There are two classifications of Interruptible Site. These sites form part of the transporters strategy for the safe and secure operation of their networks under different circumstances.
Detailed below are the different classifications of Interruptible Sites:-
Shipper Nominated Interruptible
Sites which are classified as 'Shipper Nominated Interruptible' (SNI) receive a reduction in the transportation charge which may be included in the price of gas. In return National Grid can interrupt these supply points for up to 45 days in each Gas Year. SNI is not a uniform Network Code term and care must be taken in not confusing Transporter interruption at an SNI with a commercial interruption carried out by shippers, sometime referred to as "shipper interruption".
Transporter Nominated Interruptible
A Transporter can designate a site as Interruptible (TNI) by giving at least 12 months notice. This gives the relevant transporter the ability to interrupt these sites for more than the usual 45 days (for example, up to 60 days in a 1-in-50 cold winter). This number of interruptible days is determined by Network Planning and is a specified number for each site.
In addition sites which are located in a particularly vulnerable part of the network are described as Network Sensitive Loads. These can be either TNI or SNI.
Network Sensitive Loads
Sites which are designated as Network Sensitive Loads (NSL) are those which are on a part of the network that has limited transportation capacity. These sites are required to interrupt in order to maintain supply pressures to firm end users, at a level which avoids an actual or potential supply emergency, as defined by the Gas Safety (Management) Regulations 1996. As a result, a NSL may have a higher probability of being interrupted, particularly during mild weather conditions, than other sites.