MCB Introduction
Miniature Circuit Breakers have precision formed moulded case & cover of flame retardant high strength thermo plastic material having high melting point, low water absorption, high dielectric strength and temperature with stand.
The Switching mechanism is independent, manual, and is trip free, i.e., the breaker trips internally even if the operating knob is held in ON position.
The Contact mechanism comprises of fixed and moving contacts made of silver graphite for reliability, long life and anti-weld properties.
The Arc Extinguishing Device comprises of 10-15 plates arc chute. The arc under the influence of the magnetic field and arc guide is moved into the arc chute where it is rapidly split and quenched.
The tripping mechanism is Thermal Magnetic type.
Thermal Operation
The thermal operation provides protection from moderate overloads. Under overload condition a thermo-metal element (bimetallic strip) deflects until it operates a latching mechanism allowing the main contacts to open.
Magnetic operation
In magnetic operation, large overloads or short circuit current actuates a solenoid causing a plunger to strike the latching mechanism rapidly opening the main contacts.
Internal view
Characteristics curves
| |
Thermal Tripping |
Magnetic Tripping |
| As per |
No tripping |
Tripping |
Time |
Hold |
Trip |
Time |
| IS:8828:96 |
current |
current |
Limits |
current |
current |
Limits |
| |
I1 |
I2 |
t |
I4 |
I5 |
t |
| B Curve |
1.13*IN |
|
≥1h |
3*IN |
|
≥0.1s |
| |
|
1.45*IN |
＜1h |
|
5*IN |
＜0.1s |
| C Curve |
1.13*IN |
|
≥1h |
5*IN |
|
≥0.1s |
| |
|
1.45*IN |
＜1h |
|
10*IN |
＜0.1s |
| D Curve |
1.13*IN |
|
≥1h |
10*IN |
|
≥0.1s |
| |
|
1.45*IN |
＜1h |
|
20*IN |
＜0.1s |
| l3=2.55*IN |
1s ＜ t ＜ 60s for IN ＜ 32A 1s ＜ t ＜ 120s for IN ≥ 32A |
Tripping characteristics
Based on the Tripping Characteristics, MCBs are available in ’B’, ’C’ and ’D’ curve to suit different types of applications.
’B’ Curve: for protection of electrical circuits with equipment that does not cause surge current (lighting and distribution circuits)Short circuit release is set to (3 - 5) In
’C’ Curve: for protection of electrical circuits with equipment that causes surge current (inductive loads and motor circuits)Short circuit release is set to (5 - 10) In
’D’ Curve: for protection of electrical circuits which causes high inrush current, typically 12-15 times the thermal rated current (transformers, X-ray machines etc.) Short circuit release is set to (10 - 20) In | |
