At all sites, there is either or both a main service disconnect and a fused disconnect. A main service disconnect may be located at a meter location away from the building. A main disconnect located within the shelter, equipment room or area may be fed by a feeder circuit originating at a main service disconnect located in an electrical room in a different location in the building or even a separate building. Typically, the neutral and ground conductors are bonded in the main service disconnect.

When the main service disconnect is located remotely from the equipment room or area, a separately derived system should be installed in the equipment room. (See NFPA 70-2005, Article 250.30 and 250.32 for additional information.)

One of the reasons for the separately derived system is to reestablish the neutral/ground bond, thereby improving the effectiveness of normal mode suppression.

-- Motorola R56-2005, p 6-2