Protective Devices for Circuits
In an electrical distribution system for a building, each electric service must have a means of disconnection, but it may not consist of more than six service switches.
Each service switch may disconnect service to a panelboard from which lighter feeders extend to other distribution points, up to the final branch circuits with the minimum size wire, No. 12. This panelboard contains switches with lower disconnecting ratings than that of the service switch and that serve as disconnecting means for the light feeders. The rating and type of each switch must correspond to the size and kind of load and the wire size.
There must also be in every circuit some protective device to open the circuit if there is an unexpected overload, such as a short circuit or a jammed motor that prolongs a high inrush current. These protective devices may be fuses combined with knife switches, or circuit breakers, which provide both functions in one device.
In addition, electrical systems should be protected against power surges caused by lightning strokes. See Art. 15.19.1.
Fuses in lighting and applicance circuits with loads up to 40 A may be the screwed plug type, with a metallic melting element behind a transparent top. For each rating, plug fuses are given different colors and the screw size is made intentionally different, to prevent errors.
Cartridge fuses are another type of fuse. They are cylindrical in form and are available in any size. They are classified for special purposes, and the rating is clearly marked on the cylinder. So-called HI-CAP fuses (high capacity) are used for fused service switches that have the capacity to interrupt very high short-circuit currents.
Short circuits in the heavy copper conductor immediately following a service
switch can be very high, because of the high power potential of the street transformers.
The currents may be 25,000 to 100,000 A. Upon inquiry, the utility will advise as to the maximum short-circuit current for an installation. The service switch fuse should be selected to suit this capacity.
Circuit Breakers
Circuit breakers operate on a different principle. A circuit breaker is essentially a switch that is provided with means to sense a short circuit or overload and then to open the circuit immediately. Circuit breakers have ratings that are equal to the current, A, that will cause the breaker to trip. When a circuit breaker is closed, a spring is compressed that provides the energy to trip or open the circuit breaker on overload. In its simplest form, a circuit breaker senses an overload by means of a bimetallic element that expands from the heat caused by excessive current. Also, it senses a short circuit by means of a solenoid or coil. In either case, an internal mechanism releases the spring to trip the breaker. After the cause of the trip has been removed, the circuit breaker is simply reset. More sophisticated circuit breakers use current-sensing coils and either microprocessor-based trip units or protective relays to initiate breaker trip.
Both fuses and circuit breakers have time-current characteristics; that is, both will operate in a predictable time for a given current, and both will operate more quickly for a higher value of current. The importance of this is that fuses and circuit breakers must be selected and their time-current characteristics coordinated so that, if a short circuit or overload occurs, only the fuse or circuit breaker directly upstream will operate. This selectivity will isolate the problem without causing an unnecessary outage elsewhere in the building.
Current limiters may also be used at the service connection. These are strips of metal that have a high rate of increase in electrical resistance when heated. If a short circuit occurs, this limits the flow of short-circuit current even before the short is cleared.
Current-Limiting Reactors. A coil with high inductive reactance may be placed in series with the service. If a heavy short-circuit current occurs, the impedance limits the current by temporarily storing energy in the magnetic field.
Protective Relays
There are several kinds of faults that can occur in an electrical system. Over- or undervoltage, reverse flow of power, and excessive currents are but a few. Protective relays are available for application at critical locations in the electrical system to protect against these faults. Originally, protective relays were, and most still are, intricate electromechanical devices. However, many solid-state devices have been introduced that match the performance and dependability of the electromechanical relays.
Protective relays do not directly trip a breaker; they close a contact to provide an electrical signal to the breaker trip circuit. When protective relays are used to trip a circuit breaker, it is always necessary to provide a source of electrical power, usually dc, from a battery bank at 48 V or 125 V, to provide tripping power.
