands.

It's a Honeywell and the wiring diagram essentially the same as the one ARW has just posted, but without the earth. It has an unused pin 5 so I guess one could that for the earth.
I'm not sure what ARW means by me misunderstanding the diagram. The connection to pin 3 is clearly a switched neutral (switched through the load). There is no switched live feed to the thermostat.
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Jack

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Maxwell Boltzmann wrote:

I'd say he's right :-P

No, the neutral is permanently connected to the heating system (and the anticipator) with the permanent live Pin1 switched to Pin2 when heating is on.
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OK, then. In the diagram, the wires coming out of the wall are represented by the coloured lines. Which is the switched live?

Which is switched.

You don't say.
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Max

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Maxwell Boltzmann wrote:

In the diagram Adam supplied - the yellow wire is switched live.
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