Short Circuit and Overload Protection
Short circuits and overloads put different demands on circuit breakers. It is imperative that engineers know how to protect their designs against both dangers. Circuit breakers are used in a variety of ways. They are mounted in panel boards to protect branch circuit wiring and they are built into equipment to protect it. With this range of applications, it's not surprising that a circuit breaker must provide both short circuit and overload protection.
1. Short Circuit
The circuit that allows the electric current to pass through the random path which has low resistance is known as the short circuit. The short circuit causes the heavy current which damages the insulation of the electrical equipment. It mainly occurs when the two wire touches each other or when the insulation between the conductor breaks down.
The magnitude of the short circuit current becomes thousand time large than the normal current. During the short circuit, the voltage at the fault point diminishes to zero and high magnitude current flow through the network. The short circuit has various harmful effects on the power system. They are-
- The short circuit causes a heavy current in the power system which produces excessive heat and hence results in fire or explosion.
- The short circuit produces ARC that causes the major damage to the elements of the power system.
- The short circuit affects the stability of the network which disturbs the continuity of the power supply.
To protect the device and people from short circuit hazards, protecting devices are used in electrical circuits. These devices can detect the faults and trip the circuit immediately before the surge current reaches to the maximum. There are two popular protecting devices used frequently in every electrical circuits.
Fuse is operated once in the circuit and then must be replaced after the trip occurs. It is helpful for phase by phase protection. It offers a high breaking capacity at low volume, which limits electro- dynamic stress.
(ii) Circuit Breaker
Circuit breakers can be reset either manually or automatically. It automatically breaks the circuit within a short cutoff time and separates the load from the power supply that protects the circuit from any damage. It works faster than a fuse . For example, Molded Case Switch (MCS), Molded Case Circuit Breaker (MCCB). Vacuum Circuit Breaker.
|Cross sectional part of MCB|
Circuit breakers are available in three types. Systems designers will choose among inverse time trip, adjustable trip or instantaneous trip circuit breakers, depending on the protection sought.
Inverse Time Trips:- These breakers trip faster as current increases. This provides overload protection but also allows equipment and conductors to carry excessive loads briefly.
Adjustable Trips:- These breakers are used when the operation of several protection devices in a system must be coordinated. Designers place the lowest rated trips nearest to the devices being protected so that a fault in one area is isolated but allows current elsewhere in the system to continue to flow.
Instantaneous Trips:- These use only the magnetic element of the trip and provide no overload protection. Also known as motor circuit protectors, or MCPs, they normally are used to protect large motors from short circuits and ground faults.
The over load means more than the desired load imposed on the power system network. The voltage at the overload becomes diminished to a very low value but it cannot be zero. The current in the overload condition is high but considerably lower than the short circuit current. The overloaded increases the temperature regarding jouls, which causes burns and hence damage the electrical equipment. The overload condition damage the equipment of the power system. For example- Consider the inverter has a rating of 400 watts and if the load of 800 watts is connected across it, then this will cause an overload.
Overload protection is a protection against a running over current that would cause overheating. Overload protection typically operates on an inverse time curve where the tripping time becomes less as the current increases. This means an overload relay isn’t going to trip on momentary or short-term over current events that are normal for the piece of equipment it is protecting. For example some equipment may increase inrush currents as it starts up. These inrush currents typically last only a few seconds and rarely cause a problem. So overload relays are used in a motor circuit to protect motors from damage caused by prolonged periods of over-current.