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Discuss odd one seen today, three FCUs each for a 3KW immersion on one 6mm radial in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. GBDamo
    Offline

    GBDamo Regular EF Member

    Location:
    UK
    As title,

    Megaflo hot water cylinder has three 3KW immersion elements. Each fed from an FCU in 2.5mm 3093Y flex.

    On the surface all looked good.

    Initial testing showed no earth continuity on two of the elements.

    Closer inspection showed all FCU covers were loose.

    Further inspection showed all three dry lining back boxes were smashed. The CPC from the first FCU to the second was broken, probably when the face plates were screwed on.

    The FCUs are fed in 6mm T&E and each spurred off the previous, so two 6mm conductors in the terminals and the 2.5mm flex. When the face plates had been screwed on the inflexibility of the 6mm conductors had pushed the back out of the back boxes and broken some of the screw lugs.

    MKs documentation state the FCUs are good two 6mm in each terminal but also state each FCU is good for 20Amps. My calcs put the current going through the last FCU at 12Amps, 24 for the second and 36 for the third. Now, this is only current through the terminals and not that being switched, would this challenge the FCU?

    I have been tasked with getting it working....

    I cannot see how the original set up can be safely put back into service, more from the physical constraints in the back boxes than anything else so intend to use the 6mm to feed a small CU and run each FCU off its own 16Amp MCB with an RCD protecting all three.

    all will be tested and certified afterwards but the R1+R2s are likely to be zero as the new radial circuits are going to be less than a meter long, how would you record this?

    Hope this is clear but kind of thinking it might not be.
     
    • Agree Agree x 1
  2. Murdoch
    Online

    Murdoch Electrician's Arms

    Location:
    Woking
    The R1 + R2 Reading is what your tester records ...

    How deep are the back boxes these FCUs are on?
     
  3. spinlondon
    Offline

    spinlondon Forum Mentor

    Location:
    Harlow Essex
    Why an RCD?
     
  4. GBDamo
    Offline

    GBDamo Regular EF Member

    Location:
    UK
    I thought that but just sticking 0.00 in feels wrong but it is what it is.

    I'm guessing 45-50mm
     
  5. KEV 1 N
    Offline

    KEV 1 N Regular EF Member

    Location:
    Durham
    I was once told never to record 0.00 (despite that being the given value), instead record 0.01 as your R1+R2 value
     
    • Agree Agree x 5
    • Like Like x 1
  6. GBDamo
    Offline

    GBDamo Regular EF Member

    Location:
    UK
    without trying to be flippant, why not?
     
  7. Leesparkykent
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    Leesparkykent Super Moderator Staff Member Super Moderator

    Location:
    Kent
    Introduce a know resistance and subtract it from the reading.
     
    • Like Like x 1
  8. Andy78
    Online

    Andy78 Forum Mentor

    Location:
    Kingston upon Hull
    I did one of these unvented heaters a bit back, was a salon master with 3 x 3.5kW elements. I kept it simple with a 2.5mm radial through a 20A DP switch for each element.
     
    • Agree Agree x 2
    • Like Like x 1
  9. anthonybragg
    Offline

    anthonybragg Electrician's Arms

    Location:
    NORTHAMPTON
    Business Name:
    CHARLTEC ELECTRICAL LTD
    I would round it up to 0.01 ohms or whatever the lowest reading digit is on your tester. If compliance that no RCD protection is not required up to your new board consider RCBO's if costs permit. but as others now posted RCD could be omitted.
     
  10. spinlondon
    Offline

    spinlondon Forum Mentor

    Location:
    Harlow Essex
    RCD trips, all 3 elements off.
     
    • Agree Agree x 1
    • Winner Winner x 1
  11. westward10
    Offline

    westward10 In echoed steps I walk across an empty dream. Staff Member Moderator

    Location:
    Northamptonshire
    What is protecting this 6.0 cable which I assume to be in a partition wall.
     
  12. GBDamo
    Offline

    GBDamo Regular EF Member

    Location:
    UK
    Agree but easy enough to resolve. but then again.....

    I'll give it some thought.
     
  13. spinlondon
    Offline

    spinlondon Forum Mentor

    Location:
    Harlow Essex
    As Westward10 says, what protects the 6mm2?
    Does that have RCD protection if it’s concealed in a wall?
     
  14. GBDamo
    Offline

    GBDamo Regular EF Member

    Location:
    UK
    I've see the whole run and am happy its 'clipped direct' the whole run as it above a dropped plasterboard ceiling, no insulation.

    Edit to add, it on a 40 amp MCB, no RCD.
     
  15. Murdoch
    Online

    Murdoch Electrician's Arms

    Location:
    Woking
    Regs 314.1 and 314.2 need to be looked at ...
     
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