Table 1: Causes of Large Blackouts in the United States. [NERC, 1984-2006].

No. Category % of Events Mean Size in MW * Customers Affected *
1 Earthquake 0.8 1,408 375,900
2 Tornado 2.8 367 115,439
3 Hurricane/Tropical Storm 4.2 1,309 782,695
4 Ice Storm 5 1,152 343,448
5 Lightning 11.3 270 70,944
6 Wind/Rain 14.8 793 185,199
7 Other Cold Weather 5.5 542 150,255
8 Fire 5.2 431 111,244
9 Intentional Attack 1.6 340 24,572
10 Supply Shortage 5.3 341 138,957
11 Other External Cause 4.8 710 246,071
12 Equipment Failure 29.7 379 57,140
13 Operator Error 10.1 489 105,322
14 Voltage Reduction 7.7 153 212,900
15 Volunteer Reduction 5.9 190 134,543

(Source: Ref. [2]) * The North American Electric Reliability Corporation (NERC) requires reporting of blackouts of more than 300 MW or
affecting 50,000 customers or more.