Please be aware that the GBA Trigger level for 10th January has been modified as a result of the storage stocks for a class of storage being within 2 days of the safety monitor level and also recently observed performance at System Supply points.
Full details of what a Gas Balancing Alert entails and how the triggers are calculated can be found at: http://www.nationalgrid.com/uk/Gas/OperationalInfo/GBA/
However, a brief summary is detailed below:
The purpose of the Gas Balancing Alert (GBA) is to indicate a potential requirement for demand side response. It is based on a combination of the absolute Supply & Demand level and the impact of a potential breach of a Safety Storage Monitor. The basic trigger level for the alert is set by the anticipated available supplies into the network and is modified to reflect observed supply performance and depletion of storage stocks. Specifically in relation to storage based supplies, if any of the types of storage are forecast to be depleted to such a level that there will only be 2 days (at maximum withdrawal rate) left in that class of storage above the relevant Safety Monitor level by the end of the current gas day, the GBA trigger level will be reduced by the maximum withdrawal rate for that class of storage.
A simple flowchart showing the additional Residual System Balancer actions that National Grid can take after issuing a GBA can be found at: http://www.nationalgrid.com/NR/rdonlyres/312CC783-1814-40EC-A1D3-79CBE318389C/10984/OTCflowchartv05270906.pdf
The basis for the Safety Storage Monitors is to ensure that sufficient gas is held in storage to support those gas consumers whose premises cannot be physically and verifiably isolated from the gas network within a reasonable time period. To achieve this all gas consumers are categorised into one of two groups, Protected by Monitor and Protected by Isolation, and the stored safety gas requirement is calculated using a traditional approach of matching supplies to demand.
A detailed methodology of the calculation process for Safety Monitors can be found here: http://www.nationalgrid.com/NR/rdonlyres/B4ACC5F8-A8AF-48B4-A8AA-04BE3743E1C9/13676/20067SafetyFirmGasMonitorMethodology.pdf