How you ground a separately derived system is critical to the safety of those working on the system and the performance of your equipment.
Separately derived systems are power sources with no direct electrical connection between any of the circuit conductors and supply conductors. For example, no direct connection exists between the primary and secondary winding of a delta-wye transformer. Generators, converter windings, UPS systems, and solar photovoltaic systems are separately derived systems only when all circuit conductors — including the grounded (neutral) conductor — are independent of the supply conductors. If you wish to use a separately derived system, you must use a 3-pole transfer switch for a single-phase, 3-wire system, or a 4-pole transfer switch for a 3-phase, 4-wire system [Art. 100 definition and Sec. 250-20(d) FPN No.1].
The National Electrical Code (NEC) has several rules that contain specific requirements for grounding separately derived systems. Failure to properly ground separately derived systems can result in electric shock, electrocution, fire, equipment failure, and poor power quality.
Electric shock [Sec. 250-104(a)(4)]
To prevent a hazardous voltage difference between metal water piping systems and metal electrical equipment caused by faults or voltage transients, you must bond metallic water pipe systems in the area served by the separately derived system to the grounded (neutral) conductor of the separately derived system.
Electrocution [Secs. 250-2(d), 250-20(d), and 250-30]
Installing a separately derived system and not grounding it properly can create a dangerous condition where a phase-to-case fault cannot be removed. As a result, all metal parts of the electrical system — as well as the building structure — will remain energized with line voltage if a phase-to-ground fault occurs.
Fire hazard and electric shock [Secs. 250-6 and 250-142]
Improper neutral-to-case connections at separately derived systems create multiple neutral-to-case connections. This allows neutral current to have multiple return paths to the grounded (neutral) conductor of the power supply. Improper neutral-to-case connections can create objectionable current flow on metal parts, resulting in a fire and/or shock hazard.
Equipment failure [Secs. 250-2(a), 250-20(d), and 250-242]
You must ground a separately derived system to the earth per the NEC to stabilize the phase-to-case voltage [Sec. 250-2(a)]. According to the IEEE Std. 242 (the Buff Book), Sec. 7.2.5, an improperly grounded system could be subjected to severe overvoltage to ground — as high as six or eight times phase voltage. This can puncture insulation and result in additional ground faults. Repetitive charging of the system capacitance, or resonance between the system capacitance and the inductances of system equipment can cause these overvoltages.
Power quality. [Secs. 250-6 and 250-142]
Improper neutral-to-case connections can elevate case-to-earth voltage, which leads to power quality problems, and create neutral current flow on metal parts of the electrical system, causing electromagnetic interference.