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I have a 5-wire cable as used for 3-phase (i.e. stripes/blue/brown/black/grey) coming to a hole where a thermostat was - only the earth, live (brown), and switched live (black) were used by this battery-powered thermostat.

Assuming that the 5-wire cable had been used simply to avail the black for use as a switched live, I thought that it would otherwise be like the 3-wire cables to a standard plug, and I could use it as such.

Confusingly, I measured 115VAC between live and neutral (expecting double, I'm in the UK). The same between live and earth; live and grey.

No matter, I thought, I can use a 100-240VAC -> 12VDC converter. Connected it, and then measured mere mV across its output. Strange - so I measured its input; found 7VAC. Repeatably.

I don't understand this - how or why is the voltage falling so drastically under (0.25mA) load, preventing the transformer functioning?

Do I have any alternatives? I don't understand why these lines are at 115VAC anyway.

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    Obviously, your assumption was wrong. You are going to have to find the other end of that cable and determine exactly how it's wired. But this is off-topic here -- migrating to DIY. – Dave Tweed Jan 14 '16 at 1:02
  • It's probably induced voltage, at extremely low current. The wire probably runs near some other wire and picks up voltage from it. But you will have to find the other end and check. – Ariel Jan 14 '16 at 1:14
  • @Ariel So it may be disconnected at the other end, and just picking up the 115VAC from the adjacent 240VAC line? Bah, I didn't know this could happen, thank you. The switched live obviously goes to the heating pump and actuator, so hopefully the blue line is similarly accessible so that I can wire it to a nearby plug. – OJFord Jan 14 '16 at 1:50
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    This question refers to the UK where 24V systems are very uncommon in domestic settings. 5 conductor cables with mains voltage for heating systems are common. Thermostats that directly switch mains (230VAC) are common - some of these are battery powered (not just battery backup). I don't think assumptions in other comments are relevant to this question. – David Jan 14 '16 at 8:27
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    @Ariel The only job of this thermostat was to connect, or not, the (always) live and switched live lines. It's just operating a relay, no low voltage sent anywhere. – OJFord Jan 14 '16 at 10:24
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Beware Ghost voltages

Typical UK central heating is wired like this (Honeywell Sundial S-Plan)

enter image description here

It is not immediately obvious but essentially, the 240 live supply passes through a series of switches in this order:

  • Mains supply (there is often a wall switch plate for heating). Provides power to ...
  • Timer (AKA controller - ST9400A/C in diagram). Provides power to ...
  • Room thermostat. Provides power to ...
  • Zone valve (when motorized valve is fully open it closes a switch). Provides power to ...
  • Pump and Boiler

Unlike the arrangements common in the US, all the above equipment operates at 240VAC.

Note that, so the switches in the timer and therostats don't have to switch the high currents needed by the pump, there is a separate (grey wires) parallel live feed to the mechanical relays in the zone valves. This doesn't change the fact that, logically, the units in the diagram operate in series.

Some commercial installations and some more recent domestic installations may have low-voltage to thermostats and may have intelligent thermostats that incorporate the timing functions. This doesn't seem to be the case in your installation.

As shown above, for installation convenience the equipment is wired radially and all connections are made in a central wiring box (often a backbox of the timer). So physically star-wired but logically in series (with parallel branches for hot water and heating).

Normally the thermostat needs 240 VAC neutral, 240VAC live (from timer) and switched 240V AC live (to zone valve). Some traditional-type thermostats include a heating anticipator (a resistor across switched-live & neutral that warms the thermostat).

UK wiring insulation colours are

            old           current

neutral     black         blue
live        red           brown
earth       green         yellow/green

Earth wires are sometimes bare copper (as in "twin & earth" single-core cabling inside walls/floor voids etc) but inside a junction box, back box or patress should be covered by the electrician in a yellow/green sleeve at time of installation.

If your thermostat has a ground connection but no neutral connection, it is wrongly wired and you should consult an electrician if you are not confident working with lethal voltages.

If the timer (controller) is set to "heating: always on" you should measure 220-240 volts on live at thermostat. Some multimeters have a low-impedance setting to avoid "ghost-voltage" indications. An alternative might be to temporarily wire a 60W incandescent light bulb across live and neutral, using a connector block, then measure voltage when the bulb is on (fully).

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I don't understand this - how or why is the voltage falling so drastically under (0.25mA) load, preventing the transformer functioning?

I don't understand why these lines are at 115VAC anyway. Generally when you see this sort of thing in an AC system it means that the line is floating.

Lines running in close proximity have some stray capacitance between them. Digital multimeters nearly always have a 10 megohm input impedance so in an AC system it doesn't take much to make a voltage appear on them.

As soon as you actually put a load on the line the voltage drops away because the capactive coupling is very weak. Why it only dropped to 6V and not 0V I don't know for sure but I would speculate that the power supply you used has some kind of undervoltage cutout.

Do I have any alternatives?

You need to find the other end of that cable.

What I expect you will find is that one wire is an incoming switched live from a timer. One is an outgoing switched live to a motorised valve and two wires are not connected at all. You should be able to connect those two not-connected wires to a power source so you can use them to power your equipment.

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