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This device is designed to open the
circuit by breaking the current flow when it exceeds a given value for a sufficient time. The fuse
element is designed to melt when the excessive current exists.
2. Circuit breaker
This term can be applied to a switch, but for circuit protection the device is designed to
operate automatically when abnormal currents such as short-circuits occur. Most work
electromechanically. Lighting circuits usually carry relatively low currents and thus the
components used are often referred to as Miniature Circuit Breakers (MCBs).
3. Residual Current Device (RCD)
An electro mechanical switching device automatically operated when the residual current
rises. It is normally used where a reduction to the risk of electrical shock is important, such as
portable equipment, where the connecting leads could be accidentally damaged, such as electric hand
tools.
Opposition to Current Flow
As discussed, opposition to current flow is called resistance. Another opposition to AC
current flow is called reactance. This opposition to current flow is caused by such devices as
inductors (motors), ballasts for fluorescent lamps, and capacitors. Like resistance, reactance is
measured in ohms.
Although resistance and reactance both oppose current flow, they do so in different ways and
in different amounts.
It is not necessary to understand exactly how inductors and capacitors impede current flow.
It is important, however, to know that the net effect of all resistance and reactance in a circuit
is called impedance which is given the symbol
"Z".
Circuit Losses
Every part of the electrical circuit has some resistance and this consumes electricity in a
non- productive way. To limit the amount of current flowing through a circuit a resistance can be
used, but with the normal AC supply a ballast will do the job more efficiently. For example it is
possible to run a 40W fluorescent tube in series with a 60W filament lamp, where the resistance of
the lamp controls the current in the circuit.
With an inductive ballast in place of the filament lamp, the same current limiting takes
place because of the reactance, and the ballast will consume less than 10W. When considering a
complete commercial or industrial installation the saving can be substantial.
Power Factor
Most electrical systems have two power components: productive and non-productive power. The
power needed to magnetise the coils and iron core of a motor, for example, is one type of non-
productive power. Total power is the sum of the two components.
The relationship between the ideas of productive, non-productive and total power can be more
easily understood from the analogy of a horse pulling a barge along a canal. See
Figure 1.25.
The total power is applied by the horse to the rope. The productive power pulls the barge
along the canal.
The non-productive power pulls the barge toward the bank and must be countered by the
rudder.
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