Rule 10-106 Alternating-Current Systems
(1) Except as otherwise provided for in this Code, alternating-current systems shall be grounded if:
(a) By so doing, their maximum voltage-to-ground does not exceed 150 V; or
(b) The system incorporates a neutral conductor.
(2) Wiring systems supplied by an ungrounded supply shall be equipped with a suitable ground detection device to indicate the presence of a ground fault.
Rationale for Rule 10-106.
Many components of ac systems are suitable only for voltages up to 150 V, and we need assurance that such systems will not be stressed beyond that point. When a system having no neutral and operating at a voltage above 150 V (eg, a 220 V delta 3-wire system) experiences a ground fault, no current flows to operate the overcurrent protection and the circuit can continue to function. However, should a subsequent fault to ground occur on a different phase, current will now flow between the two grounded fault points. There can be considerable damage at both fault locations and, on occasion, fire or other damage may occur along the random path that the fault current has followed.
Intent for Rule 10-106
In Subrule (1), whenever the voltage on an ac circuit can be limited to not more than 150 volts-to-ground or wherever a neutral exists, we connect the system to ground to provide that voltage limitation, thereby protecting the insulation and the devices connected to the circuit. This also provides a fault current path, to allow the operation of the overcurrent devices.
In Subrule (2), we recognize that some industries use delta systems which operate at voltages greater than 150 V between phases, allowing the plant to continue operating without a shutdown, even when there is a ground fault. Ground detection is essential for such systems, to inform maintenance personnel that a ground fault has occurred. Normally, this fault will be repaired during a scheduled maintenance shutdown. If the fault is not found and corrected before a second ground fault occurs, the plant will experience an unscheduled shutdown, and the damage will usually be considerably more extensive than it would have been had it been a grounded system. A note in Appendix B intends that the ground detection device be labeled as to its purpose and be visible to the people responsible for the system.
Tony Moscioni
Electrical Inspector
Electrical Safety Authority
