Basics of MCCB

The traditional molded-case circuit breaker uses electromechanical (thermal magnetic) trip units that may be fixed or interchangeable. An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device. Both these devices act mechanically on the trip mechanism.

Depending upon the application and required protection, an MCCB will use one or a combination of different trip elements that protect against the thermal overload, short circuit and ground fault.

Thermal overload, in an overload condition, there’s a temperature buildup between the insulation and conductor. If left unchecked, the insulation’s life will drastically reduce, ultimately resulting in a short circuit. This heat is a function of I²RT.

Short-circuit condition: Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system. This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system. To stop this current flow quickly so that major damage can be prevented, the short circuit element of an MCCB is used.

Ground fault condition: A ground fault actually is a type of short circuit, only it’s phase-to-ground, which probably is the most common type of fault on low-voltage system (600 V or less). Today’s modern electronic CB has the ground fault protection as an integral part of the trip unit.Overload trip action: Overload or thermal trip action uses a piece of bimetal heated by the load current. This bimetal is actually two strips of metal bonded together, with each having a  different thermal rate of heat expansion. They are factory-calibrated and not field-adjustable.

Short-circuit trip action: Short-circuit trip action uses an electromagnet having a winding that’s in series with the load current. When a short circuit occurs, the current flowing through the circuit conductor causes the magnetic field strength of the electromagnet to increase rapidly and attract the armature. When this happens, the armature rotates the trip bar, causing the CB to trip.

The only time delay factor involves the time it takes for the contacts to physically open and extinguish the arc, this usually is less than one cycle.

Magnetic elements are either fixed or adjustable, depending upon the type of CB and frame size. For example, most thermal magnetic breakers above the 150 A frame size have adjustable magnetic trips.

A thermal magnetic trip unit is best suited to most general purpose applications as it’s temperature sensitive and automatically will follow safe cable and equipment loadings. These loadings will vary With ambient temperatures. Thermal magnetic units do not trip if the overload is not dangerous, but will trip instantly with heavy short-circuit currents.

Electronic trip units: Electronic trip units typically consist of a current transformer (CT) for each phase, a printed circuit board, and a shunt trip. The CTs monitor current and reduce it to the required ratio for direct input into the printed circuit board, the brains of the electronic trip unit. The circuit-board then interprets current flow information, makes trip decisions on predetermined parameters, and tells the shunt trip unit to the trip the breaker.

Sn	Fuse	Sn	MCB
1	Action prompt and quick	1	Action takes short time.
2	Replacement of element is required.	2	Resetting is done.
3	Simple construction.	3	Construction is complicated.
4	Low cost.	4	Higher cost.
5	Replacement manually.	5	Resetting or automatic resetting.
6	No other protection.	6	Protection against different faults.
7	Used in old house connections.	7	Now universally used for domestic as well as industrial purposes.

EARTH LEAKAGE CIRCUIT BREAKER (ELCB)

It is used for the following purpose.

Many electrical installations have a relatively high earth impedance. This may be due to the use of a local earth rod (TT systems), or to dry local ground conditions.

These installations have a major problem. If no ELCB or RCD is used. During live to earth fault current, two conditions occur. Because earth impedance is high:

1.    No enough current exists to trip a fuse or circuit breaker, so the condition persists un-cleared indefinitely.

2.    The high impedance earth can not keep the voltage of all exposed CPC connected metal work to a safe voltage – all such metal work may rise to close to live conductor voltage.

This is a dangerous conditions, and was a safety risk in historic electrical installations. The ELCB makes such installations much safer by cutting the power if these dangerous conditions occur.

History of ELCB: ELCBs were mainly used on TT earthing systems. Now-a-days, ELCBs have been mostly replaced by Residual-Current Devices (RCDs). However, many ELCBs are still in use.

ELCBs responded to sine wave fault currents, but not to rectified fault current. Over time, filtering against nuisance trips has also improved. Early ELCBs thus offer a little less safety and higher risk of nuisance trip. The ability to distinguish between a fault condition and non-risk conditions is called discrimination.

Types of ELCB: There are two types of ELCB.

(1)  Voltage operated,  (2) Current operated.

1. Voltage operated ELCBs were introduced in the early 20^th century, and provided a major advance in safety for mains electrical supplies with inadequate earth impedance. V-ELCBs have been in widespread use since then, and many are still in operation.

2. Current-operated ELCBs are generally known today as RCDs (Residual Current Device). These also protect against earth leakage, though the details and method of operation are different.

When the term ELCB is used it usually means a voltage-operated device. Similar devices that are current operated are called Residual-current devices.

Connection of ELCB: The earth circuit is modified when an ELCB is used; the connection to the earth rod is passed through the ELCB by connecting to its two earth terminals. One terminal goes to the installation earth CPC (Circuit Protective Conductor, aka earth wire), and the other to the earth rod (or sometimes other type of earth connection). Thus, the earth circuit passes through the ELCB’s sense coil.

Operation of ELCB: An ELCB is a specialized type of latching relay that has a building’s incoming mains power connected through its switching contacts so that the ELCB disconnects the power in an earth leakage (unsafe) condition.

The ELCB detects fault current from live (hot) to the earth (ground) wire within the installation it protect. If sufficient voltage appears across the EWLCG’s sense soil, it will switch off the power, and remain off until manually reset. An ELCB however, does not sense fault currents from live to any other earthed body.

Advantages of ELCB: ELCBs have one advantage over RCDs: They are less sensitive to fault conditions, therefore they have less nuisance trips. (This does not mean they always do, as practical performance depends on installation details and the discrimination enhancing filtering in the  ELCB). Therefore, by electrically separating cable armour from cable CPC, an ELCB can be arranged to protect against cable damage only, and not trip on faults in down line installations.

Disadvantages of ELCB: ELCBs have the some disadvantages:

1.    They do not detect faults that don’t pass current through the CPC to the earth rod.

2.    They do not allow a single building system to be easily split into multiple sections with independent fault protection, because earthing systems are usually bonded to pipe-work.

3.    They may be tripped by external voltages from something connected to the earthing system such as metal pipes, a TN-S earth or a TN-C-S combined neutral and earth

4.    As with RCDs, electrically leaky appliances such as some water heaters, washing machines and cookers may cause the ELCB to trip.

5.    ELCBs introduce additional resistance and an additional point of failure into the earthing system.

Earth bypassing: It is not unusual for ELCB protected installation to have a second unintentional connection to earth somewhere, one that does not pass through the ELCB sense coil. This can occur via metal pipe-work in contact with the ground, metal structural framework, outdoor appliance in contact with soil, and so on.

When this occurs, fault current may pass to earth without being sensed by the ELCB. Despite this, perhaps counter intuitively, the operation of the ELCB is not compromised. The purpose of the ELCB is to prevent earthed metal work rising to a dangerous voltage during fault conditions.

How the ELCB Works When Earth Faults Occur?

If there is no leakage of electricity in the equipment, vector quantity of the current flowing is each phase becomes I_a + I_b + I_c = 0, and no voltage will be induced in the secondary winding of the zero-phase current transformer (some results even when the current of each phase is not in balance). However, if a ground fault current I_g, should run due to deterioration of the insulation of the equipment and circuit, vector quantity of the current will be I_a + I_b + I_c +I_g, thus including a voltage equivalent to I_g in the secondary winding of the ZCT. On the other hand, this voltage is so little that it cannot directly trip the breaker. Consequently, it is fed to the amplifier. Through amplification, it excites the shunt trip coil and activates breaking motion of the breaker.

RESIDUAL CURRENT CIRCUIT BREAKER (RCCB)

Earth leakage is an electrical hazard, which is responsible for electric shock and fire risk. Earth leakage and its associated hazard can be prevented by the use of Residual Current Circuit Breaker (RCCB), ALSO POPULARLY KNOWN AS THE Earth Leakage Circuit Breaker.

Principle of Working: The RCCB works on he current balance principle. The supply conductors, i.e. the phase and the neutral, are passed through a torrid and form the primary windings of a current transformer. Its secondary winding is connected to a highly sensitive electromagnetic trip relay, which operates the trip mechanism.In a healthy circuit, sum of the currents in phases, is equal to the current in the neutral and vector sum of all currents is equal to zero. If there is any insulation fault in the circuit and leakage current flows to earth, the currents do not balance and their vector sum is not equal to zero. This imbalance is detected by the current transformer, the RCB is tripped and supply to load is interrupted. The trip mechanism is operated at a residual current of between 50-100% of its rated tripping current.

According to the IS: RCCBs must automatically disconnect an electrical installation within 0.2 sec. in the event of an earth fault. Havel’s RCCBs break is less than (0.03 sec) when the fault current, which flows to earth exceeds the tripping current.