21. NIE has descr
ibed the storm as "the most severe network emergency we have ever had to deal with."
Nearly 4,000 faults in all were reported over the period 26 to
31 December, over three quarters of them on 26 December. The overwhelming
majority of faults causing loss of supply to customers occurred
on the distribution system.
Numerically, the bulk of the faults were on the low voltage distribution
lines, but the damage patterns were complex and multiple faults
the exception of the Belfast area, where relatively little of
the electricity supply system is overhead, damage to the high
voltage linesthe key element in the distribution networkwas
common across Northern Ireland. As we commented earlier, around
162,000 customers were affected on December 26, and, at the peak,
around 110,000 customers were off-supply simultaneously.
22. Other electricity supply undertakings also
had serious supply interruptions. ESB reported
that, at the peak of the storm on 26 December, 185,000 customers
lost supply. In ESB's Northern Region, which includes Ulster counties,
North Connaught and North Leinster, 40% of customers lost supply.
In Great Britain, 120,000 customers supplied by Northern Electric
were affected, as
were more than 240,000 supplied by Scottish Power
(at the peak, 125,000 simultaneously) and about 130,000 supplied
by Norweb. Northern
Electric commented that "the extent of the damage in this
storm was particularly challenging as it hit at all voltage levels
on the network with numerous faults being created between a customer
and a bulk supply point."
23. The complexity of the faults was cited by
NIE as one of the reasons for the delays in restoring some consumers'
supplies. By midnight on 26 December, around 63,000 customers
remained off supply, and the last of them were not reconnected
until New Year's Day. The progress of supply restoration for all
customers over the period 26 to 31 December is set out in the
|Time band||0-3 hrs
||3- 6 hrs
|Time band||24-36 hrs
||36- 48 hrs
24. In the Republic of Ireland, there was a
broadly similar pattern of supply restoration, as the Table below
|Date||Supply restored to
||Still without supply overnight
25. It is not possible to make detailed comparisons
between the performance of the various electricity suppliers in
restoring supplies to consumers following the storm without much
more information about the nature of their supply systems, of
the faults, and of the resources available to tackle them. However,
it is clear that the experience of small numbers of consumers
in Northern Ireland in being off-supply for long periods as a
result of the storm was shared by consumers in Great Britain and
in the Republ
ic of Ireland in areas affected by similar storms.
26. NIE has published an analysis of the faults
causing power failures and their attributed causes. The majority
(about 64%) were attributable to the wind, with just under a quarter
being attributable to trees. The latter were of disproportionate
significance in relation to faults on the low voltage network:
92% of all faults caused by trees affected the low voltage network.
Where damaged components were the direct cause of power failures,
the majority of cases were attributable to conductor damage, and
only 10% to pole damage. As a result of the storm, 1,133 poles
were replaced out of a total of around 400,000about 0.3%.
The vast majority
had been broken
rather than pulled out of the ground. The typical age of damaged
components was in the range of 20-40 years.
27. NIE drew attention to the significantly
better performance of fully refurbished 11kV distribution circuits.
It had previously concentrated its efforts in improving network
reliability on the 11kV distribution circuits, as previous studies
had shown that 70% of all fault-related customer interruptions
occurred due to faults on this part of the network. The programme
was prioritised to address the worst performing circui
ts first, with circuits being ranked according to a combination of their fault rate and the number of customers affected.
NIE maintains that the refurbished circuits performed significantly
better in the storm than the unrefurbished circuits.
26 NIE storm report, p. 3. Back
27 See paragraph 14 above. Back
28 Appendix 19, p. 83. Back
29 Appendix 19, p. 83. Back
30 Appendix 18, p. 83. Back
31 Appendix 16, p. 83. Back
32 Appendix 10, p.75. Back
33 Appendix 18, p. 83. Back
34 NIE storm report, p. 19. See Appendix 33, p. 119. Back
35 Appendix 19, p. 83. Back
36 NIE field staff reported that many of the tree-related faults were a direct consequence of overgrown trees in private gardens affecting low voltage overhead mains and services; see NIE storm report, p. 13. Back
37 0.47% on 33kV circuits, 0.23% on 11kV circuits and 0.45% on low voltage circuits. Back
38 Appendix 21, p. 100. Back
39 For NIE's comments on the relationship between the age of the network and its vulnerability to storm damage and an analysis of the correlation between age and failure rates in relation to poles damaged in the storm, see Ev. p. 29-31 and Appendix 21, p. 100-1. Back
40 NIE storm report, p. 10 and 14. Back