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  • Information for electrical contractors

  • Supply characteristics and earthing arrangements

  • The following typical characteristics apply to residential and light commercial/industrial premises for low voltage supplies up to 630 amps. You may find these useful when completing BS 7671 Electrical Installation Certificates.

    Nominal Voltage U/Uo = 400/230v
    Nominal frequency f = 50 Hz
    Prospective fault current Ipt: 25kA (3 phase), 16kA (1 phase)
    External loop impedance Ze:
    0.35 ohms typical maximum for TNCS system
    0.8 ohms typical maximum for TNS system
    The Ze values may be higher on older networks and should be verified by measurement.

    Supply protective device characteristics
    Type BS1361: up to 100 amps
    BS88 part 5: 200 amps and above

    Nominal current rating
    100 amps maximum residential single and three phase
    100, 200, 315, 400 and 630 amp - industrial /commercial
  • Removal of fuses

  • Key points

  • Only members of the following trade associations are permitted to remove cut-out seals:

    • National Inspection council for Electrical Installation Contracting (NICEIC)
    • Electrical Contractors' Association (ECA)
    • Electrical Contractors' Association of Scotland (SELECT)
    • National Association of Professional Inspectors and Testers (NAPIT)
    • ELECSA (owned by the ECA)

    The arrangement has been approved on the understanding the trade associations will take formal disciplinary action against members who abuse this privilege.

    Every year the trade associations will provide us, free of charge, up-to-date copies of their membership rolls. Our revenue protection staff will carry out a random sample check to confirm our procedure is being followed.

    Trade Association members must phone us on 0800 048 3516 (England) 0800 048 3515 (Scotland), before removing company cut-out seals.

    Trade Association members must apply seals to the company cut-out when they have completed their work. Cut-out seals may be of the adhesive-label or plastic seal type and may be collected (and signed for) from the nearest Scottish and Southern Electricity Networks distribution depot. Stock number 03154701 (England); 03154730 (Scotland).

    We will reassess this arrangement if there is any evidence of abuse of this procedure.