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
There is no earth connection. Using a multimeter, what is the simplest
and safest way of identifying each wire?
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.
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.
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?
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.
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.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.