We're talking about line-to-ground faults that may damage equipment. The NEC, per Sec. 215-10, requires that ground-fault protection for equipment (GFPE) be provided for any feeder disconnect rated 1000A or more in a solidly grounded wye system with greater than 150V to ground, but not exceeding 600V phase-to-phase. In other words, you're protected if your system is as described above.
What if the manufacturing process is one that requires power not at the mercy of a GFPE trip? Then, high-resistance grounding is employed to limit the amount of fault current to a level such that the manufacturing process can continue while troubleshooting and fault locating is done, all without damage to equipment.
IEEE 142-1991, Recommended Practice for Grounding of Industrial and Commercial Power Systems (the Green Book), defines a high-resistance grounded system as follows.
A grounded system with a purposely inserted resistance that limits ground-fault current such that the current can flow for an extended period without exacerbating damage. This level of current is commonly thought to be 10A or less. High-resistance grounded systems are designed to meet the criterion [R.sub.o] [less than or equal to] [X.sub.co] to limit the transient overvoltages due to arcing ground faults. Ro is the per-phase zero-sequence resistance of the system, and [X.sub.co] is the distributed per-phase capacitive reactance-to-ground of the system.
NEC Sec. 250-5(b), Ex. No. 5 permits the use of a high-impedance grounded neutral for 3-phase AC systems rated 480V to 1000V, where all of the following conditions are met:
* The conditions of maintenance and supervision assure that only qualified persons will service the installation.
* Continuity of power is required,
* Ground detectors are installed.
* Line-to-neutral loads are not served.
Troubleshooting example
A good example is provided in the book The Electrical Troubleshooting Pocket Guide, a McGraw-Hill publication, by John E. Traister. The example used is a 240V, 3-phase, delta system in a relatively small industrial application, with the service equipment as shown in the accompanying diagram. Obviously, GFPE is not required by the Code. And, high-resistance grounding can't be used. With no problems on the system, voltage readings between phases should be around 240V and around 150V phase-to-ground.
Suppose a problem arises, and we begin our troubleshooting by taking voltage measurements. In doing so, we measure 230V between two phases and ground and 50V between the remaining phase and ground. Immediately, we can deduce that the phase with the lowest phase-to-ground measurement has a partial ground or ground fault.
The best way to troubleshoot the problem is to follow this step-by-step procedure.
First, connect one of the voltmeter's leads to the main switch enclosure, which should be grounded, and connect the other lead to the phase thought to have the ground fault.
Second, open Switch A and check the phase-to-ground voltage. If there's no change, open Switch B, C, and D until there's a change to 150V. Depending on which switch opening causes the voltage change, you can deduce that the ground fault is located on the specific switch's circuit. Let's say we find that Switch D causes the voltage change to 150V.
Third, close Switch D and connect one of the voltmeter's leads to its enclosure, which should be grounded. Then connect the other lead to each of the phases and measure voltage until the phase with the ground fault is located.
Fourth, disconnect the motors fed from Switch D one at a time until the one causing the problem is found. You'll know which motor is causing the problem when the phase-to-ground voltage returns almost to 150V. The motor then can be removed and repaired or replaced as needed.
