SHORT-CIRCUITS
The term short-circuit
refers to the uncontrolled flow of electrical current
along unintentional paths. This situation usually
occurs as a consequence of the failure of electrical
insulation at one or more locations on a circuit.
Modern power systems are required
to supply their normal loads at a constant, stable
voltage. To meet this criterion, they are, inevitably,
also capable of delivering - for short periods of
time - short-circuit currents that are hundreds of
thousands of times their normal load current.
The uncontrolled release of such energy
during a short-circuit results in a violent explosion
that is a hazard to life, property, and the power
system itself. To minimize such consequences of a
short-circuit, it is imperative that the flow of the
short-circuit current be interrupted in as short a
time as possible. This can be accomplished with overcurrent
protective devices such as fuses, or relays and circuit
breakers.
However, it does require time for
these devices to detect the existence of a fault current,
and still more time for them to accomplish the total
interruption of a short-circuit current. In addition,
some systems operate with one or more automatic reclosures
of the breakers after the initial interruption.
Obviously, all of the cables that
are on the source side of a short-circuit must carry
the fault current from the time of the initial fault
until the overcurrent protective devices ultimately
interrupt the current. The temperature that a cable
conductor will attain as the result of the fault current
depends upon:
- The conductor temperature prior
to the fault.
- The magnitude of the fault current.
- The time the fault current is permitted
to flow.
To help the system designer plan for
this type of system emergency, power cables carry
an allowable short-circuit temperature rating. This
is the maximum allowable conductor temperature that
may be attained in the very short times that are usually
associated with system short-circuits. These times
are measured in cycles, normally on the basis of 60
Hz systems, i.e., a 16 cycle fault current will exist
for 16/60ths or 0.2667 of a second.
Most power cables that are rated for
90C continuous operation carry a 250C short-circuit
rating.
The magnitude of the short-circuit
current that a given cable circuit can carry within
its allowable temperature limitation depends upon
the time required for the fuses or circuit breakers
to interrupt the flow of the fault current. The greater
the time required to open the circuit, the less short-circuit
current a cable can tolerate without exceeding its
temperature limitation.
As mentioned above, the standard 250C
short-circuit ratings of cable applies for a time
period of seconds at most. In a laboratory test, a
length of 4/0 AWG copper 15 kV EP UniShield was current
loaded for 4 hours to 252C conductor temperature,
and then passed all factory voltage and corona level
tests. A similar cable insulated with XLPE would be
ruined long before 4 hours had elapsed.
