The Basics of Current-Sensing Relays

Preventing power outages from ground faults

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.