In this article we will discuss about the general characteristics of electrical apparatus.
All electrical machines, apparatus and other forms of electrical equipment must satisfy two main requirements – they must be able to operate continuously under normal service conditions and must be able to withstand short-time over-currents and over-voltages such as may arise during emergent conditions.
For operation of electrical machines and apparatus with full reliability in normal operating conditions, the following two requirements must be met.
Rated current ≥ Actual load current …(1.1)
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and Rated voltage ≥ Working voltage of the piece of equipment …(1.2)
Nominal or rated current is that maximum current with which a given piece of electrical apparatus or machine will be able to operate continuously throughout its service life (provided the equipment is rated for continuous operation).
When a piece of apparatus or machine operates with a load current that does not exceed its current rating, the temperature rise of any part will never exceed the permissible limits, thus providing continuous and reliable operation.
The rated voltage of a piece of electrical apparatus or a machine is the value of the line voltage given on its name plate, this value being equal to the nominal line voltage of the circuit it must operate in. The rated voltage must be selected so that the maximum working voltage during operation does not exceed rated voltage by more than 10-15 per cent.
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Owing to the fact that conditions under which the insulation of equipment intended for operation in the open air are much more severe than that of the equipment which is installed within enclosed premises, due to atmospheric precipitations, corrosive chemical deposits, fly ash and dust, electrical apparatus and machines are designed for either indoor or outdoor service.
All electrical installations and circuits are subject to the danger of a fault leading to short circuiting and flow of current in the equipment greatly exceeding the rated value.
In normal operating conditions all the circuit elements of an electrical system carry currents, whose magnitude depends upon the value of the generator voltage and the effective impedances of all the power transmission/distribution system elements, including the impedances of the loads usually relatively large.
However, when, due to a fault (known as a short circuit) the conductors of different phases come into contact with each other in a power line, power transformer, or any other circuit element, part of the impedance of the system will be shunted out of the circuit. This reduces the effective impedance of the system while the current rises. The lesser the effective impedance in a faulty circuit, the greater the short circuit current.
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Short-circuit currents are harmful for two reasons-the first is that even a short-time flow of heavy current will overheat the equipment, the second is that the flow of short circuit currents through the current carrying parts produces forces of electro-dynamic interaction which may destroy or damage the equipment.
This is why all the elements of any electrical installation are designed and selected for a thermal and a dynamic stability sufficient to withstand the largest possible flow of short circuit current that may occur in the given installation.
The severest conditions in service are those of the switching devices during a short circuit. These devices are circuit breakers and the fuses, whose function is to interrupt the fault current within a short period of time (0.05 – 0.3 second) and thereby switch out the faulty section.
The ability of a switching device to interrupt a short circuit current is expressed by its rated breaking current, the maximum value of the current it can safely interrupt at the normal service voltage of the installation in which it is designed to operate.
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For satisfactory operation of a circuit-breaking device the following condition should be fulfilled:
Rated breaking current ≥ Maximum current flowing through the circuit during the short circuit …(1.3)
The ability of a switching device to interrupt short circuits is expressed by their rated MVA breaking capacity.
Rated breaking MVA = √3 x rated breaking current in kA x rated voltage of the given device in kV …(1.4)
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Another condition to be fulfilled in order that a switching device will operate reliably is that Rated breaking MVA ≥ Maximum MVA developed in the circuit during the short circuit-
(also called the fault power) …(1.5)
The fault power is given as –
Smax sc = √3 x maximum value of short circuit current in kA x rated voltage of the given installation in kV …(1.6)