Circuit Breaker

CIRCUIT BREAKER

Electrical circuits breaker is a switching device which can be operated manually and automatically for the controlling and protection of electrical power system, respectively. The modern power system deals with a huge power network and huge numbers of associated electrical equipments. During short circuits fault or any other type of electrical fault, these equipments, as well as the power network, suffer a high stress of fault current, which in turn damage the equipment and networks permanently. For saving these equipment and the power networks, the fault current should be cleared from the system as quickly as possible. Again after the cleared, the system must come to its normal working condition as soon is possible for supplying reliable quality power to the receiving ends. The circuits breaker is the special device all the required switching operations during current carrying condition.

A circuits breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. The contacts can be opened manually or by remote control whenever desired. When a fault occurs in any part of the system, the trip coils of the breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuits.

The main types of circuits breakers are

(i) Miniature circuits breakers (MCB)

(ii) Earth leakage circuits breakers (ELCB) or Residual Current Breaker (RCCB)

(iii) Air blast Circuits Breaker (ACB)

(iv) Molded Case Circuits Breakers (MCCB)

(v) Vacuum Circuits Breaker (VCB)

(vi) SF₆ Circuits Breaker

1. Miniature Circuit Breaker (MCB)

Minimum circuits breakers are electromechanical devices which protect an electrical circuits from over currents. Over currents in an electrical circuits may results from short circuits overload, or faulty design. An MCB is better alternative than fuse, since it does not require replacement once an overload is detected. An MCB functions by interrupting the continuity of electrical flow through the circuits once a fault is detected. In simple terms, MCB is a switch which automatically turns off when the current flowing through it passes the maximum allowable limit. Generally MCB is designed to protect against over current and over temperature faults (over heating).

Working Principle

There are two contact ‒ one is fixed and the other is moveable. When the current exceeds the predefined limit, a solenoid forces the moveable contact to open (ie., disconnect from the fixed contact) and the MCB turns off, thereby stopping the current from flowing in the circuits.

Operation

An image of MCB is shown in figure (7.12) and internal parts of an MCB are shown in figure (7.13). It mainly consists of one bi‒ metallic strip, one trip coil and one hand operated on‒off lever. Electric current carrying path of a MCB is as follows‒ as follows - first left hand side power terminal‒then bimetallic strip ‒ then current coil - then moving contact - then fixed contact and - lastly right hand side power terminal, and all are arranged in series.

If circuits is overload for a long time, the bi‒metallic strip becomes over heated and deformed. This deformation of bi‒metallic strip causes displacement of latch point. The moving contact of the MCB is so arranged by means of spring, with this latch point, that a little displacement of latch causes releases of spring and makes the moving contact to move for opening the MCB. The current coil or trip coil placed in such a manner that during SC faults, the MMF of that coil causes its plunger to hit the the same latch point and force the latch to be displaced.

Hence, the MCB will open in the same manner. Again when operating lever of the MCB is operated by hand, that means when we make the MCB at off position manually, the same latch point is displaced as a result moving contact separated from fixed contact in same manner. So, whatever may be the operating mechanism, i.e., may be due to deformation of bi‒metallic strip or may be due to increased MMF of trip coil or may be due to manual operation actually the same latch point is displaced and the deformed spring is released, which is ultimately responsible for movement of the moving contact. When the moving contacts is separated from fixed contact, there may be a high chance of arc.

This are then goes up thorough the arc runner and enters into arc splitters and is finally quenched. When we switch on the MCB, we actually reset the displaced operating latch to its previous on position and make the MCB ready for another switch off or trip operation.

These are available in single pole, double pole, triple pole, and four pole versions with neutral poles, if required. The normal current ratings are available from 0.5‒63 A with a symmetrical short circuits rupturing capacity of 3‒10 kA, at a voltage level of 230/440 v. MCBs are generally designed to trip within 2.5 millisecond when an over current fault arises. In case of temperature rise or over heating it may take 2 seconds to 2 min. For the MCB to trip.

Advantages

(i) MCBs are replacing the re‒wireable switch i.e., fuse units for low power domestic and industrial applications.

(ii) The disadvantages of fuses, like low SC interrupting capacity (say 3 kA). Arc overcome with high SC breaking capacity of 10 KA.

(iii) MCB is combination of all three functions in a wiring system like switching, overload and short circuits protection. Overload protection can be obtained by using bi‒metallic strips where as shorts circuits protection can be obtained by using solenoid.

2. Earth Leakage Circuits Breaker (ELCB)

None of the protection devices like MCB, MCCB, etc. Can protect the human life against electric shocks or avoid fire due to leakage current. The human resistance noticeably drops with an increase in voltage. It also depends upon the duration of impressed voltage and drops with increase in time. As per IS code, a contact potential of 65 V is within tolerable limit of human body for 10 seconds, where as 250 V can be withstood by human body for 100 milliseconds. The actual effect of current thorough human body varies from person to person with reference to magnitude and duration. The body resistance at 10 V is assessed to be 19 kΩ for 1 second and 8 kΩ for 15 min. At 240 V, 3 to 3.6 kΩ for dry skin and 1‒1.2 kΩ for wet skin.

An Earth Leakage Circuits Breakers (ELCB) is a device used to directly detect currents leaking to earth from an installation and cut the power. There are two types of ELCBS:

(i) Voltage Earth Leakage Circuits Breaker (voltage‒ELCB)

(ii) Current Earth Leakage Circuits Breaker (Current‒ELCB)

(i) Voltage Earth Leakage Circuits Breaker (voltage ‒ELCB)

Voltage ‒ELCB is a voltage operated circuits breakers. The device will function when the current passes thorough the ELCB. Voltage‒ELCB contains relay coil and one end of the coil is connected to metallic load body and the other end is connected to ground wire as shown in figure (7.14). If the voltage of the equipment body rises (by touching phase to metal part or insulation failure of equipment), which could cause the difference between earth and load body voltage and the danger of electric shock will occur. This voltage difference will produce an electric current from the load metallic body and phase through the loop to the Earth.

When voltage on the equipment metallic body rises to danger level i.e., which exceed to 50 V, the flowing current through relay loop could move the relay contact by disconnecting the supply current avoid from any danger electric shock. The ELCB detects fault currents from line to the earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCB's sensing coil, it will switch off the power, and remain off until manually reset. A voltage

- sensing ELCB does not sense fault current from line to any other earthed body.

(ii) Current Earth Leakage Circuits Breaker (Current‒ELCB)

Current ‒ELCB is a current operated circuits breaker which is a commonly used ELCB. Current‒ELCB consists of a 3‒winding transformer, which has two primary windings and one secondary winding as shown in figure (7.15). Neutral and line wires act as the two primary windings. A wire wound coil is the secondary winding. The current thorough the secondary winding is zero at the balanced condition. In the balanced condition, the flux due to current through the phase wire will be neutralized by the current through the neutral wire, since the current which flows from the phase will be returned back to the neutral. When a fault occurs, a small current will flow to the ground also. This makes an unbalanced between line and neutral currents and creates an unbalanced magnetic filed. This induces a current through the secondary winding, which is connected to the sensing circuits. This will sense the leakage and send a signal to the tripping system and trips the contact.

3. Molded Case Circuits Breaker (MCCB)

Molded case circuits breakers are electromechanical devices which protect a circuits from over current and short circuits. They provide over current and short circuits protection for circuits ranging from 63A up to 3000 A. Their primary function are to provide a means manually open a circuits and automatically open a circuits under overload or short circuits conditions respectively. The over current, in an electrical circuits, may result from short circuits, overload of faulty design.

MCCB is an alternative to a fuse, since it does not require replacement once an overload is detected. Unlike a fuse, an MCCB can be easily reset after a fault and offer improved operational safety and convenience without incurring operating cost.

Molded case circuits breakers generally have a

(i) Thermal element for over current and

(ii) Magnetic element for short circuits release which has to operate faster.

The MCCBS are comprised of five major components such as molded case or frame operating mechanism, arc extinguishers, contacts and trip components as shown in figure (7.16) MCCB are manufactured such that the end user will not have access to internal workings of the over-current protection device. Generally constructed of two pieces of heavy‒duty electrically insulated plastic, these halves are riveted together to form the whole. Inside the plastic shell is series of thermal elements and a spring‒loaded trigger. When the thermal element gets too warm, from an over current situation, the spring trips, which in turn will shut off the electrical circuits.

Operating mechanism

At its core, the protection mechanism employed by MCCBS is based on the same physical principles used by all type of thermal - magnetic circuits breakers.

Overload protection is accomplished by means of a thermal mechanism. MCCBs have a bimetallic contact that expands and contacts in response to changes on temperature. Under normal operating conditions, the contact allows electric current through the MCCB. However as soon as the current exceeds the adjusted trip value, the contact will start to heat and expend until the circuits the circuit is interrupted. The thermal protection against overload is designed with a time delay to allow short duration over current, which is a normal part of operation for many devices. However, any over current conditions that last more than what is normally expected represent an overload, and the MCCB is tripped to protect the equipment and personnel.

On the other hand, fault protection is accomplished with electromagnetic induction, and the response is instant. Fault currents should be interrupted immediately, no matter if their duration is short or long.

Whenever a fault occurs, the extremely high current induces a magnetic field in a solenoid coil located inside the breaker ‒ this magnetic induction trips a contact and current it interrupted. As a complement to the magnetic protection mechanism, MCCBs have internal arc dissipation measure to facilitate interruption.

As with all types of circuits breakers, the MCCB includes a disconnection switch which is used to trip the breaker manually. It is used whenever the electric supply must be disconnected to carry out field work such as maintenance or equipment upgrades.

Applications

Molded case circuits circuits breakers can have very high current ratings, which allows them to be used in heavy duty applications such as main electric feeder protection, capacitor bank protection, generator protection, welding applications, low current application that require adjustable trip setting and motor protection.