NIE explains power cut havoc

The worst affected areas were Claudy, Limavady, Dungiven, Plumbridge, Donemana, Cookstown, Martinstown, Glenwherry, Buckna, Glarryford and Clough, with some homes left without power for several days.

A statement from NIE said: “An Ice Storm on March 30, 2010, caused unprecedented damage to parts of the electricity network in Northern Ireland. In total 100,000 customers were affected by power interruptions.”

So, what is an Ice Storm?

An Ice Storm is caused by wet snow combined with cold northerly winds creating a build up of ice on electricity equipment and lines causing them to sag and in some cases will pull them down. This phenomenon is known as ice accretion. Ice accretion is an entirely weather-related problem. It occurs only where a specific set of weather conditions coincide.

There has to be a high wind, the temperature has to be between 1C and - 1c and it must be snowing.

The effect of these three conditions is that the wind blows wet snow onto power lines. The wind-chill factor promptly freezes the snow to the wire. As snow builds up on that side of the conductor facing the wind, its weight twists the conductor round, exposing more conductor to ice build up.

The end result can be that an overhead conductor substantially less than half an inch in diameter ends up encased in several inches diameter of solid ice.

There have only been four experiences of ice accretion in Northern Ireland. In 1979, East Antrim was badly affected and in 1985 The Glenshane Pass felt the full effect of the metereological phenomenon, in 2001 the Downpatrick area was affected and of course last Monday saw the whole of the country affected.

What impact can an Ice Storm have on the electricity network?

One of three things can happen-The weight of the ice stretches the wire, bringing it to the ground, the weight of the ice snaps the wire, with similar results or the force of the wind against a large cross-section of wire, ice and snow topples, or snaps, the pole. Again, the line is brought down.

In all three cases, the result is the same. Electricity for safety reasons has to be switched off automatically, unless other circuits can be reconfigured to take the load of the damaged circuit. Supplies will remain off until repairs can be made.

Ice accretion can occur in any country where the appropriate weather conditions can occur. Since the last incident in Northern Ireland in 2001, ice accretion has occurred in England, Wales, France and Canada, in each case bringing widespread disruption to power supplies.

According to most meterologists, just one quarter of an inch of ice accumulation can add about 500 pounds of weight per line span. Damage from ice storms is capable of shutting down entire sections of electricity networks.