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Nest Thermostat E trips RCD

Discussion in 'Electrics UK' started by Serialforeigner, 5 May 2021.

  1. Serialforeigner

    Serialforeigner

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    I installed the Nest Thermostat E, all went well until I turned it on. As soon as the power goes on, it trips the RCD unless the boiler breaker is off.

    I have an all electric heating setup, the Nest only controls heating, not hot water. The Heatlink replaced a Honeywell 6360 thermostat which was wired as shown below. I connected the wire in the No. 1 terminal to the C terminal on the Heatlink, the wire in the No. 3 terminal to the NO Heatlink terminal and capped the wire in the No. 2 terminal - it's not connected to anything on the Heatlink. This was based on the guidance in the Nest app as I installed it, which also agreed with the Nest support site.

    [​IMG]

    There was an earth wire in the wall behind it which wasn't attached to anything so I capped that too just to be safe. My understanding is that it's just a switch between the C and NO poles, so I don't understand why the breaker trips.
     
    Last edited: 6 May 2021
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  3. Serialforeigner

    Serialforeigner

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    Ok, there was a photo here of the Honeywell wiring. I'm not sure how useful the question is without it. Apparently a moderator removed it, so what do I need to do to replace it? What rule do I need to be aware of in order to include that image?
     
  4. JohnW2

    JohnW2

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    Welcome to the forum.

    If it really were just a photo 'of wiring', then no 'rule' of which I am aware - countless people post 'photos of wiring' here.

    Are you sure that it wasn't removed for a technical reason (e.g. image was excessively large)?

    Kind Regards, John
     
  5. Serialforeigner

    Serialforeigner

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    It was 3MB, I've reuploaded a smaller version.
     
    Last edited: 6 May 2021
  6. ericmark

    ericmark

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    I really don't understand why Nest E should trip out a RCD there is no earth connected to Nest E and no neutral, 2 one with blue sleeve I have checked and should be neutral to the resistor and you say not connected, so there is nothing you say you have done which should trip a RCD.

    Is the RCD part of a RCBO or have you separate RCD to MCB? Wondering if definably RCD or could there be an overload?

    I wonder if the neutral could be connected to earth? If a neutral is connected to earth while no current is being drawn neutral and earth are same voltage so no current flow and no trip, but as soon as load goes on circuit neutral goes slightly towards line voltage so now current flows causing an imbalance and the RCD trips.

    So I would be checking you have not allowed neutral and earth to join together.
     
  7. CBW

    CBW

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    Any photo of nest wiring?
     
  8. Serialforeigner

    Serialforeigner

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    This is the Nest Heatlink wiring.
    [​IMG]
    Behind that, the blue sleeved wire in the initial image is capped, and the random earth wire is also capped (separately).

    I have a fusebox which has a main breaker and an RCD, then the usual array of individual breakers for each circuit.
    [​IMG]

    I don't understand how the earth and neutral could be joined, there is nowhere that they share any kind of connection.
     
    Last edited: 6 May 2021
  9. ericmark

    ericmark

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    Nest wiring has 5 connections, two are for OpenTherm and not used, neither is bottom terminal so this upload_2021-5-6_10-8-43.png
    is all we have connected. And is what it should look like.
     
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  11. Serialforeigner

    Serialforeigner

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    That's what mine looks like...
     
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  12. ericmark

    ericmark

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    I have a feeling the RCD tripping is not connected with the thermostat change, I was posting same time as you, but having all circuits on one RCD is asking for problems, each circuit is permitted to leak up to 2.5 mA, and a 30 mA trip should trip between 15 and 30 mA and 8 x 2.5 = 20 mA so the RCD can trip with no fault, and from bitter experience with last house they do.

    The problem is the MCB does not isolate neutral, so although in the main turning all other MCB's off should show if fault is the boiler circuit, it still could be a neutral earth fault on any circuit.

    Did you try returning to old thermostat? I think if you return to old one it will still trip?
     
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  13. Serialforeigner

    Serialforeigner

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    Ericmark, you are correct. I took the thermostat off, capped all the wires and it still trips. Replaced the old Honeywell and it trips as well. As soon as the boiler circuit is engaged, the RCD pops. The good news is that it's now my landlord's problem. The bad news is that I have no heating until they get hold of someone.
     
  14. ericmark

    ericmark

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    I don't know where you live, but here in Mid Wales had a lot of rain with wind last few days, could water have been blown into flue, or could it be some think else with water in like outside light.
     
  15. Serialforeigner

    Serialforeigner

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    The electrician came out and discovered that the motorised valve is leaking which is causing a short. It was replaced last December when I moved in here because the heating wasn't working and the original one had failed. Is it likely that connecting the Heatlink caused that fault somehow?

    It seems weird that it was working just fine until I wired up the Heatlink.
     
  16. CBW

    CBW

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    Unlikely.

    Seems to be the leaking zone valve.
     
  17. Serialforeigner

    Serialforeigner

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    Right, I meant is it likely that the heatlink could have created a condition that led to the motorised valve leaking?
     
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