June 1992


Vol. 2, No. 3

Ground Fault Circuit Interrupters: Facts & Fiction

Ground Fault Circuit Interrupters (GFCIs) are devices designed to protect personnel from electrical shock hazards due to faults in electrical equipment or when working in wet or damp locations. GFCIs are not designed to provide circuit overload protection for equipment and wiring. That is the job of a fuse or circuit breaker. Conversely, circuit breakers and fuses are designed to provide overcurrent protection for equipment and wiring, and do not provide personnel protection.

How Do GFCIs Work?

GFCIs are designed to operate when they sense an imbalance in the current flow between the "phase" or "hot" conductor and the "neutral" conductor of an electrical circuit. In a typical GFCI, the phase and neutral conductors pass through a sensing transformer. If more current flows through one of the conductors than the other due to a fault a trip device is operated, and set of contacts opens and removes power from the circuit.

At What Levels Do GFCIs Work?

For "Class A" GFCIs used by Sandia, operation occurs when a current imbalance of 5± 1 milliamperes (mA) is sensed (according to American National Standards Institute (ANSI/Underwriters laboratories (UL) Standard 943).

A formula determines the allowable time to operate the trip device in the GFCI (how long you may get shocked). The operating time varies with the fault current level. At 6 mA of fault current, which is above the perception level (i.e., you will feel it!), the allowable time you may get shocked before the GFCI interrupts the circuit is 5.59 seconds. At 264 mA of fault current (a potentially lethal level), the allowable shock time drops to about 25 milliseconds.

Thus even GFCIs do not provide absolute protection from shock. They just reduce "shock" time to a "nonlethal" duration.

Note that these interrupting current levels are much lower than typical circuit breaker or fuse interrupting levels of 15 or 20 Ampere or higher. That is why you can receive a fatal shock of a few hundred mA without a GFCI and never trip a standard circuit breaker or blow a fuse.

Where Should GFCIs Be Used?

It is the Electrical Safety Committee's opinion that the level of protection provided to Sandia employees and onsite contractors should be at least the same level of protection from electrical shock as they would have in their own homes. Therefore, it is the committee position that the intent of the NEC is to provide personnel protection from hazardous shocks in the workplace as well as dwellings, garages, construction sites, etc.

The National Electric Code (NEC) requires the use of GFCIs on all 125 volt (rated) single-phase 15 and 20 amp receptacles in bathrooms and within 6 feet of sinks (NEC Article 210-8 and also DOE 6430.1A, Design Criteria, data 4-6-89), in garages and other automotive repair facilities (NEC Article 511-10), on construction sites (where an assured grounding program is not used, NEC Article 305-6), or outdoors (NEC Article 210-8 and DOE 6430.1A). In addition, DOE order 6430.1A requires that newly installed receptacles within 6 feet of outside doors have GFCI protection. All GFCI protected receptacles should be labeled.

GFCIs are also required when receptacles are located within 3 to 6 feet of a safety shower (no receptacles are allowed within 3 feet of a safety shower) and within 6 feet of an eyewash (Safety & Environmental Engineering Bulletin - Division 7853 (now 7953), Issue No. 1:3/91)

Where Should GFCIs Be Used?

GFCI protection should also be provided in static grounded areas where personnel may come in contact with ac-powered electrical equipment (DOE Explosive Safety Manual, May, 1990).

The intent of the NEC is that GFCI protection is required for circuits supplying portable electric hand tools (except for double insulated or battery powered tools) when they are used outdoors or in wet or damp indoor locations. Use of (1) portable GFCIs plugged into receptacles or (2) extension cords containing GFCIs is permissible where permanent GFCI protection is not provided.

While it is not a requirement to immediately change out all receptacles not meeting the NEC GFCI requirements, you should consider using portable GFCIs or extension cords containing GFCIs when using receptacles as specified above that are not GFCI protected.

How Often Should GFCIs Be Tested?

GFCIs should be trip tested using the test button on the unit at least once a month. Results and dates of tests should be recorded on a test record label (NFPA 70B). (Note: The Electrical Safety Committee is investigating the possibility of having test record labels available through JIT or your ES&H Coordinator. You will be advised when a source of labels has been established.) For portable GFCIs that are plugged into a receptacle, or are part of an extension cord, the trip test should be performed by the user each day prior to use. It is not necessary to record this daily (before use) test. If there is any doubt by the user concerning the GFCI operations, the user should press the test button before use.