Components of LT switch gear: Switch Fuse Unit (SFU), MCB, ELCB, MCCB.

Switchgear: The apparatus used for switching, controlling and protecting the electrical circuits and equipment is known as switchgear.

Classification of Switchgear: Switchgear can be classified on the basis of voltage level into the following: 

1. Low voltage (LV) Switchgear: up to 1KV 

2. Medium voltage (MV) Switchgear: 3 KV to 33 KV 

3. High voltage (HV) Switchgear: Above 33 KV.

Components of LT Switchgear: The term LT Switchgear includes low voltage Circuit Breakers, Switches, off load electrical isolators, HRC fuses, Earth Leakage Circuit Breaker, Miniature Circuit Breakers (MCB) and Molded Case Circuit Breakers (MCCB) etc i.e. all the accessories required to protect the LV system. The most common use of LV switchgear is in LV distribution board.

FUSE: Fuse is the simplest and cheapest device used for interrupting an electrical circuit under short circuit, or excessive overload, current magnitudes. The action of a fuse is based upon the heating effect of the electric circuit. The fuse has inverse time-current characteristics.

The part which actually melts and opens the circuit is known as the fuse element. Fuses have following advantages and disadvantages: 

Advantages: 

1. It is cheapest form of protection available. 

2. It needs no maintenance. 

3. Its operation is inherently completely automatic unlike a circuit breaker which requires an elaborate equipment for automatic action. 

4. It interrupts enormous short circuit currents without noise, flame, gas or smoke.

Disadvantages: 

1. Considerable time is lost in rewiring or replacing a fuse after operation. 

2. On heavy short circuits, discrimination between fuses in series cannot be obtained unless there is considerable differences in the relative sizes of the fuse concerned. 

3. The current-time characteristics of a fuse cannot always be correlated with that of the protected device.

Fuse is provided only in phase or live pole, never on neutral pole. 

FUSE UNITS: The various types of fuse units, most commonly available are: 

1. Round type fuse unit. 

2. Kit-Kat type fuse unit. 

3. Cartridge type fuse unit. 

4. HRC (High Rupturing Capacity) fuse units.

5. Semiconductor fuse units.

Switch Fuse Unit (SFU): Switch fuse is a combined unit and is known as an iron clad switch, being made of iron. It may be double pole for controlling single phase two-wire circuits or triple pole for controlling three-phase, 3-wire circuits or triple pole with neutral link for controlling 3-phase, 4-wire circuits. The respective switches are known as double pole iron clad (DPIC), triple pole iron clad (TPIC), and triple pole with neutral link iron clad (TPNIC) switches.

MCB: An MCB is an automatically operated electrical switch. Miniature circuit breakers are intended to prevent damage to an electrical circuit as a result of excess current. They are designed to trip during an overload or short circuit to protect against electrical faults and equipment failure.

MCBs are widely used as isolating components in domestic, commercial, and industrial settings. They form part of a broader family of more powerful circuit-breaking components.

Working:

Excess current causes the bimetallic strip within the MCB to heat, bend, and trip. This releases a switch that moves the electrical contact points apart to confine the arc (electrical discharge). The arc is divided and cooled by an insulated metal strip called the arc chute. The contacts close again once the fault has been fixed and the MCBs are reset.

An MCB is designed to protect against both overloading and short-circuiting. These are detected differently using separate processes. Overload protection is provided by the bimetallic strip using thermal operation, whereas short-circuit protection is provided by the tripping coil via electro-magnetic operation.

If the discharge is especially high, the MCB will trip (activate) very quickly – within one-tenth of a second. When the overcurrent is closer to the safety limits, the component will be slower to respond.

Earth Leakage Circuit Breaker (ELCB): It is a device that provides protection against earth leakage. These are of two types. 

1. Current operated earth leakage circuit breaker:

It is used when the product of the operating current in amperes and the earth-loop impedance in ohms does not exceed 40. such circuit breakers is used where consumer’s earthing terminal is connected to a suitable earth electrode. A current-operated earth leakage circuit breaker is applied to a 3-phase, 3-wire circuit. In normal condition when there is no earth leakage, the algebraic sum of the currents in the three coils of the current transformers is zero, and no current flows through the trip coil. In case of any earth leakage, the currents are unbalanced and the trip coil is energized and thus the circuit breaker is tripped.

2. Voltage operated earth leakage circuit breaker: 

It is suitable for use when the earth –loop impedance exceeds the values applicable to fuses or excess-current circuit breaker or to current operated earth leakage circuit breaker. When the voltage between the earth continuity conductor (ECC) and earth electrode rises to sufficient value, the trip coil will carry the required current to trip the circuit breaker. With such a circuit breaker the earthing lead between the trip coil and the earth electrode must be insulated; in addition, the earth electrode must be placed outside the resistance area of any other parallel earths which may exist. The tripping operation may be tested by means of a finger-operated test button which passes a predetermined current from the line wire through a high resistance to trip the coil and thus to earth. This test operation should be performed regularly.

Molded Case Circuit Breaker (MCCB) : It is a type of electrical protection device that can be used for a wide range of voltages, and frequencies of both 50 Hz and 60 Hz, the main distinctions between molded case and miniature circuit breaker are that MCCB can have current rating up to 2500 amperes, and its trip setting are normally adjustable. MCCBs are much larger than MCBs. 

An MCCB has three main functions: 

• Protection against overload. 

• Protection against electrical faults. 

• Switching a circuit ON and OFF. This is a less common function of circuit breakers, but they can be used for that purpose if there is not an adequate manual switch.

Operating Mechanism:

• Overload protection is accomplished by means of a thermal mechanism. MCCBs have a bimetallic contact which expands and contracts in response to changes in 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 expand until the circuit is interrupted. The thermal protection against overload is designed with a time delay to allow short duration overcurrent, which is a normal part of operation for many devices. However any over current conditions, that lasts 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 is interrupted. As a complement to the magnetic protection mechanism, MCCBs have internal arc dissipation measures to facilitate interruption.