Circuit breakers

Circuit breakers for over current protection

The circuit breaker is a device for making and breaking a circuit (under normal and abnormal conditions). A
circuit breaker is selected for a particular duty taking the following into consideration (a) the normal current it will
have to carry and (b) the amount of current which the supply system will feed into the circuit under a fault (which
current the circuit breaker will have to interrupt without damage to itself). It is able to provide a more accurate
degree of over current protection than that normally provided by either semi-enclosed or cartridge fuses.
The circuit breaker has a mechanism which, when it is in the closed position, holds the contacts together. The
contacts are separated when the release mechanism of the circuit breaker is operated by hand or automatically.
Miniature Circuit Breakers (mcb), which are commonly used in domestic installations, incorporate most of the
features of the circuit breaker in a compact form and are being fitted in place of fuses in consumer units in the home
or office. An MCB eliminates the cost of fuse replacement and may be used as a switch for isolating circuits.
In the mcb, the automatic operation is by magnetic or thermal means. The reason for the two characteristics is to
have proper operation during both short circuit and overload conditions


Circuit Breakers for earth leakage circuit protection:

These are used to detect electrical faults to earth in equipment and to clear the fault by tripping and may be
classified into two types:
Voltage operated protection −Earth Leakage Circuit Breaker (ELCB)
Current operated protection −Residual Current Circuit Breakers (RCCB) or Residual Current Device (RCD)
The earth leakage protection device is commonly known as Trip Switch by electricians.

Earth Leakage Circuit Breaker (ELCB):

It is to be noted that for the proper operation of the ELCB, two
earth terminals are required. These are the frame earth to which
all non-conducting metallic parts of equipment are connected, and
the ELCB reference earth. The ELCB will normally operate when
the voltage across the coil, which corresponds to the voltage of
the frame earth with respect to the reference earth, exceeds about
40 V. [Note: Up to about 50V has been traditionally considered
as a safe voltage. However, it is now known that what is
important is the current that may pass through the human body
rather than the voltage, and that too is time dependent. Thus the
RCCB is now preferred to the ELCB.

Residual Current Circuit Breaker (RCCB):

The operation is based on a fault current, causing a difference
between the line current and the neutral current (the difference
need not actually flow to earth but back to the circuit through
an unplanned path). This difference is used to energize the
solenoid, which causes the switch to open. Under normal
operating conditions, two identical windings, m1 and m2, will
carry the main current. Since the currents are equal and
opposite through the two windings, there is mmf balance and
there will be no induced emf on the detector winding. Thus the
operating coil will not be energized. However, in case of a
fault the line and neutral currents will not equal and the trip
coil will be energized due to the induced currents in the
detector winding.
In both the ELCB and the RCCB, a test switch ‘T’ is provided to create an artificial fault for test purposes.