- The principle of operation of the RCD
- RCD types
- By the nature of the leakage current
- By triggering technology
- By response speed (delay)
- By the number of poles
- Leakage current
- Operating current
- Installation and connection of RCD
- Checking the RCD
In this article, we will consider the purpose and principle of operation of an RCD. Let’s figure out how devices of different types differ, we will determine in what conditions they are used. Let’s talk separately about connecting these protective devices..
An RCD is a switching (switching off) device that, when the differential current (leakage current) reaches and exceeds the set value, opens the contacts and disconnects the network or its section from the power supply. This product has several names: “residual current circuit breaker”, “residual current circuit breaker”, “protective circuit breaker”. One way or another, but hundreds of millions of RCDs used in the world perform two tasks – they protect a person from electric shock during direct and indirect contact and prevent a fire from igniting wiring. In many developed countries, the use of differential switches is mandatory.
Residual current devices are designed to neutralize currents in case of all kinds of damage to electrical installations. Despite the fact that this is only a part of comprehensive measures, in some cases the RCD remains the only means of protection, for example, when: lowering the insulation level, breaking the neutral protective conductor or at low values of the fault current. So fuses (circuit breakers) break the circuit at current values (short circuits or overcurrents) that are several times higher than the critical threshold for a person, at which a malfunction of the heart muscle occurs, while RCDs are triggered in milliseconds and react even to the smallest current.
It can be fatal to touch the live elements in the electrical panel or the enclosures of electrical devices that are energized, for example, if the insulation is damaged, there is always a risk of damaging the sheaths of the hidden wiring cables with a tool. A current of 5 mA is already felt by a person, at 10 mA, the muscles contract, and the threshold of “not letting go” sets in, 30 mA causes respiratory failure, 50 mA causes cardiac arrhythmias, 100 mA – a lethal outcome is possible. That is why, according to US standards, an RCD designed to protect people should operate at currents of 4-5 mA, in Europe – 10 mA. In Russia, there are no strict standards – residual current devices, according to state requirements, must be used in metal structures or buildings with a metal frame. However, after the publication of the seventh edition of the PUE, the attitude towards RCDs in our country changed dramatically for the better.
It should be noted that the residual current device cannot replace the circuit breakers that protect the wiring, since it “does not notice” malfunctions that are not accompanied by leakage currents, for example, in the event of a short circuit between the line and neutral.
The principle of operation of the RCD
The operation of any RCD is based on monitoring the balance of currents between the conductors that it includes. Possible current differences are detected and compared with the specified values. Balance violation is an indication for the actuation of the executive part (breaker).
The main “tracking” unit of the RCD is a differential transformer with three windings of the ferromagnetic core: inlet, outlet, and control. The current flowing through the device (from the phase conductor going to the consumer’s power supply to the neutral conductor coming from the consumer) excites magnetic fluxes with opposite poles on the windings. If household appliances, wiring accessories are in good working order, the wiring in the protected area is not damaged, and there are no leaks to the ground, then the sum of the currents is zero. If, for example, a person standing on a wet floor touches a bare wire, then part of the current will go through his body to the ground, the sum of the flows in the device will be greater than zero (the current flows into the RCD more than it leaves it). The appearance of a positive sum of currents means that the current also passes by the RCD, that is, there is a leakage, damage in the circuit. In this case, the balance in the control winding of the transformer is disturbed, a force arises that is transmitted to the EMF relay, breaking the contact between the line and neutral. The electromotive force can be detected by the tracker, which becomes a signal to turn off the solenoid (power actuator) holding the contacts – the circuit is opened.
Residual current devices (RCDs) can differ in many characteristics, from the way they are installed to their general purpose. The classification includes hundreds of types of RCDs with their own characteristics. We propose to consider the main ones in order to choose the right device that will function correctly in certain conditions..
By the nature of the leakage current
According to this criterion, RCDs are divided into devices of the AC, A and B types. AC devices break the circuit with AC leaks if they grow suddenly or smoothly. These RCDs are inexpensive, they are most common, and are considered acceptable for most operating conditions..
RCDs of type A are triggered not only by alternating current, but also by pulsating direct current, which suddenly increases or rises smoothly. Such devices are more preferable for residential premises, since some household appliances are the source of a constant pulsating current, for example, computers, dimmers, televisions, some washing machines (all with semiconductor power supplies). By the way, the instructions for some of these consumers indicate that they must be connected only through an RCD of type A. These protective devices are significantly more expensive than the AC class.
Type B is used for direct, alternating and rectified current, mainly such RCDs are used in industrial facilities.
By triggering technology
Depending on the principle on which the circuit is broken, an RCD is distinguished:
Electromechanical differential protection devices do not need a full power supply from the mains. They are only triggered by a leakage current that drives a high-precision mechanical actuator. These devices are relatively expensive, few manufacturers produce them, but they are considered the most reliable, since they work under all conditions and are not dependent on power parameters..
Electronic RCDs are several times cheaper than electromechanical ones, so they make up the lion’s share of our market. For the functioning of these devices, external power is needed, which “revives” its electronics with an amplifier. The main problem is that with voltage drops in the network, the efficiency of the electronic RCD (there is a dependence of the trigger moment) is noticeably reduced. In addition, there is always a danger that direct or indirect contact with an energized element (wire, terminal or device case) will occur when the neutral conductor is damaged and, accordingly, the RCD will not be energized – and will not work. Electronic RCDs do not protect from all risks, but from most, so if you need to save money, then this is also a good option. It also makes sense not to spend money on an electromechanical device if the in-house network includes an uninterruptible power supply or voltage stabilizer.
By response speed (delay)
The letter S denotes RCDs, which operate with a set delay of up to 0.5 seconds – “selective”. This type of device allows you to create multi-level “cascade” protection systems with several protected circuits. Each emergency section of the network, depending on the tasks and the implementation of the scheme, will be disconnected separately, while the general power supply to the room will remain. RCDs with index G also have a delay, but it is much less.
1 – lead-in cable; 2 – introductory machine; 3 – counter; 4 – RCD type S; 5 – machines; 6 – zero bus; 7 and 8 – RCD type AC; 9 – three-core electrical wiring; 10 – grounding bus
Selective RCDs are usually installed at the top of the cascade, therefore, in case of leaks, non-selective devices first operate, without de-energizing all circuits that are protected.
High quality modern non-selective RCDs operate in less than 0.1 seconds.
By the number of poles
For a three-phase network, RCDs with four poles are used. They protect several single-phase networks, or separate three-phase consumers (electric motor, hob …). In tandem with this type of RCD, a four-pole automatic device should work.
For a single-phase network of residential premises, devices with two poles (line and neutral) are usually used.
Leakage current (rated residual current or “setpoint”) under specified operating conditions is one of the main parameters characterizing the functional features of an RCD. The boundary barrier for classification is a current of 30 mA. RCDs that operate at lower creepage points are considered to protect a person from electric shock. Devices, the operating current of which is higher than 30 mA, are considered fire-fighting, since a rather large load can be connected to them, but the differential currents that they allow are dangerous to humans. Sometimes 30 mA RCDs are considered universal, they are most common.
Fireproof RCDs are the first stage of protection located in the switchboard, they are usually installed on the entire internal network, but they can also be used to protect individual heavy-duty and dangerous consumers from ignition (for example, a fan heater with an open spiral). The leakage current of fire-fighting RCDs is usually taken at 100-300 mA, sometimes 500 mA devices are also used as fire-fighting devices. RCDs with a lower current cannot work normally in these positions, since false alarms occur due to exceeding the permissible loads.
RCDs with a leakage current of 10 mA are usually used in the second or third stage of protection, they are used either to connect lighting elements, or for individual electrical appliances that are located in hazardous areas, for example, in a bathroom, shower, pool … power through them, most likely, will not succeed, since the workload will be limited to 1.8 kilowatts.
Note that the current rating shows only the lower trip limit, so a 30 mA RCD will not disconnect the circuit with a leakage of 25 mA, but will trip at any currents exceeding the 30 mA threshold.
With what leakage current is it necessary to apply an RCD in a particular case? First, the leakage current of the circuit or device is determined, this can be done by measurement or according to the current regulations. According to SP 31-110-2003, the leakage current of the device is taken equal to 0.4 mA for each 1 A of its power. This also adds 10 μA for each meter of phase conductor. For example, for an electrical appliance with a power of 16 A, powered by a twenty-meter wire, the expected leakage current should be taken equal to 4.2 mA. Now you can pick up an RCD, but this is done so that the leakage current of the device is no more than 33% of the operating current of the residual current device. In our case, this is 12.6 mA. A 10-ampere device is no longer suitable, which means it is necessary to supply an RCD with a trigger current of 16 mA.
The operating current of the RCD (or the maximum permissible load) determines how much and what power the consumers can be powered through this device. This characteristic shows the current that can pass through the RCD for a long time without destroying it..
The calculation of the required RCD is made from the characteristics of the consumers connected to it. In the electrical networks of residential premises, low-power RCDs with an operating current of 10 A are often used. Differential protection devices with a permissible load of 16–32 A are considered medium-power. Devices for 40 A and more are called powerful.
It is noteworthy that in practice there is a clear relationship between the shutdown current and the operating current. Manufacturers produce RCDs, in which the higher one indicator, the higher the other..
It is not difficult to calculate the required operating current of the RCD, in any case, it must be equal to or exceed the rated power of the circuit breaker of the protected circuit.
If possible, the regulation of the rated leakage current of the RCD are:
- adjustable (continuous adjustment, step adjustment)
By the presence of short-circuit protection, there is an RCD:
- with overcurrent protection (differential circuit breakers)
- with overheating protection
- without overcurrent protection
According to the method of installation, the RCD is divided into:
- stationary in the form of an automatic machine, which are mounted on a rail in the mounting panel;
- portable – mounted on an extension cord or in a break in the supply cord;
- RCD in the socket (widely used in the USA).
Installation and connection of RCD
Further, we will talk only about the protection devices that are installed in the shield, since in our country they are used most actively.
In a household network, two-pole RCDs are usually used, which occupy two places (36 mm) on a din rail. They are usually located near the lines of the protected circuits, with the exception of fire-fighting devices with a shutdown current of 100-500 A, which are installed near the input machine. RCDs can also be located in group ASUs of apartment buildings, and floor panels of a private house.
If the wiring is divided into groups, it is recommended to install one RCD at the input and several devices in different groups, while ensuring their selectivity – cascade disconnection. To do this, an RCD with a lower trip current rating or a higher trip rate is installed on each next tier below.
An RCD is connected in accordance with a pre-developed leakage current protection scheme. The protection system is designed depending on the functions performed by the device and the specific characteristics of the network. Below is a simple diagram for connecting an RCD to an electrical installation with grounding, it can be used to protect individual circuits in multi-tiered cascade systems:
1 – lead-in cable; 2 – introductory machine; 3 – counter; 4 – RCD; 5 – machines; 6 – zero bus; 7 – three-core electrical wiring; 8 – grounding bus; 9 – ground wire
As you can see, there is nothing complicated, let’s draw your attention to some points:
- For the correct operation of the RCD, in the protected circuits there should be no contact of the working neutral conductor with grounded elements or the protective PE conductor. For each of them, its own bus is used in the shield (GOST R 50571.3-94).
- The grounding conductor “does not participate” in the connection of the RCD.
- The power supply for the RCD is connected to the upper terminals. Connectors for phase input on the RCD are usually designated “1”, for the output – “2”.
- The neutral of the power supply (zero, wire with blue insulation) must be connected to the connector marked “N”. This rule must be observed for RCDs of any brand, rating and purpose..
- The most important point! The rated operating current of the RCD must be the same or greater than the operating current of the circuit breakers. Only then will the machines be able to protect expensive RCDs from overload..
- The installed RCD must be checked for operability.
Checking the RCD
After switching all circuits, the intra-house network must be powered. If the circuit breakers or RCDs have not tripped, then there is no short circuit, and the neutral conductor does not contact ground.
Then press the “TEST” or “T” button located on the front panel of the device. Thus, we forcibly simulate the occurrence of a leakage current. A serviceable RCD should instantly work and de-energize the protected area. If this does not happen, then in the event of an emergency, the device will not help to cope with the problem..
The last stage of the check can be considered the supply of a load to the RCD. It is necessary to turn on one by one all devices that will work in a particular circuit and the network as a whole. In case of possible malfunctions, it is necessary to make changes to the protection circuit or change the ratings of the residual current devices.
RCDs are not the only way to protect a person from electric shock and network overloads, which can lead to a fire. But often it is these devices that save lives and ensure the safety of citizens’ property..