By Mike Holt, Published in EC&M Magazine
Available short-circuit current (SCA) is the current in amperes that is available
at a given point in the electrical system. This available short current is first determined at the
secondary terminals of the utility transformer. Thereafter the available short-circuit current is
calculated at the terminals of the service equipment, branch circuit panel and branch circuit load.
The available short-circuit current is different at each point of the electrical
system; it is highest at the utility transformer and lowest at the branch circuit load. The available
short-circuit current is dependent on the impedance of the circuit, which increases downstream from
the utility transformer. The greater the circuit impedance (utility transformer and the additive impedances
of the circuit conductors) the lower the available short-circuit current.
Factors that impact the available short-circuit current at the utility
transformer include the system voltage, the transformer kVA rating and itís impedance (as expressed
in a percentage). Properties that impact the impedance of the circuit include the conductor material
(copper versus aluminum), the conductor size, and itís length.
Authorís Comment: The impedance of the circuit increases the further
from the utility transformer, therefor the available short-circuit current is lower downstream from
the utility transformer.
Interrupting Rating. Overcurrent protection devices such as circuit breakers and fuses are intended
to interrupt the circuit and they must have an ampere interrupting rating (AIR) sufficient for the
available short-circuit current in accordance with Sections 110-9 and 240-1.† Unless marked otherwise,
the ampere interrupting rating for branch-circuit circuit breakers is 5,000 ampere [240-83(c)] and
10,000 ampere for
† branch-circuit fuses [240-60(c)].
Extremely high values of current flow (caused by short-circuits or line-to-ground
faults) produce tremendous destructive thermal and magnetic forces. If the circuit overcurrent protection
device is not rated to interrupt the current at the available fault values, it could explode while
attempting to clear the fault. Naturally this can cause serious injury, death as well as property
Protection of Electrical Components. In addition to interrupting rating for overcurrent devices, electrical
equipment, components, and circuit conductors must have a short-circuit current (withstand) rating
that will permit the circuit overcurrent protective device to clear a fault without extensive damage
to any of the components of the electrical system [110-9, 110-10, 250-2(d), 250-90, 250-96(a) and
Table 250-122 Note].
If the available short-circuit current exceeds the equipment/conductor
short-circuit current rating, then the thermal and magnetic forces can cause the equipment to explode
and/or the circuit conductors as well as grounding conductors to vaporize. The only solution to the
problem of excessive available fault current is to
(1) Install equipment that has a higher short-circuit
(2) Protect the components of the circuit by a current-limiting
protection device such as a fast-clearing fuse, which can reduce the let-thru energy.
Authorís Comment: For more information on how to properly protect electrical equipment, click here.