The purpose of system warnings is to communicate to all parties (via BMRS / Fax) possible or actual future power shortages, system problems or imminent demand reductions. There is an expectation on all parties not to make the situation worse (do not start outages), if possible to make the situation better (make more plant available), to take preparatory action if required (applicable to DNO), and to review imbalance positions and trade accordingly. In the case of a Risk of System Disturbance, users are requested to warn their operational staff and maintain plant and apparatus in a condition best able to withstand the anticipated disturbance.

There are 5 categories of system warning:

• Inadequate System Margin (NISM) OC7.4.8.5 - there is insufficient system margin to meet demand and reserve requirements - contingency reserve has been eroded.
• High Risk of Demand Reduction (HRDR) OC7.4.8.6 - there is insufficient margin to meet operating reserve requirements.
• Demand Control Imminent (DCI) OC7.4.8.7 - there is still insufficient margin to meet operating reserve requirements close to real-time. Demand Control is expected in the following 30 minutes.
• Risk of System Disturbance (RSD) (e.g. severe storm) OC7.4.8.8 - Likelihood of system disruption, often localised warning targeted at Generators / DNOs in a specific area.
• Cancellation of GB System Warning OC7.4.8.9.

System warnings are shown on the summary page only while they remain in force. Details of cancelled warnings are available on the History page.

Please note that NISMs are only the first stage of the system warning process, and the least severe category of margin warning. They normally indicate an erosion of the contingency reserve requirement several hours ahead of real-time, and should not necessarily be interpreted as an indication that demand is not likely to be met in full.

The Demand Forecast is the National demand forecast made by National Grid. This forecast is based on historically metered generation output for Great Britain. The values shown here are derived from the '08:45' forecast, which takes into account transmission losses, but does not include station transformer load, pump storage demand or interconnector demand; this forecast is therefore comparable with the Initial National Demand Out-Turn INDO. Forecasts other than the '08:45' forecast (as published elsewhere on the BMRS) do include station transformer load, pump storage demand and interconnector demand, and are not therefore directly comparable with INDO. All the forecasts are unrestricted, i.e. any notified customer demand management is included in the history.

The Initial National Demand Out-Turn is the average megawatt value of demand for a Settlement Period. The INDO is made available by the System Operator within 15 minutes after a Settlement Period, based on their operational metering. The exclusions from National Demand Out-Turn match those in the '08:45' Demand Forecast; that is to say, it takes into account transmission losses but does not include station transformer load, pumped storage demand or interconnector demand. This figure is derived by National Grid for submission to the BMRS but is not formally defined in the Grid Code.

The Triad is used as a short hand way to describe the three settlement periods of highest transmission system demand within a Financial Year. These represent the half hour settlement period of system peak demand and the two half hour settlement periods of next highest demand. These two periods of next highest demand must be separated from the system peak demand and from each other by at least 10 Clear Days, between November and February of the Financial Year inclusive.

The Triad Demand Information on the BMRS does not constitute a forecast of when the Triads are expected to occur, but is a summary of publicly available demand data that may be of assistance in parties' own triad forecasting activities.

Highest Demands so Far

Between November and February, this table shows the three settlement periods of highest demand (subject to the 10 Clear Day rule) in the triad period so far (i.e. since the start of November). Between March and October it shows the three settlement periods of highest demand in the previous triad period. In both cases the demand figures are based on operational metering, not settlement metering, and are therefore indicative only.

Highest Forecast Demands

Between November and February, this table shows the three highest demand peaks in the remainder of the triad period (i.e. up to the end of February), based on the 2-52 week ahead demand forecast. Between March and October it shows the three highest demand peaks for the next triad period, again based on the 2-52 week ahead demand forecast.

Because the source data is at a weekly resolution rather than a daily resolution, the 10 Clear Day rule cannot be accurately applied. Instead, as an approximation to it, only demand peaks in non consecutive weeks are shown.

Following the end of each Settlement Period two Imbalance Prices, known as System Buy Price (SBP) and System Sell Price (SSP) respectively, are calculated and published on the BMRS. These prices are purely indicative, as they are calculated before all the necessary information for definitive calculations, such as actual transmission losses, is available. The definitive calculations for Settlement purposes are carried out separately, several days later. These definitive calculations may also include amendments to the Balancing Mechanism data following post event analysis.

The Imbalance Prices are used for the settlement of the energy imbalances incurred by participants - that is to say, the differences between the volumes of energy which a participant contracts for (in all trading, not just in the Balancing Mechanism) and the volumes actually generated or consumed. A participant whose imbalance is a net spill on to the system will be paid for the energy which is put on to the system, at the System Sell Price. A participant whose imbalance is a net shortfall will have to pay for the volume of energy which is in deficit, at the System Buy Price. It is thus equally possible for generators or consumers alike to be out of balance in either direction in a given half hour.