Under current agreements, Transco can interrupt for several reasons:-

Network Capacity Constraints

If Transco expects that the gas demand in any part of the transportation system will exceed the available capacity, it will instruct the shippers supplying the gas to interrupt some, or all, of their interruptible consumers in that locality to ensure there is sufficient capacity to deliver gas to firm consumers. If a capacity constraint occurs in the National Transmission system, interruption is generally invoked over a wide area. However, 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 weather conditions.

High System Demands

When forecast demand increases to very high levels, (currently above 85% of the maximum peak day demand for which the transportation system is designed), the system reaches full capacity and becomes reliant upon strategic storage at certain locations. This particular storage is generally only sufficient to support firm demand. Consequently when demands are above 85% of peak day some interruption becomes increasingly likely. This type of interruption can be called at demand levels below 85% in order to avoid an emergency scenario.

Testing Interruption Capability

Transco has 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 we have doubts that effective interruption can be achieved.

Note: Depending on the contract with the consumer, gas suppliers or shippers may also interrupt the consumer for their own reasons. This could be for supply/demand matching or other commercial reasons.