ELECTRICAL PROTECTION DEVICES

ELECTRICAL PROTECTION DEVICE

ELECTRICAL PROTECTION DEVICE

RESIDUAL CURRENT CIRCUIT BREAKER

RCCB is provided mainly for the protection of human being from accidental electric shock that can occur due to faulty wiring or earth faults. In normal working conditions of the circuit the current flowing into it through the live wire is same as that of the current coming out the neutral wire.IN case of an earth fault the current finds path to earth. For example a person comes into accidental contacts with live wire then current flows to earth and the amount of current returning through the neutral is reduced and this reduced current is known as residual current.

The RCCB continuously monitors the amount of current flowing and if there is any difference in current detected then it trip out.

 

 

A torroidal transformer is contained within the RCCB body this helps to detect the current imbalance within the system. There is current path for live and neutral wires connected to the transformer. A magnetic flux is generated proportional to the current drawn through the live and neutral wire. The construction of windings is such that they produce magnetic fields in the opposite direction. Net magnetic flux is zero in the case when there are no faults, since the magnetic fields cancel out each other.  Even if there is a small residual current then the net magnetic flux will be greater than zero which will result in the tripping of circuit to avoid accidental shocks.

There are two types of RCCB

TWO POLE RCCB
FOUR POLE RCCB

TWO POLE RCCB

Two pole RCCB is used in case of a single phase supply. It has two points to which a live wire and neutral wire is connected.

FOUR POLE RCCB

Four pole RCCB is used in case of 3 phase supply. It has 2 ends to which 3 phase wires and one neutral wire is connected.

LIMITATIONS OF RCCB

If none standard waveform are generated by loads, RCCB does not guarantee to operate since they are designed to operate on normal supply waveforms.
 Current overload protection is not provided by RCCB. It provides protection only when there is a difference in current in live and neutral. But a current overload however large cannot be detected.
Unwanted tripping of RCCB, due to sudden changes in electrical load there can be small current flow to earth especially in old appliance.
RCCB will not protect against the overheating that occur when conductors are not screwed properly into terminals.
RCCB will not provide protection against line-neutral shocks since current in them is balanced. As both terminals are been held together the current gets balanced.

MINIATURE CIRCUIT BREAKER

There is a bi-metallic strip which becomes over heated when circuit is overloaded for long time. Overheating results in deformation of the strip and causes displacement of latch point. Even a slight movement of latch causes release of spring and makes the moving contact to move for opening the MCB. During short circuit the MMF of current coil causes its plunger to hit the latch point and displaces the latch, which results in the opening of the MCB.

Arc generated by high currents will reach the arching chamber through the guide plate. The arc is separated into smaller arcs by the arc splitter and extinguished there.

CLASSIFICATION OF MCB BASED ON TRIPPING CHARACTERISTICS

TYPE B

These are mainly used for residential purpose. They trip between 3 and 5 times full load current.

TYPE C

This type of MCB is used where there can be chances of higher values of short circuit current in the circuit like commercial or industrial type of application. Loads connected are mainly inductive in nature. They trips between 5 and 10 times full load current.                                  

TYPE D

This type of MCB is used for commercial purposes where there is high current in flow. They trip between 10 and 20 times full load current.

                                

RCCB, MCB, fuses, ELCB, are all devices used to protect users and equipment from fault conditions in an electrical circuit by isolating the electrical supply.

Fuses

 Fuse is a protection device which melts and breaks the circuit when the current rating exceeds the rating of the fuse. Fuse must be replaced ones it has melted. Fuses are becoming rare as electrical installations are updated.

ISOLATORS

Electrical isolators isolates a part of the system from rest this is useful when circuit breaker trip the circuit but the open contacts are not visible from the outside so it’s not safe to touch the circuit just by switching of the circuit breaker. Therefore the isolator is opened only after the circuit breaker is opened and closed before the circuit breaker is closed. Electrical isolators must be operated only when there is no current flowing through the circuit. Isolator can be operated by hand as well as by motorized setup. But motorized set up costs more than hand operation.

 TYPES OF ISOLATORS

DOUBLE- BREAK ISOLATOR

There are three post insulators. The central post insulator can be rotated. There is a flat male contact on central post insulator which rotates as central post insulator rotates. Female contacts are fixed up on the other two post insulators.

The rotating movement of male contact makes it in contact with the female contact and isolator becomes closed.  And as the isolator rotates in opposite direction it loses the contact with female contact and isolator becomes open. Driving lever mechanism enables rotation of the central post insulator.

SINGLE- BREAK ISOLATOR

There are two post insulators which rotate. And due to their rotation the male and the female contacts fitted over them also rotates. Rotation of both post insulators stacks in opposite direction causes to close the isolator by closing the contact arm. Counter rotation of both post insulators stacks open the contact arm and isolator becomes in off condition.

Isolators are classified depending upon their position in power system.

1-Line side isolator is at the line side of any feeder.

2-Bus side isolator is directly connected with the main bus.

3-Transfer bus side isolator is directly connected with transfer bus.

RCBO

When there is need of combined protection against over current (over load and short circuit) and protection against earth leakage current RCBO is used.

It has wide range of breaking capacities and can be used for industrial and residential purposes.it has the combined effect of MCB and RCCB in one device. Every circuit has its own RCBO therefore if any device is tripped due to residual current, only the faulty circuit is disconnected.

The internal residual current detecting element will trip the circuit when there is a current leakage fault. The internal thermal/magnetic circuit breaker parts are able to trip the device when the circuit is over loaded. RCBO come in single pole and dual pole variations. The single pole is used as a direct replacement for MCB.

ELCB

An ELCB is a voltage sensing device which is used to prevent shock threats as well as protection of instruments. Earlier it was widely used but now a days RCCB’s are in use for protection. An ELCB has a relay coil one of the terminals is connected to the equipment and other terminal is given to ground directly. In case of faulty conditions like if insulation of the equipment fails or live phase wires touches the metal part of the equipment, a voltage difference appears across the coil which is connected to device and earth. After this voltage reaches a limit, current developed in the relay trips the circuit breaker and the power supply to the device is stopped.

 

 

Disadvantages

• · Wire break in the fault to load section will result in failure of operation of ELCB and consumer may get electric shock during Earth fault.
• · There can be nuisance trips for example two adjacent building can have earth rod placed close to each other then a high earth leakage current in one building can rise the ground potential near and cause voltage difference in the other too which results in the unwanted tripping of the circuit.  
• Grounding cannot be done for every single device separately.