Introduction to Short Circuit Analysis
A. Bhatia, B.E.
The protection for an electrical system should not only be safe under all service conditions but, to insure continuity of service, it should be selectively coordinated as well. Even the most sophisticated and well-designed facilities have experienced the effects of electrical system failure or misoperation. Since unplanned outages can cost millions of dollars in lost production, information, and customers, it pays to explore how outages typically occur and to better understand how you can prevent them.
The purpose of
this course 4-hour course is to gain a basic understanding of short circuit
The course includes
a glossary and multiple-choice quiz at the end, which is designed to enhance
the understanding of the course materials.
This course will help you understand the following questions:
This course is aimed at students, electrical engineers, facility managers, architects, H &S professionals, O&M personnel, energy auditors, technical and sales representatives.
A short circuit
is an accidental path of low resistance which passes an abnormally high amount
of current. At an instant short circuit occurs, a very low impedance path is
created through which the full system voltage is applied. The resultant current
will be many times greater than the normal circuit current and if the circuit
is not opened and the current interrupted, extensive damage can occur. To protect
from adverse affects of short circuits, it is important to estimate or calculate
the value of prospective current likely to occur under short circuit conditions
and ensure that the protective devices provided to interrupt that current are
rated to withstand and interrupt it.
This course discusses some of these aspects followed by an example on how to calculate short circuit currents.
The course content is in a PDF file Introduction to Short Circuit Analysis. You need to open or download this document to study this course.
A short circuit
results in excessive current flow in the power source through the 'short,' and
may even cause the power source to be destroyed. In electrical devices, unintentional
short circuits are usually caused when a wire's insulation breaks down, or when
another conducting material (such as water) is introduced, allowing charge to
flow along a different path than the one intended. A short circuit may be in
a direct- or alternating-current (DC or AC) circuit.
Damage from short circuits can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, which disconnect the power in reaction to excessive current. If a fuse is in the supply circuit, it will do its job and blow out, thus opening the circuit and stopping the current flow. If a circuit breaker is in the supply circuit, it will trip and open the circuit.
Overload protection must be chosen according to the maximum prospective short circuit current in a circuit. For example, large home appliances (such as clothes dryers) typically draw 10 to 20 amperes, so it is common for them to be protected by 20 - 30 ampere circuit breakers, whereas lighting circuits typically draw less than 10 amperes and are protected by 10 - 15 ampere breakers. Wire sizes are specified in building and electrical codes, and must be carefully chosen for their specific application to ensure safe operation in conjunction with the overload protection.
Electrical switchgear may be rated on a symmetrical current basis or an asymmetrical basis. It may be necessary therefore to determine other short circuit values such as the "average asymmetrical value", the "maximum RMS asymmetrical value" or even the "peak value" of the short circuit current.
Short circuit analysis of facility electrical systems should be performed on 3-phase bolted condition since this will cause highest fault currents. Periodically, the short- circuit analysis should be reviewed to ensure that modifications to the facility electrical system and changes in equipment over time do not result in potential fault currents which exceed device interrupting ratings. Literature containing a detailed discussion of short circuit capacity calculations is available within the electrical power transmission industry. Considering the fact that the cost of a short circuit and coordination study is typically a small fraction of the electrical system cost, it's a wise investment that can pay dividends in the form of increased safety and availability.
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.