Determining the Electrical Grounding for Mission Critical Facilities’ Engine-Generators
The Uptime Institute LLC considers the engine-generators as a primary source of power for the mission critical facilities.
By Syed Ahsan Baqi11/13/2017
Usually in the electrical schemes and the associated sequence of operations the transfer switches in the ATS Panels switches the grounded (neutral) conductor; therefore, it is recommended that the engine-generators must be grounded as a separately derived system in accordance with NEC requirements, because in the normal power mode, the grounded (neutral) conductor for the load is connected to the grounding electrode conductor at the service. In alternate mode, the grounded (neutral) conductor for the load is switched over to the generator source, which must be grounded as a separately derived system. Thus, in either position of the transfer switch, the electrical system is grounded. However, if there is no switching action in the grounded (neutral) conductor through the transfer equipment, then the generator system remains grounded with the transfer switch in either position: normal or standby by the grounding electrode conductor at the service.
As the generator is a separately derived system and usually located outdoors, a grounding electrode connection is required at the source location to comply with NEC 250.30(C).
Also, note that an on-site generator, is not a separately derived system if the grounded conductor (neutral) is solidly interconnected to a service-supplied system grounded conductor (neutral). An example of such a situation is where alternate source transfer equipment does not include a switching action in the grounded conductor (neutral) and allows it to remain solidly connected to the service-supplied grounded conductor (neutral) when the alternate source is operational and supplying the load served.
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Syed Ahsan Baqi is an experienced engineering consultant, business analyst and solution architect for mission-critical and data center facilities.
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