Under current agreements with Shippers, National Grid or other transporters can interrupt for three specific reasons
- Capacity Constraints
- Testing
- Emergency
Capacity Constraints
NTS
If National Grid expects that the gas demand in any part of the National Transmission System will exceed the available capacity, it will instruct shippers supplying gas to interrupt some, or all, of their interruptible NTS and LDZ customers to ensure there is sufficient capacity to deliver gas to firm consumers. National Transmission system interruption is generally invoked over a wide area.
Distribution Networks
When a constraint occurs in a local distribution system it may only affect a small number of consumers in that locality. As a result of this some consumers may experience more frequent interruption than others, depending on their location within the gas transportation system and the severity of local system constraints .
Testing Interruption Capability
Transporters have the right to interrupt for up to three days per annum to check the effectiveness of the interruption process. Such tests will be used when there are doubts that effective interruption can be achieved.
Emergency
Transporters also have the right to interrupt to avoid, or manage a gas supply emergency incident as defined by the Gas Safety (Management) Regulations 1996. This may be caused by unforeseen circumstances such as pipeline, plant or equipment failure, or where demand exceeds either total supply or planned firm capacity. The normal notice period for interruption is wavered for emergency interruption and sites should stop using gas as quickly as possible.