UL's new GFCI classes
Ground fault circuit interrupters (GFCIs) are available in three new classes, which electrical engineers should become familiar with.
- Understand UL’s new GFCI classes.
- Understand how GFCIs for 240 to 600 V applications differ from the familiar Class A GFCIs.
- Know where to apply these GFCIs to provide shock protection.
NFPA 70: National Electrical Code (NEC) mandates ground fault circuit interrupters (GFCIs) wherever electricity, water, and people are likely to meet. Their required use in residences, which dates to the 1970s, has saved a great many lives. But the use of household GFCIs has been limited in commercial and industrial buildings because of the voltages involved. GFCIs for residential use are listed as Class A devices under UL 943, and are limited to circuits at 240 V or less.
Of course, Class A GFCIs do find use in commercial buildings; NEC requires them on all 5-amp and 20-amp 125-V utility outlets used in locations such as restrooms; commercial kitchens; roofs; commercial garages; outdoor receptacles in the vicinity of hydromassage bathtubs, spas, hot tubs, and spas; elevator machine rooms and machinery or control spaces; in the vicinity of swimming pools; and many more places.
But not all equipment is powered by 120 or 125 Vac. Submersible pumps in large decorative fountains use integral horsepower motors running up to 460 V, while pumps in large commercial water slides may run to 500 hp or more and at 460 V or greater. Wastewater plants often use pumps running at these voltages. Other high-voltage equipment that presents an elevated risk of electric shock includes wet saws, processing equipment handling wet material, and arc welding receptacles. Shock risks are also present in wet work areas with large portable fans or heaters and in any area where machinery is subject to washdown cleaning. All these applications would benefit from GFCI protection, yet Class A GFCIs are unsuitable for these applications for a variety of technical reasons.
UL 943C classes
Fortunately, in 2009 UL published UL 943C that established three new classes of special purpose GFCIs intended for higher voltage:
- Class C: for use in circuits with no conductor more than 300 V to ground (i.e., systems where line-to-line voltage is 480 V or less) where reliable equipment grounding or double insulation is provided.
- Class D: for use in circuits with one or more conductors more than 300 V to ground (i.e., 600 V systems), and with specially sized, reliable grounding, to provide a low impedance path so that the voltage across the body during a fault does not exceed 150 V.
- Class E: for systems similar to Class D, but with special high-speed tripping required that eliminates the need for the oversized ground of Class D.
Class C, D, and E GFCIs trip at 20 mA rather than the 6 mA trip current mandated for Class A GFCIs. This increase in GFCI trip level is allowed by UL assuming the availability of a reliable ground in parallel with the body. During a fault, the grounding conductor will shunt the fault current around the body and cause the device to trip. This provides the let-go protection, while the 20 mA threshold provides protection against fibrillation. (If there is no grounding conductor, such as in two-wire household products, then the GFCI must provide both let-go and fibrillation protection, and a Class A device is required.) See Figure 1.
The danger caused by electric current passing through the human body depends on both current and time (see Figure 2). Currents of 0.5 mA or less (the green zone, AC-1) are harmless and generally imperceptible. Currents from 0.5 mA to 10 mA or more, depending on duration (the orange zone, AC-2) may cause involuntary muscle contractions and possible injury from striking nearby objects. Currents in the red zone, AC-3, will likely cause difficulty in breathing, while still higher currents cause an increasing likelihood of ventricular fibrillation, in which the heart loses its internal coordination and ceases to beat effectively, with a fatal outcome unless defibrillation is done immediately.
The trip curve published by UL for GFCIs was designed to keep current through the body to safe levels, prescribing GFCI trip times that decrease with increasing current, as defined by the equation , where T is in sec and I is in mA, and shown in Figure 3. Note that at currents greater than 300 mA, the delay is fixed at 20 msec.
Selecting and monitoring the grounding conductor
The importance of the grounding conductor in determining the class of GFCI to be used cannot be overstated. The purpose of the equipment ground is to keep the case or cabinet from reaching a dangerous voltage if an energized conductor faults to it. Because the grounding conductor is so important, UL 943C requires that Class C, D, and E GFCIs monitor its integrity and trip if it opens.
At voltages greater than 150 V, the resistance of the human body decreases dramatically and the fault current flowing through the body increases to unsafe levels requiring impossibly fast trip time. The purpose of the grounding conductor is to prevent the case or cabinet of a piece of equipment from exceeding 150 V if an energized conductor faults to it. The resulting current through the grounding conductor will also trip the GFCI to eliminate the danger of fibrillation.
If the system voltage is less than 150 V and the equipment is neither grounded nor double insulated, a Class A GFCI with 4 to 6 mA trip current is required to provide both let-go protection and protection against fibrillation.
If the system voltage is less than 150 V and the equipment is either grounded or double-insulated, a Class C GFCI, with 20 mA trip level, is allowed, as it needs only to provide protection against fibrillation, with let-go protection optional.
If the system voltage is between 150 V and 300 V, then either double insulation or a separate safety ground with the same gauge wire as the hot conductor is required; the purpose of the safety ground is to keep the equipment case or cabinet from reaching 150 V in case of a fault. In this case a Class C GFCI may be used to provide fibrillation protection; again, let-go-protection is optional.
If the system voltage is 300 V or greater, then the grounding conductor becomes more critical. Here UL allows what at first appear to be two choices. The first choice is to install a low-impedance grounding conductor made of larger gauge wire than the hot conductor, to prevent the touch voltage from exceeding 150 V (see Appendix A of UL 943C for a sample of the calculation to determine the required wire gauge). Under these conditions a Class D GFCI with a 20 mA trip level may be used. The second choice is to install a Class E GFCI with high-speed tripping. Such a unit must follow the same UL 943C trip curve required of Classes A, C, and D up to 300 mA, but at currents between 300 mA and a current defined by , where V is the line to ground voltage, it must change to a trip time curve defined by .
In practical terms, this means that in a 480-V corner-grounded delta system the trip time would have to be less than 1 msec, and no GFCI capable of so interrupting current that quickly has been invented. At this time no Class E GFCIs are commercially available.
Characteristics of special-purpose GFCIs
While UL published UL 943C in 2009, it was not until 2013 that UL Listed GFCIs became available. These devices work in circuits running 208, 240, 400, and 600 V, and have the trip curve shown in Figure 3. In accordance with UL 943C, they monitor the equipment ground conductor for continuity. To be listed, they must also pass a number of tests including high-resistance ground fault, grounded neutral, endurance, surge current, overvoltage, overcurrent, and short circuit.
The minimum short circuit interrupting capacity is 5000 A. Some units are now available with 10 times that rating, and can be used to provide overcurrent protection as well as personnel protection.
The advent of special-purpose ground-fault circuit interrupters listed under UL 943C has made it possible to provide reliable shock protection for workers and the public in places where it was not previously possible. Knowing that electric shock is often fatal, and that GFCI protection is now available for higher power applications, consulting-specifying engineers should consider adding it to their designs. Some industry experts believe that, in the future, the NEC will require GFCI protection in more and more applications.
Nehad El-Sherif is the technical product specialist for the protective relay products line for Littelfuse. El-Sherif has software and hardware design experience and has been involved in working on certification of various products with the Canadian Standards Association and UL. El-Sherif is a member of the UL STP 943, CSA C22.2 No.144 standard committee, IEEE IAS, IEEE PES, and the IEEE Working Group on Industrial and Commercial Power Systems Communication Based Protection and SCADA.