T40 Voltages

31 Mar 2017
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United Kingdom
Hi all
I am a layman so sorry if these are silly questions
I want to replace a Honeywell T40 stat with a Honeywell DT90E
The T40 has three wires connected
terminals 1 2 3
My meter is showing 243v between 1 & 2
Between 2 & 3 I am getting 2v

Q1 Is this OK?
I was expecting 0v between 2 & 3

Q2 Is my multimeter wrong?

I understand the DT90E uses the live and Switched live 1 & 3
Q3 Why then does the diagram on the backplate show the wires as L & N
That is confusing to me
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The T40 requires a Neutral supply as well as Live. The Neutral supply is for the accelerater heater which makes the mechanical thermostat more accurate

The DT90 does not need Neutral as it uses batteries to power the electronic temperature sensor. You will need to terminate the redundant Neutral in a terminal block ( which you supply ) so that it cannot make contact with anything in the DT90
Odd low voltages are normally as a result of capacitive pickup!

They are from a very high impedance source so would not actually power anything.

Although sometimes an LED bulb will glow slightly from the same source.
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The voltage you see indeed is 'picked up' from the cables nearby
If you use a low impedance meter, you will not see any voltage
Digital meter has high input impedance, thus it does not 'load' the circuit, hence you see these spurious voltages

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