Designers and Installers make costly grounding mistakes by not following grounding requirements for more than one building or structure supplied from a common service.

A change in the l990 NEC© keeps catching designers and installers. The change applies to the grounding requirements where two or more buildings or structures are supplied by a common service, and where the disconnecting means is located in a separate building or Structure. Normally a disconnecting means is required to be located at each building or structure, but the 1993 NEC© Section 225-8(b) Exception No. l gives permission for the disconnecting means to be located elsewhere on the premises other than at a building or structure for large capacity multibuilding industrial installations under single management, and where it is assured that disconnecting can be accomplished by establishing and maintaining "safe switching procedures." This exception is often used where hazardous locations are encountered so that the disconnecting means can be located in a nonhazardous area. When this design is used, NEC© Section 250-24(c) that was added to the 1990 NEC© must be complied with by bonding the grounding conductor at each separate building or structure to a grounding electrode. The 1990 NEC© required that this bonding be done at a junction box located immediately inside or outside of the separate building or structure. The 1993 NEC© elaborates on this by allowing the bonding to take place in a junction box, panelboard, or similar enclosure located immediately inside or outside the separate building or structure. The purpose of this bonding is to shunt lightning discharges to the earth at the separate building or structure. There are some practical limitations that should be applied. For instance, these requirements may not apply to a lighting pole or to a unistrut frame used at a lift station. However, these requirements would surely apply to a truck loading facility, a tank farm, or a process facility found at refineries.