Installing Circuit
Protection Devices

[lesson title]

INSTALLING GROUND-FAULT CIRCUIT INTERRUPTERS

Ground-fault circuit protection devices are required by the NEC in places where the user may come in contact with moisture, such as bathrooms, basements, garages, and outside area. Ground-fault protection is not overcurrent protection.

Leakage Paths

Ground faults can cause leakage paths of very small amounts of current and still be very dangerous. Leakage paths occur when current flow is altered by a short circuit. The leakage current will flow from the short to a ground point. A simple loose wire can cause a leakage path; for example, a loose hot wire in a light switch. Small amounts of electric current may be passing through the cover plate of the light switch if the hot wire touches it even very slightly.

Fatal Shocks

As little current a 0.1 ampere can cause a fatal shock This can only happen, however, if the victim is in contact with the ground or a grounded conductor. For example, if an electric hair dryer has a break in the insulation near the plug on the power cord, current can flow from that break to any ground point. In bathrooms, many exposed points, such as water faucets, metal sink, and decorative metal trim, are all possible ground points.

Shock factors. The degree of shock that a victim receives depends on two factors:

• The amount of current
• The length of time that the victim receive the current.

As stated previously, small amounts of current (0.1 to 0.2 ampere) can cause fatal shocks, but this small amount of current will not cause a circuit breaker to trip. If a user receives a shock of 0.1 to 0.2 ampere, it can cause the muscles to freeze, so the user will be unable to release the live conductor. Under these conditions, the current flow will continue to pass through the user's body and cause serious injury and perhaps even death.

Shock protection. GFCI devices have been developed to protect against shock hazard. Short circuits that create leakage paths of only 0.2 ampere will cause the GFCI to trip and cut off the power. The NEC requires that GFCI protection be installed on all 120-volt, 15-and 20-ampere receptacle circuits outdoors and in areas such as bathrooms and garages where the user may come in contact with moisture.

Neutral and Fault Paths

Under normal conditions, the current flow in any two-wire circuit is exactly the same in the hot wire and the neutral (white) wire. When a ground fault happens, current flow can follow two paths—the neutral path and the fault path. As in any parallel resistance circuit, the current now divides, with the heavier current flowing through the lower resistance.

In the example previously described, when the user provided a path for current flow from the insulation break to a ground point, current could flow through the device (hair dryer) to the neutral wire and also through the fault path (the user) to the ground point

This second path (fault path) required an increase in the hot-wire current flow. The current flow in the hot wire and the neutral wire was no longer equal. This imbalance in current flow resulted from a ground fault and was sensed by the GFCI. The circuit was interrupted before serious injury occurred.

Parts

The GFCI contains a differential transformer, a sensing-and-testing module, and a magnetic switch (Figure 1-17).

Differential transformer. The differential transformer consist of a circular, iron-core secondary winding. The circuit power conductors act as the primary power source of the differential transformer. These conductors pass through the center of the circular core. As current flows through the hot and neutral conductors, a magnetic field is created around each conductor. The strengths of the opposing magnetic fields are equal and remain in balance. The fields cancel, and no current flows in the secondary.

Sensing-and-testing module. When the current in the hot wire becomes greater than the current in the neutral wire, the field of the hot wire increases, and current is induced in the differential secondary. This secondary output is sensed and amplified by the sensing-and-testing module.

Magnetic switch. The module output activates the magnetic switch that cuts off the power to the load. The GFCI also contains a test button to check the operation of the module and the switch.

Types

There are three basic types of GFCI. All have two common features--a test button and a reset button. The test button simulates a leakage condition and ensures that proper tripping or turnoff occurs. The reset button restores current flow after a test shutoff or an actual ground-fault shutoff.

Plug-in. The plug-in GFCI provides the simplest form of ground-fault protection. It consists of a small, rectangular unit with plug prongs on the back side (Figure 1-18).

Figure 1-17.   Components of a GFCI

Figure 1-18.   Plug-in GFCI

The front of this unit contain the test and reset buttons and either one or two three-prong receptacles. This GFCI is available for both two-wire, 120-volt and three-wire, 240-volt circuits in current ratings up to 30 amperes. It requires no special installation. It simply plugs into a receptacle. It has the advantage of simplicity and portability. If many receptacles require ground-fault protection, using this unit at every receptacle would be quite costly. In such a case, other methods of providing ground-fault protection should be considered.

Receptacle. To provide ground-fault protection for several receptacles on the same circuit, install a receptacle GFCI unit in the electrical box instead of a standard receptacle (Figure 1-19).

This provides ground-fault protection not only for the devices plugged into the GFCI receptacle but for all devices plugged into receptacles between the GFCI receptacle and the end of the branch circuit. This GFCI receptacle is also known as a feed-through unit. A receptacle GFCI can fit any electrical box 1 1/2 inches deep or deeper. It is available for two-wire, 120-volt circuits of 15 or 20 amperes.

Figure 1-19.   Receptacle GFCI

Combination. For ground-fault protection on any circuit, a combination GFCI unit is made to include both circuit-breaker and ground-fault protection in a single unit. This unit is installed into a service panel (Figure 1-20).

This GFCI has a white pigtail lead that must be connected to the neutral bus bar. The neutral and hot wires of the circuit must be connected to the GFCI unit. A combination GFCI is manufactured for two-wire, 120-volt and three-wire, 240-volt circuits in 15 to 30 ampere ratings.

Figure 1-20. Service-panel GFCI

Copyright © 2005, SweetHaven Publishing Services
All Rights Reserved.