Select Committee on Northern Ireland Affairs Minutes of Evidence


Further Memorandum submitted by Northern Ireland Electricity plc

  Thank you for your letter dated 23 April 1999 in which you sought our response to two additional points, with a third point included in a cover note.

  We are pleased to reply as follows:


  You asked for an indication of the number of customers whose supplies might be at risk as a result of failures on various parts of the distribution system.

  The 33kV network is essentially a sub-transmission system providing supply to approximately 200 33/11kV substations. The 33kV network is designed so that the majority of 33/11kV substations can be resupplied from an alternative 33kV circuit in accordance with standards required under NIE's licence. The system redundancy so provided reduces the effect of 33kV faults on customers' supplies by either minimising the duration of supply outages or, where a substation is supplied by duplicate 33kV circuits, ensuring that customers fed from such substations are unaffected by a fault on one 33kV circuit.

  Faults on the 33kV network do have the potential, however, to result in the loss of supply to one or more 33/11kV substations potentially affecting several thousand customers.

The 11kV network as described in our earlier submission, is made up of main arterial lines with normally open points, running between adjacent 33/11kV substations and multiple subsidiary radial lines which have progressively developed to facilitate new customer connections. Faults on the 11kV and low voltage network therefore impact more directly on customers' supplies. Approximately 70 per cent of all customer interruptions are due to faults on the 11kV network.

  You asked for a breakdown of 11kV faults between arterial lines and spur lines. As an indication of the split, we would advise that the 11kV overhead network comprises over 800 circuits and some 20,000km of overhead line and experiences typically 3,000 faults annually. Approximately 30 per cent of these faults affect 11kV arterial lines.

  The approximate range of customers affected by a single fault on either of a 33kV, 11kV or low voltage line is provided in Table 1.


Scale of customers per fault by network
Customers affected
Network AffectedRange Average
33kV line0-7,0001,000
11kV arterial line1-3,000 300
11kV subsidiary line1-300 30
Low voltage line1-300 15


  You asked to what extent could the accelerated refurbishment programme be accommodated without deferral of other projects.

  Our 14 April submission to the Committee[5] illustrated the benefits for customers in maintaining a system of capex regulation which incentivises the company to invest efficiently to achieve the required outputs at lowest cost. Whilst the capex allowance for the second regulatory period was determined after detailed and careful scrutiny by the MMC, it would be reasonable to assume a modest level of savings in the capex programme by the end of this period on the assumption that the efficiency incentives remain effective.

  To date, firm efficiency savings have been possible in respect of that part of the transmission plan which relates to the reinforcement of the north and west of the transmission network by the decision to establish a new 275kV substation at Dungannon as an alternative to the previously proposed 275kV substation at Omagh. In the normal course of events, NIE would have argued that it should retain the benefit of the savings in financing costs which occurred over the period (so as to maintain the ongoing incentive for efficient investment) and customers would get the benefit in the form of lower prices at the next review.

  However, in the light of the storm experience, NIE proposes to use these savings as a contribution towards the funding of the accelerated refurbishment programme where they will be sufficient to cover around 50 per cent of the funding required.

  The remaining 50 per cent must be covered through our review of investment priorities which has identified areas where investment can be deferred with a level of risk which is minimal when measured against the significant risk which arises if the network's resilience to storm conditions is not enhanced.

  As explained in Annex A5 of our April submission,[6] we have re-prioritised our performance improvement strategies in the context of the identified risks and, consequently, we have rephased the programme by bringing forward additional line refurbishment into the current regulatory period at the expense of deferring some 11kV automation to the next period. A similar reappraisal of our priorities for the undergrounding of overhead lines has resulted in the deferment of part of the high voltage undergrounding programme to allow for the undergrounding of low voltage lines during the current period. A review of asset replacement priorities has facilitated a rephasing of the meter replacement programme in a way which enables NIE to remain compliant with our meter recertification obligations.

  As regards your question on technical resources, the amendment of the transmission plan to an alternative more cost-effective reinforcement project will not, in itself, release any significant technical resources to cover the accelerated refurbishment programme. Also the resource capacity released by the deferral of 11kV automation, HV undergrounding and meter replacement would be insufficient to cover the expanded refurbishment programme. Additional technical resources for design, management and operational switching will be required. We have therefore taken the necessary steps to provide the required resources, as detailed in Annex A3 of our April submission.[7]


  You requested clarification of the initials AAAC in your cover note. AAAC refers to All Aluminium Alloy Conductor.

4 May 1999

5   Appendix 23. Back

6   Appendix 21, p. 99. Back

7   Appendix 21, p. 98. Back

previous page contents next page

House of Commons home page Parliament home page House of Lords home page search page enquiries

© Parliamentary copyright 1999
Prepared 29 July 1999