For an easier understanding of the differential current, one physical process should be considered. When touching an insulated live line, why is there no electric shock? The answer is obvious: isolation does not allow current to flow through the human body. But if you expose the core, stand on the insulating substrate and touch the wire? The effect is the same - there is no electric shock. The substrate prevents the circuit from closing through the body to the ground.
Differential current concept
In nature, there is no such physical process as 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. For a differential current to appear, a physical process called a leakage current must occur. But it is necessary that one condition is met: the equipment case, where the leakage current appears, must be connected to the ground. Otherwise, if the body is not grounded, then the occurrence of a leakage current does not lead to the appearance of a differential current. And the differential current switch (VDT) will not work.
The relationship between differential and leakage current
When a current leakage occurs in the circuit, it switches to elements that have conductive material (metal housings for appliances, heating pipes, etc.) from live parts (electrical circuits, wires). There are no short circuits during these leaks. And therefore, there is no fact that the circuit is not working (its obvious damage).
Since the differential current, if expressed mathematically, is the difference (in the vector value) 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, of isolation, of increased humidity of the medium through which it can pass, or something else, then the differential current appears when connected to the ground.
Tripping and non-tripping differential currents
Under the response current (or breaking) is understood such a differential current, the flow of which leads to the shutdown of the RCCB in case of leaks in the circuit.
The current, the flow of which is permissible in the circuit of the residual current device (RCD) and does not operate, is called a differential non- disconnecting current.
In a loaded circuit, where pulse-type devices operate: rectifiers, digital digital devices for adjusting power - all this is modern household appliances, there are differential background currents. But such currents are not damage currents, and the electric circuit in this case cannot be turned off. Therefore, the operating threshold of the RCD is selected so as not to respond to the operating value of the background, but to disconnect the leakage current in excess of this value.
RCD or differential machine
In order to protect the circuit from earth faults of large currents, special circuit breakers have been developed . The circuitry of the device 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 device passes, it will immediately disconnect the circuit. Such systems are installed in 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 there are:
- VDT devices (differential switch), where there is no protection against high currents. They respond to leakage currents, but to protect their circuitry, fuses must be connected in series.
- AVDT device, where an automatic type switch is provided. These are universal devices with a dual function - for protection against short-circuit and overload, as well as leakage control.
- The device BDT with the possible connection of the automatic circuit breaker at the point of connection. A device designed for joint installation with a circuit breaker. Its design is designed in such a way that allows only a one-time connection to the machine.
Depending on the shape of the leakage currents, groups of protective devices of the following modification have been developed:
- AC - devices operating with alternating sinusoidal current. They do not respond to differential pulsed currents that occur at the time of switching on, for example, fluorescent lamps, x-ray devices, devices for processing information signals, converters on thyristors.
- A - devices for protection against constant pulsating and alternating current. Peak differential leakage currents are not recognized. They work in circuits of electronic type rectifiers, phase-shift converters. Prevent leakage of pulsating electricity to the ground, in which there is a constant voltage component.
- B - systems working with alternating, constant and pulsating leakage currents.
In sensitivity, the differential switch has the following types:
- Low-sensitivity systems that disconnect the circuit when indirectly touched.
- Systems with high order sensitivity. Protect if there is a direct touch to the current lead.
- Fire action.
By the time it takes for the device to operate:
- Actions are instant.
- High-speed.
- For general purpose.
- Delayed - selective type.
Current protection devices of a differential selective device are capable of turning off only that part of the equipment where a violation has occurred.
How the differential current switch works
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 the same cross section and the number of turns is identical. Current flows through one winding in the direction of the load input, and then returns to the second winding through the load. Since the rated current passes in each load, the summed currents at the input and output, according to Kirgoff, should be equal. As a result, the currents create the same magnetic flux in the windings, directed in the opposite direction. These flows cancel each other out and the system remains stationary. If only a leakage current appears, then the magnetic fields will be different, the differential current relay will trip, which will lead to the opening of the electrical contacts. The electric line will be completely de-energized.
Where applicable residual current protective device
In modern construction and electric equipment of areas, as well as during reconstruction, devices that disconnect differential current are increasingly being used. This is justified by increasing the safety of operation of electric 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, dachas, dormitories, utility buildings;
- trading floors, especially those made on the basis of metal structures;
- administrative buildings;
- industrial enterprises.
Options for connecting RCDs
The protective device of the differential current is released at 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 you need to connect an RCD and a single circuit breaker to it, then there is no fundamental difference what to put in the first place. All these devices are placed at the input of the circuit. It is simply more convenient to first put the machine in phase, and the differential current switch after. Since the load is then connected to both contacts of the RCD, instead of a phase - to the machine, and instead of zero - to the protective device.
If the main line is divided into several lines with loads, then the RCDs are installed first, and then each circuit breaker has its own circuit breaker. It is important that the rated current that an RCD can pass is greater than the trip current of the machine, otherwise it will not work to protect the device itself.
Conclusion
All work on the organization of electrical wiring and circuit protection systems is best left to professional electricians! With your own hands you can collect only simple electrical circuits, and when connecting protective devices, clearly follow the instructions. Usually each contact is marked accordingly.