Ground faults contribute to outages in industrial plants, especially further from the service equipment, and nearer to the point of use. While ground-fault protection is required by Sec. 230.95 of the 2011 NEC for solidly grounded wye services of more than 150V to ground, but not exceeding 600V phase-to-phase (we know them as 480/277V systems), it doesn’t require ground-fault protection downstream of that point (except in healthcare facilities). The maximum setting can’t exceed 1,200A, and the maximum time delay can’t exceed 
1 sec. for ground-fault currents greater than 3,000A.

Ground-fault protection is installed to prevent arcing ground faults. While ground-fault protection only on the main breaker offers maximum protection for the electrical system, the entire building electrical system may be shut down due to a low-level ground fault and is not conducive to efficient production. Numerous large systems have been tripped out by a ground fault produced by an electrician’s having grounded a light switch inadvertently when working, which raises two cautions:

  1. De-energize circuits prior to performing work.
  2. Adjust ground-fault systems when installing equipment. Manufacturers ship equipment with settings at minimum.

A better choice may be to provide low-level ground-fault protection on both individual motors and the main breaker. Figure 3 below illustrates a simple example of this design concept.

Fig. 3. In this type of electrical arrangement, both the low-level motors and the main breaker are equipped with ground fault protection.

In this example, the main breaker contains ground-fault protection within its trip unit. Ground-fault protection has now been added to a motor as well. Notice the three motor leads pass through the window of GFCT, a “donut-style” current transformer. Some ground-fault relays incorporate a CT in their construction while others may have it mounted separately, especially for larger motor leads.

GFCT is operating as a zero-sequence ground-fault detector. A zero-sequence CT operates by algebraically summing the currents through its core; that is, all current flowing out through the core must also return through the core. If a ground fault occurs on one of the conductors, part of the current returns to the source through the ground path. The difference will be detected by the ground-fault relay (GFR), which will have a setpoint. If the current exceeds the setpoint, GFR’s contacts on line 1 will open, de-energizing starter M and stopping the motor. When starting, some large motors can induce temporary ground-fault current into the electrical system during the starting surge. A ground-fault relay can incorporate a time delay to prevent tripping during this period.