Building Design and Construction Handbook - Merritt - Ventech!

ELECTRICAL SYSTEMS 15.25
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.
15.7.5 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.
15.7.6 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