- Types of RCDs
- Neutral and protective conductors
- Selection of nominal parameters
- Single-phase and three-phase connection
- Correct wiring
- Checking and troubleshooting
False trips of residual current devices are usually the result of wiring errors. There are several types of RCDs with different principles of operation and minor differences in the connection diagram, which you need to know for the correct organization of power grids.
Types of RCDs
Current leakage protection devices, known under the abbreviations RCD, ADZ, VDT, RCBO, have the main function – to protect living organisms from electrical injuries, as well as to prevent parasitic dielectric losses that can lead to fire. The entire range of devices described in this review has differences in principle of operation, purpose, sensitivity, type of current in the controlled circuit, the ability to withstand the load, as well as a number of other factors. In order to have a clear and clear idea of the capabilities of a particular device, one should understand the specifics of its operation..
According to the mechanism of action, the RCD can be electromechanical and electronic. In the first case, the main functional element is a differential transformer on a ring core. The transformer has two primary windings, through which the main load passes, as well as a third control one. In normal operation, oppositely directed currents of equal value flow through the primary windings, thus, their electromagnetic induction is mutually compensated. If a leak occurs at any point in the circuit connected after the RCD, the currents in the primary windings lose their equivalence, respectively, a pickup appears in the secondary winding. When the induced current exceeds the set value, the release trips and breaks the main group of contacts.
The principle of operation of an electromechanical RCD
Electronic RCDs have a different principle of operation, their work is based on semiconductor devices. The first link of the electronic circuit is a current divider, whose task is to convert the load acting on the main contacts of the device to one that is permissible during the operation of semiconductor elements. A proportional, but smaller, current flows to the comparator (comparing semiconductor device), which, with a significant difference at the inputs, generates an output signal that activates the main circuit opening device.
Diagram of an electronic RCD: A – comparator; K – relay; Т – “Test” button; R – resistor
The practical difference between RCDs of electronic and electromechanical action is as follows:
- Electromechanical RCDs can falsely trigger at high reactive and inductive loads. In other words, the lag or advance of the current curve in one winding relative to the other generates interference to the control circuit.
- Electronic RCDs do not have a sufficiently high accuracy due to nominal errors inherent in all radio-electronic components. Also, the efficiency of electronic RCDs is significantly influenced by the voltage value acting in the controlled circuit..
Left: electromechanical RCD. Right: electronic RCD
According to its purpose, it is customary to classify RCDs into devices for protection against electric shock and devices that protect against fire-hazardous current leaks through insulation. In addition to minor differences in the device, these devices simply have different ratings of differential currents to which the protective mechanism is triggered.
Fire protection RCD type S (selective)
The load capacity of the RCD indicates, first of all, the conductivity of the elements of the main contact group. There are also differences in:
- Massive magnetic core capable of withstanding heating with mutual compensation of inductive influences.
- Power class of electronic components.
In the category of other RCD functions, the most noteworthy is the ability to turn off the power circuit when the current is exceeded. In fact, such RCDs, called differential circuit breakers, combine a power circuit breaker and a current leakage protection device..
Neutral and protective conductors
We figured out the principles of operation of the RCD, it remains only to carry out a correlation with the existing AC power supply circuits. Most of the incidents associated with improper operation of differential protection devices are caused precisely by incorrect application in various power supply schemes..
Mainly, AC circuits are distinguished by the presence and connection scheme of the neutral and protective conductors. Thus, it is possible to distinguish power supply circuits with a solidly grounded and insulated neutral. In practice, the difference lies in the place where the zero working and zero protective conductors are combined. For the correct operation of the RCD, the common zero point must be located according to the diagram before the installation site of the device.
RCD-controlled circuits must not have the potential to drop some of the current to ground, otherwise false trips are guaranteed. Therefore, leakage protection is mainly equipped with networks with an isolated neutral (IT and TT), that is, they have no connection with the protective neutral conductor throughout the entire length of the network after the ASU. The same category includes systems with TN-S and TN-C-S grounded neutral, although the installation of differential protection in them requires additional care..
However, residual current circuit breakers can still function correctly in TN-C systems. Their connection is carried out according to a 3 or 5-wire scheme, that is, the protective conductor stretches to the distribution unit to combine with a working zero to the place where the RCD is inserted. In this case, protection against differential current is limited in selectivity: it is difficult to protect entire groups of conductors, devices can be installed only on the extreme branches, that is, immediately in front of pantographs. A particular example – sockets with built-in leakage protection.
Selection of nominal parameters
The scope and purpose of the RCD are determined by two key parameters: the load capacity and the amount of leakage at which the circuit breaks. If differential protection is designed to reduce the severity of the consequences of electrical injury, its rating is selected based on the permissible values of the current acting on the body.
The first degree of electrical trauma is characterized by seizures without loss of consciousness and does not cause irreparable damage. Such a lesion is typical when tiny currents flow through the body: about 10 mA for children and up to 30 mA for adults. Therefore, an RCD with a leakage setting of such values is used to protect the main outlet groups. In this case, the most sensitive RCDs are used for sockets located near the floor, where children can access them, as well as for groups connected in a two-wire circuit. Sockets for household appliances with a protective ground contact are connected through an RCD with a sensitivity of 30 mA. To protect against electric shock, it is customary to use electromechanical devices as the most reliable.
The main characteristics of the RCD
General protection of cable power lines from leakage through insulation is provided by fire-fighting RCDs with a differential current setting of 100, 200 or 500 mA. A more accurate value is determined by the characteristics of the cable product and the line length. The worse the dielectric properties and the higher the length, the greater the total leakage value. High intrinsic capacity of the cable does not cause false alarms, since the accumulation of charge is accompanied by a proportional work of the current in both conductors.
The load capacity of the RCD is established with a safety margin of about 10–20%, depending on the operating mode of the protected line. The choice of the rating exactly according to the values of the effective current is fraught with overheating of the device, but if the margin is significantly larger, a decrease in sensitivity is possible. In turn, for differential automatic devices, the maximum current setting and the tripping characteristic are of key importance and are determined by the requirements for protecting the line from overloads..
Single-phase and three-phase connection
The most important rule for connecting differential protection devices is that all conductors along which the electric charge moves must be connected to them. For single-phase networks, two-pole devices are used: the left group of contacts is intended for the phase conductor, the right one for the working zero. The conventional direction of current flow does not matter for electromechanical RCDs, while electronic devices require the load to be connected exclusively from the bottom with power supply to the upper terminals.
Three-phase RCD connection diagram: 1 – input automatic device; 2 – three-phase meter; 3 – four-pole RCD; 4 – automatic device for connecting a three-phase load; 5 – automatic machines of two-phase load
The connection of three-phase RCDs also without fail occurs with the conduction of a working zero through the device. Ultimately, even an asynchronous motor has three linear conductors that do not have strict load balancing, so they are connected in a “star” circuit through a balun. If at the same time the motor itself is zeroed through the protective grounding system, the RCD is guaranteed not to work correctly..
Most of the RCDs belong to the category of modular technology for installation on a 35 mm DIN rail. The height of the module and the size of the neck correspond to the standard dimensions, so there are no problems with the placement of the diffuser in ordinary row boxes.
In terms of assembling the panel wiring, there are subtleties. The connection of the input working zero to the common bus or cross-module must be carried out immediately after the output from the RCD with one conductor without branches. In this case, only those lines should be connected to this bus, the protection of which is controlled by the device from which the working zero is taken. Thus, the following connection diagram operates in the standard panel:
- The input phase and neutral wire from the input cable are connected directly to the RCD terminals. On the reverse side, the working zero and phases are removed, each conductor on a separate bus.
- The following are connected to the common zero bus:
- neutral conductors of the lighting network directly;
- zero connection of RCD 1 group at 10 mA;
- zero connection of RCD 2 groups at 30 mA.
- The entire load is connected to the phase bus, including RCDs of groups 1 and 2.
RCD connection diagram: 1 – introductory machine; 2 – counter; 3 – general selective RCD; 4 – cross-module; 5 – lighting circuit breakers; 6 – circuit breaker for RCD protection; 7 – RCD of the first group 10 mA; 8 – RCD of the second group 30 mA; 9 – zero bus; 10 – grounding bus
Since the zero contact of the differential protection devices is located on the right, the devices themselves are placed on the right side of the row, in order to subsequently distribute the phases to the circuit breakers with a comb. After RCDs 1 and 2 of groups, additional buses or cross-modules are installed, to which all lines included in the corresponding protection group are connected. If a residual current device or differential circuit breaker is installed in the local group boxes, they always follow the diagram first. The exception is lighting lines, which are powered from the input terminals of the protective devices. To reduce the transfer resistance, stranded conductors should be crimped with ferrules. Tightening torque control for modular devices is not critical, however, it is required to re-tighten the contacts 48–72 hours after completion of the installation.
Checking and troubleshooting
Installing an RCD in almost any power supply system allows you to accurately check devices and lines connected to the network for insulation problems and breakdown to the case. To do this, they try to move the RCD as close to the input circuit breaker as possible: the protection area only becomes wider, while the problem point is easily detected by sequential enumeration of the connected lines.
False operation of an RCD is almost always a consequence of any human action: touching the body of the equipment, plugging the device into an outlet, etc. Thus, in most cases, the place of leakage can be localized rather quickly. If an introductory RCD, which controls several groups, is triggered, a line with weak insulation is determined by sequentially disconnecting the outlet groups and monitoring the performance of the electrical network. The detected network can switch to power bypassing the RCD, but only with reconnection of both conductors and only if such a change in the circuit is permissible from the point of view of electrical safety. In other cases, either the installation of a diffuser for a higher leakage current value, or restoration of the line insulation.
Periodically you need to test the performance of the mechanism. For this, each device has a test button that closes one output pole with the opposite input pole through a current-limiting resistance. Thus, a leak is simulated, the value of which is close to the response threshold with high accuracy. The lack of response to pressing the test button can serve as a malfunction of the device, and too low operating voltage.