Differential current. Differential automaton: characteristics, purpose

For easier understanding of the differential current, one physical process should be considered. When there is a touch to an isolated current-carrying line, why there is no electric shock? The answer is obvious: isolation does not allow current to flow through the human body. But if the vein is bare, stand on the insulating substrate and touch the wire? The effect is the same - there is no electric shock. The substrate does not allow the chain to be closed through the trunk to the ground.

The concept of differential current

In nature there is no such physical process as a differential current. This concept is a vector quantity, expressed as the sum of the currents present in the circuit, taken in the mean square value. To create a differential current, a physical process called a leakage current must occur. But it is necessary that one condition is met: the equipment body, where the leakage current has appeared, must be connected to the ground. Otherwise, if the body is not grounded, the occurrence of a leakage current does not lead to the appearance of a differential current. And the differential current switch (VDT) does not work.

Relationship between differential and leakage current

When there is a leakage current in the circuit, it goes to the elements that have a conductive material (housings made of metal for appliances, heating pipes, etc.) from the parts under voltage (electrical circuits, wires). During these leaks there are no short-circuited areas. And therefore there is no fact of a malfunction of the chain (its obvious damage).

Since the differential current, if expressed mathematically, is the difference (in the vector meaning) between the current at the source output and the current after the load, it is clear that it is almost identical to the leakage current. But if the latter really exists when there is a violation, for example, insulation, high humidity of the medium through which it can pass, or something else, then the differential current appears when connected to the ground.

Disconnecting and non-closing differential currents

Under the tripping current (or tripping) is meant such a differential current, the flow of which leads to the disconnection of the VDT with leaks in the circuit.

The current, the flow of which is permissible in the circuit of the residual current device (RCD) and does not occur, is called a differential non-closing current.

In the loaded circuit, where pulsed-type devices operate: rectifiers, discrete digital instruments for power regulation - all these are modern household appliances, background currents are differential. But such currents are not fault currents, and the electric circuit can not be disconnected in this case. Therefore, the threshold of operation of the RCD is chosen so as not to react to the working value of the background, but to disconnect the leakage current exceeding this value.

RCD or differential automaton

In order to protect the circuit from ground fault currents of large magnitude, special circuit breakers have been developed . The device circuit constantly tests the monitored circuit for electrical leaks. As soon as the sum of the vector values of the linear currents becomes greater than zero and the sensitivity limit of the instrument goes over, it immediately turns off the circuit. Such systems are put in both single-phase and three-phase lines.

Characteristics of differential switches

Various modifications of protective devices differ from each other in:

• Design features;
• Type of electricity leakage;
• Sensitivity parameters;
• Speed.

Depending on the design features are:

• Devices VDT (differential switch), where there is no protection against high currents. They react to leakage currents, but in order to protect their circuitry, the fuses must be connected in series.
• The RCBO device, where an automatic switch is provided. This is a universal device with a dual function - for protection against short-circuit and overloads, as well as for monitoring leaks.
• Device BDT with possible connection of the automatic switching device at the connection point. The device intended for joint installation with an automatic switch. Its design is designed in such a way that it allows only one-time connection with the machine.

Depending on the shape of the leakage currents, groups of safety devices of the following modification have been developed:

• AC - devices working with alternating sinusoidal current. They do not react to the differential pulse currents that occur at the moment of switching on, for example, lamps of luminescent, X-ray devices, devices for processing information signals, converters on thyristors.
• A - devices for protection against constant pulsating and alternating current. Peak values of leakage of pulse differential currents are not recognized. They work in circuits of rectifiers of electronic type, regulators of phase-impulse transformation. Prevent leakage to the ground of pulsating electricity, in which there is a constant component of voltage.
• B - systems operating with variable, constant and pulsating leakage currents.

On sensitivity, the differential switch has the following types:

• Systems are low-sensitive, which disconnect the circuit when indirectly touched.
• Systems with high sensitivity. Protect if there is a direct contact with the current conductor.
• Fire-fighting action.

By the time it takes to operate the device:

• The actions are instantaneous.
• Fast acting.
• For general use.
• With a delay - selective type.

Current protection devices of a differential selective device are able to disconnect only that part of the equipment where the violation occurred.

How the differential current switch works

The RCD consists of a core in the form of a ring and two windings. These windings are exactly the same, that is, they are made with a wire of one section and the number of turns is identical. One current passes in the direction of the load input, and then through the load it returns to the second winding. Since in each load the rated current passes, the total currents at the input and output, according to Kirgof, must be equal. As a result, currents create in the windings the same magnetic fluxes directed in the opposite direction. These flows compensate each other, and the system remains stationary. If there is a leakage current, the magnetic fields will be different, the differential current relay will trip, which will lead to the opening of the electrical contacts. The electrical line will be completely de-energized.

Where is the protective differential current device

In modern construction and electrical equipment areas, as well as in the reconstruction of more and more devices that disconnect the differential current. This is justified by increasing the safety of operation of electrical networks, as well as reducing injuries. RCDs are used in:

• Public buildings: educational institutions, cultural buildings, hospitals, hotel complexes, sports facilities;
• Buildings of individual residential and multi-apartment buildings: houses, cottages, dormitories, ancillary buildings;
• Shopping areas, especially those made on the basis of metal structures;
• Administrative buildings;
• Industrial enterprises.

Variants of connection schemes for RCDs

A differential current protective device is discharged into a different number of controlled phases. There are single-phase, two-phase and three-phase switches of differential current.

If the line is single-phase and it is necessary to connect an RCD and a single circuit-breaker to it, then there is no fundamental difference what to put first. All these devices are placed at the input of the circuit. It is simply more convenient to first set the automaton on the phase, and the differential current switch after. Since the load is then connected to both contacts of the RCD, instead of the phase - to the automaton, and instead of zero - to the protective device.

If the main line is divided into several lines with loads, then the RCDs are placed first, and then on each line their own circuit breaker. It is important that the rated current that the RCD can pass is greater than the tripping current of the machine, otherwise it will not be possible to protect the device itself.

Conclusion

All work on the organization of electrical wiring and circuit protection systems is best entrusted to professional electricians! With your hands you can collect only simple electrical circuits, and connecting protective devices, follow the instructions clearly. Usually, each contact is marked accordingly.