What is a Short Circuit Study and why do it?
A
short circuit study is necessary to ensure that equipment is rated for the
available fault current of the system.
Breakers,
cables, and bus work all are rated for a maximum electrical current. Current above this value can cause failure of
the equipment, often with disastrous results.
As current flows through a conductor, magnetic forces also act on the
conductor. These forces can pull or push
on the conductor and surrounding magnetic materials. When the forces are strong enough, they may
pull apart support structures, or bend the conductors. This can create a short circuit in the
system.
A
short circuit study is also used as part of a proper breaker and relay
coordination study. The maximum short
circuit current expected at a location is used to determine the maximum value
for an instantaneous setting on a breaker or relay. If the setting is above this point, it will
never operate.
Because
the maximum short circuit current is different from one location to the next,
this value is also used to study coordination between devices. If a downstream device operates faster than
one upstream, it may not matter if at current levels above the downstream
maximum short circuit current. Because a large number of faults are short circuits, this
distinction can be critical for device coordination for expected high fault
current levels.
Short
circuit data in a substation helps to determine step and tough potential. This is important in phase to ground faults. By determining the maximum expected phase to
ground short circuit, if the ground impedance is known, the step and touch
potential at a substation can be determined.
This determination is used to ensure personnel safety in a substation
during a fault. When high currents flow
to and through the ground, the resistance between points just a couple of feet
apart can cause a large voltage difference.
By limiting this voltage difference through proper grounding and
bonding, personnel safety can be assured.
Of
course, all of this ignores the other reason why short circuit studies are
performed: they are required. NFPA 70, the National Electrical Code (NEC),
has several articles which mention the available short circuit current. In most of these instances, it is necessary
to determine the available fault current in order to determine the minimum
rating of the equipment. Articles 110.9
and 110.10:
110.9
Interrupting Rating. Equipment intended to interrupt current at fault
levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that is available at the line
terminals of the equipment. Equipment
intended to interrupt current at other than fault levels shall have an
interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.
110.10 Circuit Impedance, Short-Circuit Current
Ratings, and Other Characteristics. The
overcurrent protective devices, the total impedance, the equipment short circuit
current ratings, and other characteristics of the circuit to be protected shall
be selected and coordinated to permit the circuit protective devices used to
clear a fault to do so without extensive damage to the electrical equipment of
the circuit. This fault shall be assumed
to be either between two or more of the circuit conductors or between any circuit
conductor and the equipment grounding conductor(s) permitted in 250.118. Listed equipment applied in accordance with
their listing shall be considered to meet the requirements of this section.
The
short circuit study is necessary to determine the minimum short current rating
of the electrical equipment. While it
could be assumed that the fault levels are extremely high, and order high rated
equipment, this can quickly increase a project cost to many times the cost of a
study.
110.24 Available Fault Current.
(A) Field Marking. Service
equipment in other than dwelling units shall be legibly marked in the field
with the maximum available fault current. The field marking(s) shall include
the date the fault-current calculation was performed and be of sufficient durability to withstand the environment involved.
Informational
Note: The available fault-current marking(s) addressed in 110.24 is related to
required short circuit current ratings of equipment. NFPA 70E-2012, Standard
for Electrical Safety in the Workplace, provides
assistance in determining the severity of potential exposure, planning
safe work practices, and selecting personal protective equipment.
(B) Modifications. When
modifications to the electrical installation occur that affect the maximum
available fault current at the service, the maximum available fault current
shall be verified or recalculated as necessary to ensure the service equipment
ratings are sufficient for the maximum available fault
current at the line terminals of the equipment.
The required field marking(s) in 110.24(A) shall be adjusted to reflect
the new level of maximum available fault current.
Exception:
The field marking requirements in 110.24(A) and 110.24(B) shall not be required
in industrial installations where conditions of maintenance and supervision
ensure that only qualified persons service the equipment.
This
article in the NEC explicitly states that equipment must be marked with the
available fault current. This fault
current is determined by a short circuit study performed
by a qualified engineer. The exceptions
apply to dwelling units, and industrial installations with qualified
electricians. It still applies to
commercial units and some industrial locations.
The Institute of
Electrical and Electronic Engineers (IEEE) has many standards, guides, and recommended
practices that outline short circuit studies and how to perform them. The most complete would be the Violet Book,
IEEE 551, Recommended Practice for Calculating AC Short-Circuit Currents in
Industrial and Commercial Power Systems.
This book is being replaced in an effort by the IEEE by IEEE 3002.3,
IEEE Recommended Practice for Conducting Short-Circuit Studies and Analysis of
Industrial and Commercial Power Systems.
These documents address short circuit analysis in new and existing
systems. They talk about the methods and
assumptions which may be made in the calculations, including motor
contributions and other data requirements.
A
short circuit study should be performed on any electrical system for the safety
of the site occupants and the people who will have to operate or maintain the
equipment. A fault with current levels
higher than expected can have disastrous effects, such as fires and
explosions. Equipment can easily be
destroyed if it is not properly rated.
By performing a study before the equipment is installed, proper ratings
can be determined. This will help keep
equipment costs down and safety up.