Protective Relay and Contractor Rack

Protective Relays:
In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, overvoltage, reverse power flow, over-frequency, and under-frequency.
Microprocessor-based digital protection relays now emulate the original devices, as well as providing types of protection and supervision impractical with electromechanical relays. Electromechanical relays provide only a rudimentary indication of the location and origin of a fault. In many cases, a single microprocessor relay provides functions that would take two or more electromechanical devices. By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays.

Types according to construction

1. Electromechanical Relay: Electromechanical relays can be classified into several different types as follows:
a. Attracted Armature
b. Moving Coil
c. Induction
d. Motor Operated
e. Mechanical
f. Thermal

2. Induction disc overcurrent Relay: Magnetic system in induction disc overcurrent relays is designed to detect over-currents in a power system and operate with a pre-determined time delay when certain overcurrent limits have been reached. In order to operate, the magnetic system in the relays produces a torque that acts on a metal disc to make contact.
"Induction" disk meters work by inducing currents in a disk that is free to rotate; the rotary motion of the disk operates a contact. Induction relays require alternating current; if two or more coils are used, they must be at the same frequency otherwise no net operating force is produced. These electromagnetic relays use the induction principle discovered by Galileo Ferraris in the late 19th century.

3.  Static Relay: The application of electronic amplifiers to protective relays was described as early as 1928, using vacuum tube amplifiers, and continued up to 1956. Devices using electron tubes were studied but never applied as commercial products, because of the limitations of vacuum tube amplifiers. A relatively large standby current is required to maintain the tube filament temperature; inconvenient high voltages are required for the circuits, and vacuum tube amplifiers had difficulty with incorrect operation due to noise disturbances.
Static relays have no or few moving parts and became practical with the introduction of the transistor. Measuring elements of static relays have been successfully and economically built up from diodes, Zener diodes, avalanche diodes, unijunction transistors, p-n-p, and n-p-n bipolar transistors, field-effect transistors, or their combinations. Static relays offer the advantage of higher sensitivity than purely electromechanical relays because the power to operate output contacts is derived from a separate supply, not from the signal circuits. Static relays eliminated or reduced contact bounce, and could provide fast operation, long life, and low maintenance.

4. Digital Relay: Digital protective relays were in their infancy during the late 1960s. An experimental digital protection system was tested in the lab and in the field in the early 1970s. Unlike the relays mentioned above, digital protective relays have two main parts: hardware and software. The world's first commercially available digital protective relay was introduced to the power industry in 1984 by Schweitzer Engineering Laboratories (SEL) based in Pullman, Washington. In spite of the developments of complex algorithms for implementing protection functions, the microprocessor-based-relays marketed in the 1980s did not incorporate them.
A microprocessor-based digital protection relay can replace the functions of many discrete electromechanical instruments. These relays convert voltage and currents to digital form and process the resulting measurements using a microprocessor. The digital relay can emulate functions of many discrete electromechanical relays in one device, simplifying protection design and maintenance. Each digital relay can run self-test routines to confirm its readiness and alarm if a fault is detected. Digital relays can also provide functions such as communications (SCADA) interface, monitoring of contact inputs, metering, waveform analysis, and other useful features. Digital relays can, for example, store multiple sets of protection parameters, which allows the behavior of the relay to be changed during the maintenance of attached equipment. Digital relays also can provide protection strategies impossible to implement with electromechanical relays. This is particularly so in long-distance high voltage or multi-terminal circuits or in lines that are series or shunt compensated. They also offer benefits in self-testing and communication to supervisory control systems.

5. Numerical Relay: The distinction between digital and numerical protection relay rests on points of fine technical detail, and is rarely found in areas other than Protection. Numerical relays are the product of the advances in technology from digital relays. Generally, there are several different types of numerical protection relays. Each type, however, shares a similar architecture, thus enabling designers to build an entire system solution that is based on a relatively small number of flexible components. They use high-speed processors executing appropriate algorithms. Most numerical relays are also multifunctional and have multiple setting groups each often with tens or hundreds of settings.

Video:

Electrical wiring and Switchgear Rack

MCCB: Molded Case Circuit Breaker is a type of electrical protection device which is used when the load current exceeds the limit of a miniature circuit breaker. The MCCB provides protection against overload, short circuit faults and is also used for switching circuits. It can be used for higher current rating and fault levels even in domestic applications. The wide current ratings and high breaking capacity in MCCB find their use in industrial applications. MCCB can be used for the protection of capacitor bank, generator protection, and main electric feeder distribution. It offers adequate protection whenever an application requires discrimination, adjustable overload setting, or earth fault protection.

MCB: Miniature Circuit Breaker automatically switches OFF electrical circuit during any abnormal condition in the electrical network such as overload & short circuit conditions. However, the fuse may sense these conditions but it has to be replaced through MCB can be reset. The MCB is an electromechanical device that guards the electric wires &electrical load against overcurrent so as to avoid any kind of fire or electrical hazards. Handling MCB is quite safer and it quickly restores the supply. When it comes to house applications, MCB is the most preferred choice for overload and short circuit protection. MCB can be reset very fast & doesn’t have any maintenance cost. MCB works on a bi-metal respective principle which provides protection against overload current & solenoid short circuit current.

Types of MCB: It is important to know about the types of MCB trip curves to decide what type to use for different appliances for the correct electrical system. This is the selection chart or the criteria to make a call on one of the MCBs. But before that, it is vital to understand what a trip curve means. Trip curves are essentially nothing but the maximum current that a particular MCB can withstand. Once the ideal current loading is breached, the circuit automatically cuts off.

There are about six different types of MCB, which are A, B, C, D, K, and Z. Firstly,
a. Type A, trips off the circuit when the current exceeds 2-3 times the actual current rating. Since this type is highly sensitive to short circuits, it is better suited for semiconductor devices.
b. Type B, which trips off when the current flow is 3-5 times the actual flow and finds a use for cable protection.
c. The best-suited type of MCB for domestic appliances, where the current load is medium, type C. Type C MCB trips off when the flow of current is 5-10 times more than normal.
d. Type D MCB has a high resistance as they can withstand up to 10-20 times the current rate. If you are looking for circuit breakers for devices with a high starting current load like a motor, then type D is the ideal choice.
e. The type K MCB withstands up to 8-12 times the initial charge and thus can be used for bulky load devices.

RCCB: Residual Current Circuit Breaker is basically an electrical wiring device that disconnects the circuit whenever there is leakage of current flow through the Human body or the current is not balanced between the phase conductors. It is the safest device to detect and trip against electrical leakage currents, thus ensuring protection against electric shock caused by direct contacts. RCCB is generally used in series with an MCB which protects them from over current and short circuit current. Both phase and neutral wires are connected through an RCCB device. These are an extremely effective form of shock protection & widely used for protection from a leakage current of 30,100 & 300mA. It is essential lifesaving equipment used to protect the human body from electrical and is mandatory in many states for domestic installation.

ELCB: Earth Leakage Circuit Breaker has the same function as RCCB but is a voltage sensor devise. However, this is an old technology & is not in common use. RCCB being a current sensitive device has a better advantage over ELCB.

Contractor: Contractors are electrically controlled switches (relays) used for switching an electrical power circuit. A contactor is typically controlled by a circuit that has a much lower power level than the switched circuit, such as a 24-volt coil electromagnet controlling a 220-volt motor switch.