does not open immediately when an overload occurs. The bimetallic element requires a short time (length depends on the size of the overload) to respond to the heat generated by the overload current.
A magnetic type of circuit breaker responds instantaneously when an excess of current flows through the breaker. A small electromagnet is used to actuate the breaker mechanism. Whenever a predetermined amount of current flows through the electromagnet, enough magnetic flux is created to attract a small armature. As the armature moves, the breaker mechanism trips and opens the circuit.
The thermal-magnetic circuit breaker, as the name implies, combines the features of both the thermal and the magnetic types. Of the three, the thermal-magnetic circuit breaker is preferred for general use. A small overload actuates the bimetallic strip to open the circuit on a time delay, while a large overload or short circuit actuates the magnetic trip to open the circuit instantaneously. Circuit breakers are rated in amperes and volts the same as fuses and you select them on the same basis. Circuit breakers are sealed units and no attempt should be made to repair them or to adjust the ampere capacity. A defective breaker must be removed and replaced.
Circuit breakers that are to be used in circuits that may pose an added hazard to the user are made with an extra safety feature. This breaker is called a ground fault circuit interrupter (GFCI). It is a thermal-magnetic breaker with an additional internal circuit that detects a current leak from the hot wire to ground and opens the breaker if that current reaches a set amount. This leakage cannot be more than 5 (±1) milliamperes (thousandths of an ampere) to ground. Most of these breakers have a test button that can be used to check the GFCI to see if it will trip when there is a fault.
To install the GFCI, you connect the circuit hot wire to the breaker the same as you do on a standard breaker. The circuit neutral is connected to another terminal on the GFCI instead of to the neutral bar in the panel. The GFCI comes with an attached white neutral wire, which you then connect to the neutral bar. The NEC© requires that GFCIs be installed for several circuits used in the home. These circuits include ALL 120-volt, single- phase, 15- and 20-ampere receptacles in bathrooms, garages, and outdoors. GFCIs may be used elsewhere when there is a need for the added protection.
Now that we have discussed the various types of panelboards, fuses, and circuit breakers, we need to discuss panelboard connections. Once the circuits have all been brought into the cabinet, the panelboard can be mounted in the cabinet. Also, the neutral bar and the equipment ground bar are attached to the cabinet. The ground bar must be bonded to the cabinet by either a bonding jumper or the more common method of running a screw through the bar into the cabinet. The equipment ground bar and the neutral bar are not bonded together unless the panelboard also serves as the service equipment.
Quite often the panelboard is not connected until the interior wiring is done and the receptacles, switches, and fixtures have been installed. The method of attaching circuit conductors is based on conductor size and type of terminals on the panelboard. Small conductors, No. 10 and smaller, are normally looped around a screw type of terminal. Larger conductors may need to have terminal lugs, attached so the connection can be made to screw terminals. Pressure types of terminals are often provided for larger conductors, neutral conductors, and equipment- grounding conductors.
Conductors should be connected in a neat and professional manner. In many cases, conductors are connected with little excess wire. Conductors brought in through the sides of the cabinet are connected directly to the overcurrent device. Those brought in from the top or bottom of the cabinet are bent neatly opposite the fuse or circuit breaker to which they are to be attached and cut just long enough to make a good connection, as shown in figure 5-66. However, many experienced electricians feel that this system of connecting conductors is not necessarily the best, even though it presents the most uncluttered look and leaves more space around each conductor. These electricians usually try to leave an end on each conductor that is equal to the height plus the width of the cabinet. Each conductor is run along the panel and looped back 180 degrees before being connected to its fuse or circuit breaker. This method is shown in figure 5-67. Little added material is needed, and the extra length on the conductor permits it to be switched to another terminal on the panel if desired. Also, in the case of conductor breakage near the terminal, the conductor can be reconnected easily.
The ungrounded conductors in a fuse panelboard are connected directly to terminals on the bus bars. In a circuit breaker panelboard, the underground conductors are usually connected to the circuit breaker. The circuit breaker is then inserted in the panelboard. In most cases, the breaker is snapped into place and is held by spring tension. Sometimes breakers are held in the panelboard by a screw.Continue Reading