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The principle of operation of the current relay: device and purpose

  1. Current relay device
  2. Purpose and methods of connecting current relay
  3. Purpose of the current relay
  4. Current relay connection diagrams
  5. Conclusion

Electromechanical current relay

What is a current relay? Such a question often arises among students and self-taught electricians. The answer is quite simple, but in textbooks and many articles on the Internet it contains a huge amount of formulas and references to various laws. In our article we will try to explain what it is and how it works literally on the fingers.

Current relay device

First, let's look at the principle of the current relay and its device. At the moment there are electromagnetic, induction and electronic relays.

We will disassemble the device of the most common electromagnetic relays. Moreover, they provide an opportunity to most clearly understand their principle of operation.

Electromagnetic current relay device

  • Let's start with the basic elements of any current relay. It necessarily has a magnetic core. Moreover, this magnetic core has an area with an air gap. There may be 1, 2 or more such gaps depending on the design of the magnetic circuit. In our photo there are two such gaps.
  • On the fixed part of the magnetic circuit there is a coil. And the movable part of the magnetic circuit is fixed by a spring, which counteracts the connection of the two parts of the magnetic circuit.

The principle of operation of the electromagnetic current relay

  • When a voltage appears on the coil, EMF is induced in the magnetic circuit. Due to this, the moving and stationary parts of the magnetic circuit become like two magnets that want to connect. The spring does not allow them to do it.
  • As the current in the coil increases, the emf will increase. Accordingly, the attraction of the moving and fixed section of the magnetic circuit will increase. When a certain value of current strength is reached, the EMF will be so high that it overcomes the resistance of the spring.
  • The air gap between the two sections of the magnetic circuit will begin to decrease. But as the instruction and logic says, the smaller the air gap, the greater the attractive force becomes, and the faster the magnetic cores are connected. As a result, the switching process takes hundredths of a second.

There are current relays of different types of performance

  • Moving contacts are rigidly attached to the moving part of the magnetic circuit. They close with fixed contacts and signal that the current on the relay coil has reached the set value.

Current Reset Current Adjustment

  • To return to its original position, the current in the relay should decrease as in the video. How much it should decrease depends on the so-called return coefficient of the relay.

It depends on the design, and can also be customized individually for each relay due to the tension or weakening of the spring. It is quite possible to do it yourself.

Purpose and methods of connecting current relay

Current and voltage relays are the main elements of almost all basic protections. Therefore, let's take a closer look at their scope and wiring diagram.

Purpose of the current relay

And first of all, let's see, but why actually need this current relay? To answer this question, we should dive a bit into theory. But we will try to make it as superficial and accessible as possible.

  • Any electrical installation has two main parameters of its work - it is current and voltage. By controlling these two parameters, it is possible to assess the performance of the equipment and possible malfunctions.
  • Current relay, as you can guess, controls the current. And if its decrease speaks only of a reduction in load, then its increase in most cases indicates a serious malfunction. In order not to consider the issue in more detail, let's take the electric motor as an example.

Relay motor protection circuit

  • The motor has a rated current, for example 50A. A slight increase in current, say up to 55A, signals an overload. In this case, the engine should not be shut down immediately, because the overload can be temporary, and according to the EI, most electric motors are allowed to be periodically overloaded.
  • But continuous operation with a higher rated current may signal mechanical failure or other problems. Therefore, after the load, after a certain period of time, the engine must be turned off.

Overload protection circuit

  • The current and time relay circuitry provides such protection. When the current increases above the nominal value of 50A, the current relay is activated. With its contacts, it starts up a time relay, which counts the permissible operating time of the engine in the quenched state. If during this period of time the current relay does not disappear, then the time relay is activated and turns off the electric motor.

Note! Overload protection must be rebuilt from the time of starting the engine. As you know, when starting, the starting current can reach up to ten times nominal (usually five or six times). Therefore, to avoid false triggering of the overload protection, the time delay must be longer than the motor reversal time.

Therefore, to avoid false triggering of the overload protection, the time delay must be longer than the motor reversal time

Current cutoff

  • Now let's take another situation . There is a short circuit on our engine. It must be disabled as soon as possible. A short circuit is characterized by a sharp increase in current. Depending on the type of short circuit, these currents may exceed 10 times the nominal value.
  • Based on this, we need to put a current relay, the circuit of which will react to such a current, and immediately turn it off. Such protection is called a current cut-off. When protection instantly turns off electrical equipment when a certain current value is reached.

Timed current relays

  • But there are short circuits which have not such big currents. In this case, the current relay and its wiring diagram changes somewhat. Its principle of operation is similar to overload protection, only the greater the current, the faster it will turn off our electric motor. This is achieved by combining in one device and the time relay and current. Such protection is called overcurrent.

Current protection built into switch

  • There are also protection against single-phase earth faults, protection against negative sequence currents, differential protection, distance protection and many other relay circuits that use a current relay.

But these are already more specific defenses that require a deeper understanding of the processes. Therefore, in our article we will not consider them.

Current relay connection diagrams

Having examined the device and the purpose of the current relay, you can go to the question of their connection. There are two main options - directly or through a current transformer.

Let's look at each of these options:

  • Relays can be directly connected to electrical installations with voltage up to 1000V. This is due to the fact that with a higher voltage, the dimensions of the relay would have to be significantly increased in order to ensure adequate insulation and the flow of large currents. And because of this, the price of the relay would also increase.

Directly connected current relay

  • Consumers up to 1000V are usually not the most responsible, therefore protection is implemented in one or two phases. But it is possible to implement the protection on all three phases. To do this, simply in series with the load, the coil of the current relay is switched on on one or several phases.

Current relay

  • Many current relays contain two coils. For them can be used in series or parallel connection of the windings of the current relay. This is necessary to change the operating limits of the relay.
  • As an example, take the relay PT 40. When the coils are connected in parallel, the response current varies from 0.1 to 100A. When the windings are connected in series, the operation limit can be adjusted in the range of 0.2 - 200A.

Note! If you need a response limit of 0.1-100A, then in principle you can not connect the second winding at all.

1-100A, then in principle you can not connect the second winding at all

Current transformer 6 - 10kV

Current transformer 6 - 10kV

Current transformer 110kV and above

  • More often, electrical circuits for connecting current relays involve the use of current transformers. These devices allow you to convert any current to values ​​of 1 or 5 A.

Wiring diagram of current relay through current transformer

  • Such consumers are usually considered responsible, therefore current protection is implemented in each phase . The principle of connection is simple. The relay coil is simply connected to the terminals of the current transformer.

Attention! But here it should be remembered that current transformers and all secondary switching work in the mode close to short circuit. Therefore, the breaking of such circuits is fraught with damage to the current transformer, as well as serious consequences for humans. Therefore, before making any switch in current circuits, they should be short-circuited with a jumper. Or to make switch on electrical equipment, derived in repair.


The current relay and the electrical circuit of its connection has many nuances. If you go into each, you get a full textbook. Our goal was to give you a general idea of ​​this relay in the most accessible language. Therefore, some questions in our article are not fully disclosed or simply. More details on each aspect should be understood on the basis of existing conditions.



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