Contingency measures to prevent electricity shortages this winter in the Republic have been put in place.

Approximately 300 megawatts of power on the North-South electricity interconnector will be reserved for the Republic in case demand in the South exceeds the supply from generators such as the ESB and Viridian. The arrangements have been put in place by EirGrid and its North equivalent, the System Operator of Northern Ireland (Soni).

Demand is traditionally high in winter months, especially in January and February. EirGrid figures show demand at peak times is close to 5,000 megawatts, with the supply from generators approaching 5,500 megawatts.

The spokesman said as a system operator it was prudent to engage in contingency planning.

Impacting on the current supply problems is the temporary closure of the thermal power plant at the ESB plant in Poolbeg. Several of the units at this plant are being overhauled, mainly because of the discovery of rust in some of the boilers. Were all stations operable, the available supply would be close to 6,000 megawatts.

Electricity from the North-South interconnector has been available for many years and this source is often called upon by EirGrid to fill gaps in supply during winter. The latest arrangement is slightly different in that the 300 megawatts is officially reserved for the South, regardless of events in the North.

Northern Ireland currently has an excess of power. Soni is a ring-fenced entity within the Viridian/Northern Ireland Electricity group.

Figures released over recent weeks by the Commission for Energy Regulation (Cer) show supply running just ahead of demand. An EirGrid spokesman last night said its analysis indicated a satisfactory outlook for the remainder of the winter months.

The national grid in the Republic has a range of options to deal with any supply shortages. Apart from conventional power generators and the North-South interconnector, there is also wind energy. The problem, however, is that wind energy is intermittent and has to be backed up by conventional generation. For example, figures this week from the Cer show that at certain times wind was only producing four megawatts of power for the grid, while at other times this increased to over 500 megawatts.

If demand does exceed supply the whole electricity system does not necessarily have to go down. Sometimes a lack of supply results in "brownouts" where power is still provided but at lower voltage. This causes lights and computer screens to flicker.

The other solution to a supply and demand problem is "load shedding". This involves cutting off certain parts of the country for a time until supply and demand comes back into balance.

The key period each day for high electricity demand is between 5pm and 7pm. EirGrid has recently been running a campaign to persuade people to curtail power use at these times.