Firm service cannot be interrupted during adverse conditions, such as periods of high demand. However, firm service may also refer to service that is covered by a long-term contract, such as a year or more. There are extreme cases when firm services may be interrupted, such as emergencies and when system reliability is threatened. Services to homes and small businesses are usually firm. Some businesses that cannot afford interruptions also have firm service. Businesses that can afford to have services interrupted or that can significantly reduce their consumption when notified by the provider can get better rates by having non-firm service. Customers that have non-firm service may have a low level or baseline firm service that is guaranteed so that they do not have to shut down completely. Firm and non-firm service is also available for companies that rent or lease pipeline or electrical transmission capacity.
Firm service for natural gas pipelines and electrical transmission lines often include two charges. The first is a reservation charge related to how much capacity the customer reserves. This charge is paid regardless of how much capacity is actually used. The second charge is based on how much capacity is used. Interruptible rates are volumetric, being based only on the volume of gas or electricity delivered.
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