Despite all the advantages that electrical energy has, it also has disadvantages. The main one is the danger of electric shock. To protect people from the action of electricity, many devices have been created, and one of them is called RCD - Residual current device. But in order to organize effective protection using these devices, it is necessary to have a good idea of how to choose an RCD and how to connect it.
Content
- Purpose and device of RCD
- How protection works
- Residual current device characteristics
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How to choose an RCD
- Differential selection
- Power and voltage
- How to connect an RCD
- Checking if the connection is correct
- Difautomat as a variant of double protection
Purpose and device of RCD
The device is an automatic device that disconnects the voltage when a differential current occurs. (differential current, leakage current). It is he who arises with a particular malfunction of electrical equipment, in most cases, is the result of accidents. Worst of all, the fault causing the leak does not manifest itself in any way. The washing machine does the washing, the computer calculates the task, the boiler diligently heats the water. But as soon as a person touches the casing of such a device or takes a shower, the irreparable will happen.
The task of the differential switch includes leak detection and emergency shutdown of faulty equipment when it appears. How does he do it?
How protection works
Electrical equipment is known to receive energy through wires. At the same time, household appliances use two conductors - phase and zero. If the equipment is industrial or just powerful, then it can use three phases, but this does not change the essence of the problem. So, roughly speaking, the current runs in phase (s), activates the electrical equipment and runs away through zero *. In this case, the incoming and outgoing values of the currents are always equal - how much energy entered the device through the phase wire, the same amount came out through the zero one.
* Since the mains voltage is variable, the direction of the current changes 100 times per second, but this is not essential for understanding the basic principle.
Suppose an abnormal situation has occurred - the oil seal leaked in the washing machine and the control board was poured inside water has entered the electric meat grinder, the wire insulation is frayed due to vibration, or a thermal breakdown has occurred electric motor. For this reason, voltage appeared on the metal parts of the unit. If the instrument is grounded, this voltage from the chassis or enclosure will drain into the ground loop, causing a differential kick. There will be no leakage in ungrounded equipment, but it will occur if a person standing on a conductive surface (concrete floor, metal or wood, but wet flooring, etc.), touches the cover.
The occurrence of a differential current through the ground loop or the human body
In this situation, the current entering through the phase conductor will no longer be equal to the outgoing current at zero. The value of the leakage flowing into the ground will be added to it: Iphase = Izero + Ileakage.
The task of the RCD is precisely to determine this imbalance. During operation, the device constantly compares the value of the currents passing through the phase and zero. As long as they are equal, everything is fine. As soon as the phase current exceeds zero, the device will cut off the voltage in an emergency.
Contrary to popular belief, leakage currents are not uncommon. Almost every housewife faced a situation when an electrical appliance began to "pinch".
Residual current device characteristics
Your health, and possibly life, will depend on how correctly you choose a protection device. That is why this issue must be approached with all responsibility. What are the characteristics of RCDs, which also have one more name - differential switches (not to be confused with automatic machines)?
- Differential current. The main parameter of the device. In essence, this is the sensitivity of the protection system. For example, the differential switch pictured below will trip with a leakage of 30 mA (position 3 in the photo).
- Working current or power. The current load in amperes that the device can withstand for a long time without damage or overheating (position 1).
- Working voltage. The maximum voltage of the network into which the RCD will be built (position 2).
- A kind of tension. The type of mains voltage into which the protection device will be built. Can be variable, constant or variable + constant (position 5).
How to choose an RCD
Obviously, each of the above parameters is equally important when choosing an RCD. A device with low operating voltage and power will simply burn out, and the wrong choice of the differential current value and kind voltages make the device completely useless - it will either falsely trigger or it won't work at all.
Differential selection
This is the main and most important selection criterion. Let's see, how to calculate an RCD for a specific object. According to the PES, the permissible leakage value in electrical installations is taken at the rate of 0.4 mA per 1 A of load current. Additionally, you need to know the length of the electrical circuit and add another 0.01 mA to the result obtained for each meter of the supply wire. Agree, such a calculation of an RCD for, say, house wiring is quite complicated and time-consuming. But you can make everything much simpler, and the level of protection will not suffer.
If the device is planned to be installed not for fire safety purposes, but for the protection of people (usually this and there is the main task in house networks), then it must confidently respond to a leak of no more than 50-80 mA. It is this value that is considered the maximum permissible for the human body. On the other hand, if the house network is very long and ramified, and even with damp premises (the same bathroom), then the amount of natural leakage distributed along the entire line may be higher than the same 50 mA.
How to be here? Install a more powerful device, as recommended by some "experts"? In no case! After all, if you, God forbid, get under voltage, powerful automation will not be able to save you - it will either work after the deadly current passes through the body, or it will not turn off at all. In any case, the person himself will not care.
The way out of the situation is not the choice of a coarser RCD, but the installation of several sensitive devices, each of which will monitor a separate section of the circuit. For example, one device for the bathroom and kitchen, another for the sockets, and the third for lighting.
Power and voltage
By these criteria, making the choice of an RCD is much easier. You know the voltage value in the networks: with a single-phase line it is 220 V, a three-phase line - 380 or 660 V. The type of voltage in both cases is variable. If your network is single-phase, then the device can also be selected single-phase. If there are three phases, then a three-phase differential switch is needed.
Single-phase and three-phase differential switches
Now about how to choose an RCD and an automatic machine for power. Why an automatic machine? The fact is that the differential switch does not operate from overload or short circuit, but only reacts to the differential current. If there is a short circuit in the house, the difavtomat will safely burn out from overload along with the wiring. Therefore, the installation of an RCD paired with a machine is required.
As for the operating current of the differential switch, it must be no less than that for which the input circuit breaker is designed. If you already have a circuit breaker, just look at what current it is rated for. If not, then you will have to put it. For an ordinary apartment without particularly powerful consumers, an automatic machine with a capacity of up to 32 A, withstanding a load of 7 kW, is usually sufficient. It should be noted here that the voltage and rated current indicated on the body of the switch and the machine may be more than necessary, but never less.
How to connect an RCD
In order for the differential switch to work for a long time and reliably, it must be correctly installed in the house scheme. Here are some prerequisites that must be met when installing an RCD:
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Correct phasing. In order for the differential switch to control the difference between the phase and zero currents, it must be able to distinguish between them. Therefore, the phase and neutral wires must be connected to strictly defined terminals, marked accordingly on the device case. If you confuse zero and phase, then the device will either constantly work, or will not turn on protection at all, which is even worse. In polyphase devices, the phase sequence can be ignored, but zero must be strictly in its place. - Obligatory protection against short circuit. As mentioned above, the RCD does not have its own short-circuit protection, therefore it must be installed in series with the machine. Where the machine will stand - before or after the RCD - does not matter. It is allowed to install one machine on several RCDs and vice versa - the output of the differential switch can be loaded on several machines serving different lines.
- Protection from external influences. Almost all RCDs are not protected from moisture, so they need to be placed in dry rooms or in special closed cabinets. Otherwise, the device may fail at the most inopportune moment with all the ensuing consequences.
Option for connecting an RCD to a house network
Checking if the connection is correct
After the protection device is installed, it is imperative to check the correctness of its operation, so as not to find out about the problems in the connection after the fact - after being energized. The check is carried out by two methods - a test button and artificial creation of a leakage current. The first one is extremely simple - press the button located on the RCD body and signed accordingly. The circuit built into the device will create a simulation of the leakage current, and the protection will trip, de-energizing the line.
If you do not trust this button (you never know what and what imitates there), then you can check the device by artificially creating a leakage current. Connect an electrical appliance - a table lamp, a soldering iron, etc. between the phase contact of the outlet and its grounding terminal.
Scheme for checking the correct operation of the differential switch
The RCD will immediately notice the leak and immediately disconnect the circuit. Everything is fine. If your outlets are not connected to the ground loop, which is very, very bad, then you can use a half-meter prong stuck into the ground as a ground contact. This, of course, is not a full-fledged grounding, but the current through it is more than enough for the RCD to fix the leak.
Difautomat as a variant of double protection
There is another device capable of operating from a leakage current. It is called a differential automaton. Its difference from a conventional RCD is that the device additionally includes a circuit breaker that reacts to a short circuit. Installing a difavtomat makes sense if you are doing an installation from scratch or your wiring does not have an automatic machine at all. Buying a difavtomat, you get two devices in one case at once - an RCD and an automatic machine. It is both cheaper and easier to install.
Difautomat looks like a residual current device, but has a built-in automatic
The choice of a differential automaton is made according to the criteria:
- Differential current.
- Rated current.
- Working voltage.
- Kind of current.
- Cut-off current.
You are probably familiar with almost the entire list - you used it to choose an RCD. He is also suitable for choosing a difavtomat. An additional criterion is only the cut-off current - the current at which the automatic device built into the device is triggered. Usually it is equal to three times the operating current, so if you choose the operating current correctly, then the cutoff current will also be optimal.
