Understanding ground fault protection
Equipment reliability is crucial for plants striving to be more competitive. Electrical ground faults can put equipment out of commission and, in serious cases, can lead to catastrophic fire. In addition, ground faults can lead to arc flash events and shock hazards for operators and maintenance workers. A burned out motor can be replaced in short time, but fire, serious injury or death could shut down a plant for several days or weeks. Fortunately, ground fault protection is easy to apply, if you understand the basics.
Simply put, a ground fault occurs when electrical current finds a bridge to ground via worn insulations, conductive dusts, water or other “soft grounds,” instead of the normal path back to its source. More than 80% of short circuits in equipment are ground faults, and 90% of these events are due to insulation deterioration on wires and cables. In cases where workers accidentally bridge power and ground, current as low as 75 mA can send their hearts into ventricular fibrillation.
Ground Fault Circuit Interrupters (GFCIs) can detect ground faults as low as 6 mA to protect people from shock. The most common types of GFCIs are single-phase 120 V, 15- and 20-A outlets used in residences near sources of water, such as in kitchens, baths and outdoor receptacles. Ground Fault Circuit Breakers also may be installed at the power panel to provide ground fault protection. Most Ground Fault Circuit Breakers are for single-phase 120 or 240 V circuits up to 60 A; however, some GFCIs are available for three-phase 480 V circuits of up to 100 A or more.
Ground Fault Relays (GFRs) typically monitor ground fault currents from 10 mA to 1200 A, and are installed in high current, multi-phase switchboards and panelboards to protect manufacturing plants, elevators and escalators, shipping docks and wastewater treatment facilities. Facilities with solidly grounded wye electrical systems rated 1000 A or more are required by the NEC to have ground fault protection on the main line. GFRs detect ground fault currents in the same manner as GFCIs, but then signal other devices such as circuit breakers, shunt trip switches or contactors, to open, protecting equipment before a catastrophic event occurs. Unlike GFCIs, current and time-delay settings may be adjusted on GFRs, per UL Standard 1053 and NEC 230.95. Arranging GFRs in subpanels from main power to individual machines provides selective coordination and aids troubleshooting %%MDASSML%% a tripped GFR device along a branch will limit outage to the faulted circuit and quickly identify where an electrical problem has occurred within the plant.
Some GFRs are manufactured to continuously monitor leakage current and insulation resistance, with an analog signal output that could be fed into a PLC or plant operations computer. Such a feature can trigger an alarm when setpoints are reached, as well as provide a valuable predictive maintenance tool because, as wire insulation ages, increasing amounts of current leaks to ground. By regularly tracking the ground fault current on circuits, low-level ground faults can be identified before they become problems, and help head off the possibility of disastrous insulation failure. Some GFRs are designed to monitor wiring integrity of a motor in an OFF state prior to start-up, and prevent that motor from operation if an undetected ground fault exists.
By using GFRs to monitor ground fault currents in facilities with ungrounded power systems, leakage currents can be identified and corrected, ensuring the maximum use of every amp that comes into the facility. Ground fault devices also can reduce the severity of an arc flash event. With ground fault protection, an arcing ground fault can be shut off in 100 ms or less, reducing arc flash incident energy and the probability of escalation into a more dangerous phase-to-phase arc flash event. As one industrial insurance company has noted, 80% of all electrical faults start as ground faults.
Certain circuits %%MDASSML%% particularly those feeding power to variable-frequency drives %%MDASSML%% may cause “nuisance” tripping of GFRs. The cause for this particular problem stems from electrical noise generated by the VFDs leaking back into power lines. Devices are now available that, added to these circuits, will block the noise and stop the nuisance tripping. Another recently introduced GFR technology minimizes nuisance tripping, yet is sensitive enough to protect both equipment and workers.
Installing ground fault protection devices in a manufacturing plant will reduce or eliminate downtime from equipment failure, fires and arc flash accidents. Sensing actual or potential ground faults before insulation failure or phase-to-phase arcing faults occur will limit damage to equipment, increase reliability and protect those working on or near energized equipment.
|Ken Cybart is a senior technical sales engineer at|