An overcurrent relay is a type of protective relay that activates when the load current exceeds a preset threshold. There are two kinds of relays: instantaneous overcurrent (IOC) relays and definite time overcurrent (DTOC) relays. When the relay is activated, one or more contacts will open and energize, causing a circuit breaker to trip. The amount of time the contact(s) remain closed after the relay has activated determines how long the excessive load current can flow before the breaker intervenes.
Overcurrent protection is important in electrical systems. An overload condition can cause damage to your equipment or fire. An underload condition can cause your equipment to fail by overheating or collapsing. Either situation can be dangerous if you aren't aware of it happening to another type of load in your system. Overcurrent protection should be functional and effective in reducing the risk of fire and other hazards.
Instantaneous overcurrent relays (IOC relays) activate at a set current level and stay active until this current drops below the setting. Thus, they provide very fast acting overcurrent protection. However, since the relay remains active as long as the current level is exceeded, an overloaded branch may continue to draw power even after the main line voltage has returned to normal. This can cause further overloads in other parts of the system. To prevent this from happening, most IOC relays have delay circuits that pause for several seconds after activation before continuing to operate the protected device.
The overcurrent relay is described as a relay that activates only when the current value exceeds the relay setup time. It protects the power system's equipment from fault current. An overcurrent condition can result from any of several things causing electricity to be passed along a circuit other than the one originally intended. For example, if a wire gets pinched between two metal pieces, it will create a short circuit that will allow much more current to flow through than should otherwise be allowed. This can happen with either aluminum or copper wires. If an aluminum wire gets pinched, it will quickly get hot and burn up; while if a copper wire is pinched, it will continue to pass current until it reaches a copper joint in another part of the system where it will stop.
Overcurrent relays were first used in electric power distribution systems but are now also found in many other applications including air conditioners, heat pumps, and refrigerators. Overcurrent protection is required by law for all appliances that use electrical wiring as support beams (i.e., overhead power lines). The amount of current needed to operate most appliances is very small compared to what you might see on a household circuit board. However, enough current can build up due to a short circuit or other cause to damage electronic components on the device itself or on nearby connected devices.
When safeguarding resistive loads, for example, the overcurrent relay can be utilized as overload (thermal) protection. The overcurrent relay, however, cannot act as overload protection for motor loads. Overcurrent relays often have a longer duration setting than overload relays. 1. Defining the Present (Instantaneous Overcurrent Relay) vs the Past (Duration Setting Overcurrent Relay)
An instantaneous overcurrent relay opens its contacts immediately upon detection of current flow. Thus, it can provide immediate protection against electrical faults that could cause injury. Duration setting overcurrent relays open their contacts after they have been closed for some period of time. This period of time can be adjusted by turning a small knob on the back of the relay. A short setting will open the contacts after just one current flow event while the maximum setting will keep the contact open for as long as the load is connected across them.
Instantaneous overcurrent relays are used in applications where there is the possibility of electrocution if the circuit is not repaired quickly, for example, when working near power tools or other equipment that can cause a fire if not taken care of promptly. These relays are less expensive than duration setting overcurrent relays but they must be replaced whenever they fail because they do not stay closed long enough to allow for replacement of the load. Durability can be improved by using higher voltage relays but this increases the cost significantly.
A "Numerical over Current Relay" is a sort of safety relay that activates when the load current exceeds a certain threshold. The IDMT's overcurrent relay is the relay that begins to work after the desired time delay. The time delay is often referred to as the operating time. The operating time can be set by the user, or it can be set automatically depending on the type of circuit being protected. The IDMT's overcurrent relay operates whether the power is off or on.
The overcurrent relay has two coils: one drives the contactor open and the other closes the contacts of the coil. When an excessive amount of current passes through the primary of the transformer, the iron core inside the relay heats up, which in turn causes the secondary coil to magnetize, thereby closing the contactor. This action cuts off the supply of current to the protected parts of the system.
Overcurrent relays are used in many applications where there is a risk of electrical damage occurring if too much current flows through a conductor. These include household wiring, lightening protection, motor control, and power supplies. Overcurrent protection devices are usually rated according to the expected maximum continuous current that will flow through them. For example, an overload relay may be specified to operate at 20 amperes for 60 minutes continuously, which means it will shut off if the current rises above 20 ampeers for more than 60 minutes.
Overcurrent relays can be used to safeguard almost any component of a power system, such as transmission lines, transformers, generators, or motors. There would be more than one overcurrent relay for feeder protection to safeguard different areas of the feeder. The overcurrent relay should be installed near the main line conductor that it is intended to protect.
An overcurrent relay uses magnetic forces to open and close contacts inside the casing. When an excessive current flows through the relay coil, the iron core around which the coil is wound causes these contacts to make or break, allowing or preventing current from flowing through other components in the circuit. Overcurrent relays may be contactors (such as motor starter contactors) or interrupters (such as transformer tap changer interrupting). Some relays have both contactor and interrupter functions.
The choice of relay type (contactor or interrupter) depends on how much current you expect to see on the line when there is a fault. If the current is very high, then an interrupter will need to open within its rated time even if some of this current is diverted into ground. Otherwise, the relay may stay closed long after the fault has been cleared. Contactors are usually chosen when less current is expected to flow during a fault. This reduces the risk of damage occurring due to prolonged exposure to voltage.
The overcurrent relay is classified into the following classes based on the time of operation: Definite Time Extremely Inverse Overcurrent Relay There is no intentional time delay in the relay's operation. When the current within the relay exceeds its operating value, the relay's contacts are quickly closed. Indefinite Time Extremely Fast Overcurrent Relay The relay will not open until a certain current level is reached; however, once this level is exceeded, the relay will immediately close itself. This type of relay is used where a low-impedance load is being controlled by the power source.
The overcurrent relay is operated by a magnetic field generated by an electromagnet when excess current flows through it. The strength of this magnetic field is such that it can be detected by another electromagnet having windings with a resistance high enough to give a voltage drop across it when exposed to the magnetic field. This other electromagnet then closes its contacts which connect it to the power source or not depending on whether or not the first magnet's contact has been activated. Since overcurrent protection depends on detecting a high current with a low-resistance winding in order to close the switch rapidly, very large currents may be carried before acting to shut off the power supply.
Overcurrent protection is usually provided by a thermal mechanism in addition to the magnetic one. This prevents excessive heating of the conductor causing it to break away from its attachment points.