Am I right in thinking an RCBO protected circuit can't switch-live a different circuit?

[cjard]

once upon a time, my heating system was to be like other underfloor systems - a bunch of thermostats connnected to a wiring centre, underfloor pump connected to wiring centre, air source heat pump also connected. Wiring centre runs off a ring final. Stat calls for heat, wiring centre sends power to underfloor pump, and closes a switch on wires X and Y to the ashp' control board to get it to fire up

I'm no longer really interested in using the stats, as the house works really well just having the command unit for the ashp manage the temperature- it has its own stat, a very precise one, and timer etc so it was wired in temporarily until I could get the ridiculously expensive Heatmiser stats going. To get around the problem of the pumps for the underfloor manifolds, they're just wired up to run continuously during the heating season

However, this particular air source heat pump can control an external pump, when producing central heating water it will make live another one of the contacts on its control board. It's not intended for running a pump, as it lacks the power to do so I presume its intended for switching. I don't think it's possible to wire it to a switch-live connector as though the heat pump were a stat, because them conceptually power will be leaving the ashp' dedicated rcbo but not returning, causing a trip so it needs to be wired up to close a contactor (there's a neutral available drom the ashp too), and the contactor links a spare thermostat live terminal to simulate a thermostat calling for heat/activating the manifold pump

Is this assumption correct?

 

[Lectrician]

Yes. Not a contactor though, a relay. IMO, I would prefer to see it wired correctly though.

 

[Adam_151]

I'm not following all of what you wrote, but I think the answer to the question asked is "Yes, the assumption is correct, but not just because the circuits are on RCBOs"

You must never have a load connected to the live from one circuit and the neutral from another, thats termed a borrowed neutral and is against BS7671, and could cause a shock to someone who is working on the circuit providing the neutral and is under the impression its isolated!

 

[cjard]

Yes. Not a contactor though, a relay. IMO, I would prefer to see it wired correctly though.

I Was under the impression that contactor and relay were synonymous. Suffice to say, using one of these was the overall idea:

https://www.screwfix.com/p/british-...gclsrc=aw.ds&dclid=CLHj_OCJt9cCFY-G7QodkYEE1w

On a din rail, in an enclosure. The heat pump would energise this and it would close, connecting the live and switch live of the thermostat port on the wiring centre, activating the manifold pump. If this isn't "wired properly" please advise?

@Adam_151 re the attached- UFH systems are typically wired as per bottom diagram - stat calls to wiring centre for heat, wiring centre energises UFH pump and calls to boiler for heat. I want to use the top diagram- boiler outside, command unit inside, with proprietary serial comms control- command unit calls for heat, boiler fires up, and then boiler energises terminal #23 of its control board (auxiliary pump). I want to use this plus neutral from terminal 22 of boiler, to close a relay, which the wiring centre will think is one of its stats, it will energise the pump. There's nothing connected to the "call for heat from boiler" contacts of the wiring centre, as the call for heat comes from elsewhere(can't tap into that call; it's rs232)