Industrial GFCIs are finally here
Will they help make your plant and workers safer?
Electric shock has always been a vital concern for plant maintenance workers. A recent IEEE white paper points out that from 1992-2002, there were 3,378 fatal electrical injuries listed in the Census of Fatal Occupational Injuries, and all but 30 were attributed to electrocution.
Anywhere plant equipment and workers interact—especially if water or moisture is involved—there is a risk of ground faults, which can injure or kill workers. As little as 50 mA can stop a human heart (or even lower in the worst of conditions).
That’s why the NEC requires ground-fault circuit interrupters (GFCIs) in homes wherever people, water, and electricity may be present (such as a bathroom). In such applications, a UL-listed device is required. However, residential GFCI devices are impractical for industrial facilities as they would cause too many nuisance trips. That’s why, until now, industry has not made extensive use of these people protection devices at higher voltages. Other devices—ground-fault relays (GFRs), for example—are frequently used, but they are intended to protect equipment, not people, and to avoid nuisance tripping their thresholds are set too high to protect workers.
Advances in GFCI technology now make it possible to provide sensitive, low-level protection without nuisance tripping at higher voltages (See Figure 1). Last year UL approved the first GFCI for applications up to 600 V, and some experts feel it’s only a matter of time before GFCIs are required by NEC and OSHA to protect industrial workers.
This article will explain these new devices, the regulations that govern their use, and where they’re best applied.
What a GFCI does
A ground-fault circuit interrupter, whether for industrial or household use, constantly monitors for dangerous ground current by comparing current levels on outgoing (hot) and return conductors; any current flowing through a person goes directly to ground, bypassing the return conductor. When the GFCI detects a current sufficient to injure a person, it instantly shuts off the power by physically interrupting the circuit.
GFCIs for residential use trip at 6 mA and up to 240 V, per UL943C Class A. Unfortunately, these units cannot be used in industrial facilities, which have higher voltage, load current, and leakage to earth. A Class A GFCI will nuisance trip unacceptably in an industrial facility.
Industrial regulations and standards
Until recently there was no UL listing for GFCIs designed for higher power use. In 2000, UL addressed GFCIs for higher voltage applications with a draft standard “Outline of Investigation” called 943C and added to it in 2009, but no commercially available product could pass its test requirements until recently. UL 943C Class C, Class D, and Class E define the characteristics expected of a GFCI operating up to 600 V.
UL 943C sets the trip threshold of special purpose GFCIs at 20 mA—low enough to provide worker shock protection but high enough in most applications to avoid nuisance tripping.
Although industrial GFCIs have higher current trip settings than residential GFCIs, they keep workers safe because of their fast reaction time. Like household devices, their response to ground current follows an inverse-time curve (See Figure 2); a current of 20 mA will cause a trip in about 1 sec (quickly enough to prevent injury at this current level), and higher currents will cause a trip within 20 ms.
Because of the wide range of industrial environments, 20 mA may not be the ideal trip threshold in all applications and units with adjustable trip setting may be needed. But because GFCIs are defined as devices having a fixed trip setting, devices with selectable trip levels cannot be called GFCIs; rather, they are called equipment ground-fault protection devices (EGFPDs). An EGFPD may offer trip levels that can be set from 6 to 50 mA.
The higher mA settings would be useful in applications where the 20 mA setting is causing nuisance tripping, and a lower mA would be useful when there is a need for additional safety. EGFPDs also offer flexibility for global companies wishing to standardize on one device across regions that have different trip level standards. Although EGFPDs can be used for people protection, UL lists them in a category for equipment protection rather than for people protection.
EGFPDs and GFCIs differ from the GFRs familiar to most maintenance managers. GFRs are intended to protect equipment in grounded systems, not people, and they do not open the affected circuit themselves, but send a signal upstream to a breaker. While the GFR reacts in 8 to 10 ms, the upstream circuit breaker may take 30 to 50 ms to open—enough time for an electrical shock to stop someone’s heart. A GFCI or EGFPD contains its own interrupting device (a relay, contactor, or circuit breaker) to interrupt power, and it interrupts power much more quickly. Per UL 943C, a GFCI must open the circuit to interrupt power in less than 20 ms for higher currents. This time is fast enough to prevent shock injury.
UL wanted the UL 943C standard to ensure a high level of safety and added the requirement that the GFCI device must monitor the ground wire for open connections, so that if the connection to ground is lost, then the device will open the circuit. Ground wire monitoring ensures that voltage on the equipment chassis is not elevated relative to ground when an internal ground-fault occurs at the load. The device cannot be reset until the connection to ground is restored. This is an important safety feature that is often overlooked until it is too late.
At present the National Electrical Code (NEC) does not require broad use of industrial GFCIs because of the perceived unreliability of previous products, but it does recommend the use of other protected devices and GFCIs for specific applications. As awareness of UL 943C-listed products increases, acceptance of GFCIs will also increase, and experts expect the NEC to be updated to require GFCI protection in more locations.
GFCIs for lower voltage applications (up to 240 V) have been on the market for a long time, but their use has not always been practical in industrial applications. GFCIs for higher voltage applications (480-600 V) did not exist until last year. Now that industrial GFCIs (See Figure 3) are available for equipment operating at this level of voltage, and are UL listed, the adoption of such protection is expected to grow.