Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 12 replies
  • Subscribers 32 subscribers
  • Views 1966 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

TT system

MrJack96
In a TT system what protects the meter tails from an earth fault?
  • 0
    Insulated and sheathed tails passing through a plastic gland in to the consumer unit. Leaving the sheathing on up to the main switch.
  • 0
    Sorry my question wasn’t clear I meant if there was an earth fault from a tail touching an exposed conductive part
  • 0
    Plastic trunking.
  • 0
    Well, your meter tails are double-insulated. How will that insulation be breached? I don't quite see how the earthing system affects the risk.
  • 0
    ? No, they are not double insulated, they are insulated and sheathed
  • 0
    https://electrical.theiet.org/wiring-matters/years/2019/75-may-2019/mythbusters-4-double-insulated-cables/
  • 0
    Sparkingchip:

    Plastic trunking.


    But according to the article you've linked to, the plastic trunking isn't required.

    "So, for meter tails or other insulated and sheathed cables, the basic insulation over the conductor and the non-metallic sheath are deemed to comply with the requirements for both basic and fault protection in Regulation 412.2 and hence don’t need to be installed in containment (unless required for mechanical protection)."


  • 0
    Sparkingchip:

    ? No, they are not double insulated, they are insulated and sheathed 


    But ... they are considered to be ... in this particular case. The OP's question is clearly covered by Regulation 531.3.5.2.201



    531.3.5.3.2.201 For Class I enclosures in TT systems where RCD protection is used on outgoing circuits, double or reinforced insulation of all live conductors (incoming cables, extension terminals, etc.) on the supply side of the incoming device, e.g. main switch, shall be used. Insulated and non-metallic sheathed cables are deemed to meet the requirements of double or reinforced insulation.



    And, as others have already pointed out, this means that no other protection against contact with exposed-conductive-parts is necessary [to comply with BS 7671] because of this.


  • 0
    Look at the differences of functional, supplementary and reinforced insulation definitions in clause 7 of IEC 335 part 1 or BS 3456. Meter tails are Class 2 insulation and touchable with the human finger is safe under the low voltage directive.. i.e  No conduit or other sheathing is required.
  • 0
    There is no automatic disconnection mechanism - which is I suspect what is worrying you. There are no RCDs on the low voltage DNO networks,indeed given the way that earth and neutral are connected,  even if we wanted to, in most cases there could not be.

    If by some means the TT earth and the core of a live meter tail do make contact, everything on that TT zone becomes live, the electricity bill goes up, but there is no other symptom, except that folk may report getting a shock off it, and the power stays on in that dangerous state until the fault has been fixed.


    Until very recently the advice was always not to bring any  pre RCD live wire and a TT earth into the same enclosure, so there was no place that a stripped meter tail and TT earth could be side by side. So the first thing meter tails did, was to go into a plastic box with an RCD in side and little else.

    With the change of rules deprecating plastic consumer units for domestic use, this has now become possible, with a live tail in an earthed metal box, and ideally other steps should be taken to restore a similar level of safety - as others mention above, special glands and spacers so even if the meter tail connection comes loose, it cannot spring very far.

    Concentric cable with the neutral on the outside would be safer, and is what the DNOs use, but BS 7671 does not recognise the neutral as safer than live. (Thought ESCQR does)

    regards Mike.

Community Rules & Guidelines