Tt earthing

Discussion in 'Electricians' Talk' started by spen123, Nov 22, 2021.

  1. spen123

    spen123 Screwfix Select

    T is my understanding that under current regs aslong as
    all outgoing circuits are protected byba 30mA rcd or rcbo
    Tails are supported throughout the run and supported at cabke entry
    Then the need for a time delayed rcd is omitted
    However should the live be damaged for whatever reason to earth then a fault will occur and no fuse will blow?
    If thus is the case how can this be safe.
    I appreciate we bond everything so in theory nothing should be above 50v but the fault wouldn't be detected until potentially to late?

    Can anyone elaborate on this and either point out what I have missed or explanation on how this is safe
     
  2. MGW

    MGW Screwfix Select

    Correct, one we no longer use fuses, and it is the RCD or RCBO which will operate if there in an imbalance.
    For an overload to operate be it a trip or fuse, it needs over the rated current to flow, most MCB's are type B so to work the magnetic part between 3 and 5 times the rating, plus 5% to allow for volt drop, so an earth loop impedance of under 1.4 ohms.

    Most earth electrodes are around 60 ohms, so will not trip the magnetic part, but will trip the RCD which is why we test the RCD regular and fit RCD's of a type unlikely to be frozen, and protect the RCD with a SPD.

    The problem arises with EV charging and solar panels, both can cause DC on the supply under fault conditions, so either a type A RCD with a 6 mA DC detection device to disconnect if there is DC or a type B RCD. The problem is we also have a type B MCB, so with RCBO's it should be curve B for the overload, and type B for the RCD part, at moment there does not seem to be any type B RCD's built into single width RCBO's so if they say type B this refers to the curve.

    Also unless a RCD is fitted before the RCBO's really they should be double pole switching type. Although overload is only on the line not the neutral.
     
  3. spen123

    spen123 Screwfix Select

    Thanks
    But this hasn't answered the whole questions as to what happens if a live tonearth fault occurs before the rcd if we don't fit one as new regs don't require us to do so if certain conditions are met
     
  4. The Happy Builder

    The Happy Builder Screwfix Select

    It is good engineering practice to fit a Double Pole upfront RCD to disconnect both live conductors supplying a TT installation for a number of reasons, you can argue about if the regs require it but it’s definitely advisable to fit one.
     

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