For an easier understanding of the differential current, one physical process should be considered. When an insulated current-carrying line is touched, why is there no electric shock? The answer is obvious: insulation prevents current from flowing through the human body. But if the core is exposed, stand on an insulating substrate and touch the wire? The effect is the same - there is no electric shock. The backing prevents the circuit from shorting through the torso to the ground.
The concept of differential current
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 RMS value. In order for a differential current to appear, a physical process called leakage current must occur. But it is necessary that one condition be met: the equipment case, where the leakage current appeared, 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 a residual current circuit breaker (RCD)won't work.
Relationship between differential and leakage current
When current leaks in a circuit, it passes to elements that have conductive material (metal cases for appliances, heating pipes, etc.) from live parts (electrical circuits, wires). There are no short-circuited sections during these leaks. And therefore, there is no fact of a malfunction of the circuit (obvious damage to it).
Since the differential current, mathematically expressed, is the difference (in vector terms) 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 in case of violation, for example, of insulation, high humidity of the environment through which it can pass, or something else, then the differential current appears when connected to the ground.
Trip and non-trip residual currents
Under the operation current (or breaking current) is understood such a differential current, the flow of which leads to the tripping of the VDT in case of leakage in the circuit.
Current, the flow of which is permissible in the circuit of a residual current device (RCD) and does not trip, is called differential non-tripping current.
In a loaded circuit, where pulse-type devices operate: rectifiers, discrete digital devices for power control - all this is modern household appliances, there are differential background currents. But such currents are not fault currents, andIn this case, the electrical circuit cannot be turned off. Therefore, the RCD threshold is chosen so as not to respond to the operating background value, but to turn off the leakage current exceeding 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 turn off the circuit. Such systems are installed in both single-phase and three-phase lines.
Characteristics of differential switches
Different modifications of protective devices differ from each other by:
- design features;
- sight of electricity leakage;
- sensitivity settings;
- performance.
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 fuses must be connected in series to protect their circuitry.
- RCBO device, where an automatic type switch is provided. These are universal devices with a dual function - for protection against short circuits and overloads, as well as leakage control.
- BDT device with the possibility of connecting an automatic trigger at the connection point. A device designed for jointinstallations with a circuit breaker. Its design is worked out in such a way that it allows only a one-time connection with the machine.
Depending on the form of 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 pulse currents that occur at the moment of switching on, for example, fluorescent lamps, X-ray machines, devices for processing information signals, thyristor converters.
- A - devices for protection against direct pulsating and alternating current. Do not recognize the peak values of the leakage of pulsed differential currents. They work in circuits of electronic type rectifiers, phase-pulse conversion regulators. Prevent pulsating electricity that has a DC component from leaking to ground.
- B - systems operating with variable, constant and pulsating leakage currents.
In terms of sensitivity, the differential switch has the following types:
- Low sensitive systems that turn off the circuit when touched indirectly.
- Systems with high-order sensitivity. They protect if there is direct contact with the conductor.
- Fireproof.
By the time it takes for the device to operate:
- Instant actions.
- Quick acting.
- For generaldestination.
- Delayed - selective type.
The current protection devices of the differential selective device are able to turn off only that part of the equipment where the violation has occurred.
How a residual current circuit breaker 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 section and the number of turns is identical. Current flows through one winding in the direction of the load input, and then returns through the load to the second winding. Since the rated current passes in each load, the summed currents at the input and output, according to Kirchoff, must be equal. As a result, the currents create identical magnetic fluxes in the windings, directed in the opposite direction. These flows cancel each other out and the system remains stationary. If only a leakage current has appeared, then the magnetic fields will be different, the differential current relay will work, which will lead to the opening of the electrical contacts. The electrical line will be completely de-energized.
Where applicable residual current protective device
In modern construction and electrical equipment of areas, as well as in the reconstruction, more and more devices are used that turn off the differential current. This is justified by an increase in the safety of operation of electrical networks, as well as a decrease in injuries. RCDs are used in:
- public buildingsdestination: educational institutions, cultural buildings, hospitals, hotel complexes, sports facilities;
- in individual residential and multi-apartment buildings: houses, dachas, hostels, outbuildings;
- shopping space, especially metal-based;
- administrative buildings;
- industrial enterprises.
Options for RCD connection diagrams
The differential current protective device is produced for a different number of controlled phases. There are single-phase, two-phase and three-phase residual current circuit breakers.
If the line is single-phase and you need to connect an RCD and a single circuit breaker to it, then it makes no fundamental difference what to put in the first place. All these devices are placed at the input of the circuit. It's just more convenient to put the machine on the phase first, and the differential current switch after. Since the load is then connected to both RCD contacts, instead of a phase, to an automatic machine, and instead of zero, to a protective device.
If the main line is divided into several lines with loads, then the RCD is installed first, and then each line has its 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 work 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 own hands, you can assemble only simple electrical circuits, and by connectingprotective devices, strictly follow the instructions. Usually each contact is labeled accordingly.
