Further Memorandum submitted by Northern
Ireland Electricity plc
Our letter of 4 June 1999 (Appendix 31) explains
that the NAFIRS system, which is our main fault recording system
is not an ideal data source for analysing the frequency with which
specific customers are affected by faults. The proper analysis
requires a significant effort in researching the circumstances
surrounding individual faults and for that reason we have limited
Table 1 to circumstances in which fuses have operated five or
more times. The figures in the columns headed 11 kV circuit breakers"
in Table 1 are still an appropriate estimate of the number of
customers affected by faults cleared by circuit breaker operations.
We consider that the data on faults cleared
by circuit breakers taken together with the revised data on faults
cleared by fuses is sufficiently accurate to draw broad conclusions
along the following lines:
Of those customers who were subject
to one or more faults, around 90 per cent experienced no more
than two interruptions per annum, and
in respect of a worst case"
situation less than 100 customers experience in excess of 10 faults
per annum. (Our previous letter suggested several hundred customers
are affected to this degree).
21 June 1999