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Could I safely replace an MCB in our consumer unit?

Discussion in 'Electrics UK' started by Peter A, 10 Nov 2020.

  1. Peter A

    Peter A

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    We recently had the electric heating in our flat replaced, and it seem the fitter forgot to replace the MCB in our consumer unit for the circuit the heating runs off. It is currently a "Type-B" MCB, but a "Type-C" is recommended by the heating manufacturer.

    We're having trouble getting the fitter back to sort this out. I'm fairly comfortable with general electrical work around the house (replacing a mains socket, etc.) but I've never replaced an MCB. We have a Crabtree Starbreaker consumer unit, and looking into how to replace the MCB, it seems fairly straight forward, but I don't want to do any work that I shouldn't be doing myself.

    So, two questions for you all:

    1 - Is replacing an MCB something a DIYer could reasonably do themselves (both with regards to legality, and competence)? Or is replacing an MCB something only a registered electrician should do?

    2 - Assuming I replace the Type-B MCB with a Type-C of the same rating (16A), is there any other consideration that should dissuade me from doing so? For instance, a number of mains sockets run on the same circuit, so could a Type-C cause problems for day-to-day household appliances?

    Suffice it to say, if this is not something a DIYer should consider doing themselves, I won't.
     
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  3. AndyPRK

    AndyPRK

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    it's only required if the MCB is giving annoying trips.
    Is it? e.g when heater turned on
     
  4. Peter A

    Peter A

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    We've had a couple of intermittent / inconsistent trips, typically around 3-5 minutes after turning on the heating. Manufacturer believes this may coincide with a feature of the heating that switches the configuration of the elements between series and parallel after an initial warm-up period. They suggest the inrush from this switch could be causing the trip, and as the install manual recommends a Type-C (specifically to handle this inrush), they suggest the MCB replacement as an initial trouble-shooting step.
     
  5. flameport

    flameport

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    Changing to a type C requires that the circuit is tested first to ensure that a type C can actually be installed and still disconnect when a fault occurs. Far outside the scope of DIY.

    Type C are used for inductive loads which have a very high starting current, such as motors, large transformers and similar.
    Heating elements do not require a type C, there is no inrush with a resistive load.
    It's rather more likely that the heater is faulty, or the MCB is the wrong rating for the heater.
     
  6. endecotp

    endecotp

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    Can you post a link to a description of or instructions for the heater?
     
  7. Peter A

    Peter A

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    Hi Flameport. Thanks for the help. We've installed a couple of runs of ThermaSkirt electric skirting board heating. The manufacturer's installation instructions state:

    "The 3 core 230v AC supply into the back box should be at least 2.5mm2 and fed from a Type C RCD or RCBO circuit breaker (BS7671)."

    They've also reiterated this to me on the phone.

    But, if you say that the circuit needs testing for suitability, this obviously is beyond the scope of a DIYer.
     
  8. Peter A

    Peter A

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  9. endecotp

    endecotp

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    This worries me; I feel it should be on its own circuit.

    Is anything much plugged in to those sockets when it trips?

    Yes the instructions do specify a type-C MCB. That surprises me; there is clearly some “intelligence” in its controller and I feel they could have engineered it to soft-start and not require the type C.

    It is still possible that there is a fault, I.e. when it changes over to full power after five minutes it connects power to something that is short circuited.

    What length of units do you have?

    I would perhaps investigate how much current it is taking with a clamp meter, or something.
     
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  11. winston1

    winston1

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    It could also be yet another example of manufacturers instructions being wrong.
     
  12. RF Lighting

    RF Lighting

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    Wow that’s ever so helpful as always. Thanks whineston.

    Peter A are you able to see a rating in watts or amps attached to your heater or in any of the literature supplied with it?
     
  13. endecotp

    endecotp

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    It will depend on the length he has.

    This page: https://www.discreteheat.com/technical-data.aspx (scroll down)
    says: “inrush 1.2 A / m” and various heat outputs between 50 and 180 W / m depending on the model and temperature setting.
     
  14. ericmark

    ericmark

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    It says 16/20/32A TYPE C 30mA RCD/RCBO well you can't get a type C RCD so clearly an error, I think it is clear it should have said a dedicated 16/20/32A curve C MCB plus 30 mA RCD or a 30 mA curve C RCBO.

    But the big question is if some thing damages the heater, will it auto disconnect? For a MCB to disconnect within the prescribed time as a curve C it needs 160 amp to flow, only for maybe 0.01 seconds, but it needs that much to flow to trip the magnetic part of the trip, but since the heater uses resistance cable only the supply to it will cause that amount of current to flow, if the skirting is damaged then only the thermal part of the trip would operate, and that could take some minutes to operate, so the system relies on the RCD tripping if the skirting is damaged.

    Now the problem with RCD's is modern equipment can desensitise them due to DC flowing, as a secondary safety device not really too much of a problem, but with the skirting heater it is the primary safety device, so the RCD needs to only supply the skirting heater to ensure no other load can desensitise it. So either a dedicated RCBO or a RCD FCU so no other load can stop the safety device working.

    This is the problem bit, you can fit a RCD FCU to ensure the skirting heating is protected, but any of those sockets will need an earth loop impedance of 1.36 Ω or less, and the equipment to measure 1.36 Ω is expensive, with a curve B the 2.73 Ω can be measured with a cheap (£35 to £50) plug in tester, but most the first light is 1.9 Ω so your looking at a full blown loop impedance meter which will cost over £100 so it is not really a DIY job because of the cost of the test equipment.

    Had there been no sockets on the circuit then may be you could have done the job as a DIY, although even then you can't test the RCD works.

    So in real terms to do any electrical work on completing it needs inspecting and testing, and clearly most DIY people can't inspect and test as they don't have the test gear, so although you should not DIY the job, the question has to be would it really be that much of a risk considering how cold it is going to get before you can find anyone to do the work. So personally I would say change to a curve C and once you can get the installer back then get him to do the job correct with a dedicated supply to heater. One would hope he can test the loop impedance and ensure it is within limits for the sockets, and ensure there is dedicated RCD protection.

    The big question is have you got 50~65w/m, 85~95w/m or 115~125w/m and depending which is the total 40 meters, 30 meters or 24 meters long, as using the manufactures data only if you exceed those lengths should the in-rush trip a curve B MCB/RCBO under those lengths it should not trip anyway. So if under the limit then no point swapping to a curve C as there must be some other fault.

    I call it curve B or C as RCD's are classed type AC, A, F, or B so we call it curve B or C so don't get confused when using RCBO's.

    So what I am saying is the same as @winston1 but I am trying to explain why the instructions are wrong. However I also realise in England there is a problem at the moment, so you may need to do some thing to keep it all running, the worry is, if some thing has been damaged causing the MCB to trip, swapping it for curve C could mean a big bang. So I am rather uneasy saying change it, without first testing.
     
  15. Peter A

    Peter A

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    Thanks all! Very much appreciated. Bottom line right now seems to be that I'm more comfortable waiting for the fitter to be able come back and test / inspect properly. Luckily we're able to manage without turning this heating on right now.

    Cheers!
     
  16. AndyPRK

    AndyPRK

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    what are you plugging into the sockets that are also run off this MCB ?

    And are they on, when it trips ?
     
  17. Peter A

    Peter A

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    From what I can see, the sockets on the same circuit are largely unused. We do have a couple of lamps plugged in (lower power LED bulbs).
     
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