UZO — purpose, principle of construction, choice
Residual current devices (RCDs) are one of the most popular devices used by both building corporations and private consumers. But how to make sure of the right choice RCD? I hope this article will make it easier for you to navigate the RCD market saturated with different models.
Residual current device. The basics
Residual current devices (RCD) or, in other words, differential protection devices, are designed to protect people from electric shock in the event of electrical faults or in contact with live parts of an electrical installation, as well as to prevent fires and fires , caused by leakage currents and earth faults... These functions are not inherent in conventional circuit breakers which only react to overload or short circuit.
What is the reason for seeking fire extinguishers for these devices?
According to statistics, the cause of about 40% of all fires that occur is "closing electrical wires".
In many cases, the general phrase "short circuit in electrical wires" often covers electrical leaks that occur due to aging or insulation failure. In this case, the leakage current can reach 500mA. It was experimentally found that when a leakage current of just such a strength flows (and what is half an ampere? Neither thermal nor electromagnetic release to a current of such a strength simply does not respond - only for the reason that they are not designed for this) for a maximum half an hour through wet sawdust, they spontaneously ignite. (And this applies not only to sawdust, but to any dust in general.)
And how do RCDs protect you and me from electric shocks?
If a person touches a live part, a current will flow through his body, the value of which is the coefficient of division of the phase voltage (220 V) by the sum of the resistances of the wires, grounding and the human body itself: Ipers = Uph / (Rpr + Rz + Rp ). In this case, the earthing and wiring resistances compared to the resistance of the human body can be neglected, the latter can be taken as equal to 1000 ohms. Therefore, the current value in question will be 0.22 A or 220 mA.
From the normative and reference literature on labor protection and safety measures, it is known that the minimum current, the flow of which is already felt by the human body, is 5 mA. The next standardized value is the so-called release current, equal to 10 mA. When a flow of such force passes through the human body, spontaneous muscle contraction occurs. An electric current of 30 mA can already cause respiratory paralysis.Irreversible processes associated with bleeding and cardiac arrhythmias begin in the human body after a current of 50 mA flows through the body. Lethal output is possible when exposed to a current of 100 mA. It is obvious that a person should already be protected from a current equal to 10 mA.
So the timely response of the automation to a current of less than 500 mA protects the object from fire, and to a current of less than 10 mA — protects a person from the consequences of accidentally touching live parts.
It is also known that you can safely hold the current-carrying part, which is under a voltage of 220 V, for 0.17 s. If the active part is energized at 380 V, the safe touch time is reduced to 0.08 s.
The problem is that such a small current, and even for a negligible short time, is not able to fix (and, of course, turn off) conventional protective devices.
Therefore, such a technical solution was born as a ferromagnetic core with three windings: — “current supply”, “current conductor”, “control”. The current corresponding to the phase voltage applied to the load and the current flowing from the load in the neutral conductor induce magnetic fluxes of opposite signs in the core. If there are no leaks in the load and in the protected part of the wiring, the total flux will be zero. Otherwise (touch, insulation failure, etc.), the sum of the two currents becomes non-zero.
The flux arising in the core induces an electromotive force in the control coil. A relay is connected to the control coil through a precision filtering device for any interference. Under the influence of the EMF occurring in the control coil, the relay breaks the phase and neutral circuits.
In many countries, the use of RCDs in electrical installations is regulated by norms and standards.For example, in the Russian Federation - adopted in 1994-96 GOST R 50571.3-94, GOST R 50807-95, etc. According to GOST R 50669-94, the RCD is installed without problems in the power supply network of mobile buildings made of metal or with a metal frame for street trade and household services. In recent years, the administrations of large cities, in accordance with the state standards and recommendations of Glavgosenergonadzor, made decisions to equip the stock of residential and public buildings with these devices (in Moscow — Order of the Government of Moscow No. 868 -RP dated 20.05.94 .).
UZO are different... Three-phase and single-phase...
But the division of RCD into subclasses does not end there...
Currently, there are 2 radically different categories of RCDs on the Russian market.
1. Electromechanical (mains independent)
2. Electronic (depends on network)
Let's consider the principle of action of each of the categories separately:
Electromechanical RCDs
RCD founders are electromechanical. It is based on the principle of precision mechanics, i.e. looking inside such an RCD, you won't see op amp comparators, logic, and the like.
It consists of several main components:
1) The so-called zero-sequence current transformer, its purpose is to track the leakage current and transmit it with a certain Ktr to the secondary winding (I 2), I ut = I 2 * Ktr (a very idealized formula, but reflecting the essence of the process ).
2) A sensitive magnetoelectric element (lockable, i.e. when triggered without external intervention, it cannot return to its initial state — a lock) — plays the role of a threshold element.
3) Relay - provides tripping if the lock is engaged.
This type of RCD requires highly precise mechanics for the sensitive magnetoelectric element.Currently, only a few global companies sell electromechanical RCDs. Their price is much higher than the price of electronic RCDs.
Why have electromechanical RCDs become widespread in most countries of the world? Everything is very simple - this type of RCD will work if a leakage current is detected at any voltage level in the network.
Why is this factor (regardless of the mains voltage level) so important?
This is due to the fact that when we use a working (serviced) electromechanical RCD, we guarantee 100% of the time that the relay will trip and the consumer's power will be cut off accordingly.
In electronic RCDs, this parameter is also large, but it is not equal to 100% (as will be shown below, this is due to the fact that at a certain level of network voltage, the electronic RCD circuit will not work), and in our each percent is a possible human life (either a direct threat to human life when it touches the wires, or indirect, in the case of a fire from burning the insulation).
In most of the so-called "developed" countries, electromechanical RCDs are a standard and a device that is mandatory for widespread use. In our country, there is a gradual transition to the mandatory use of RCDs, but in most cases the user is not given information about the type of RCD, which leads to the use of cheap electronic RCDs.
Electronic RCDs
Every construction market is flooded with such RCDs. The costs of electronic RCDs are in some places lower than for electromechanical ones up to 10 times.
The disadvantage of such RCDs, as already mentioned above, is not a 100% guarantee, if the RCD is in good condition, that it will be triggered as a result of the occurrence of leakage current. The advantage is cheapness and availability.
In principle, the electronic RCD is built in the same way as the electromechanical one (Fig. 1). The difference lies in the fact that the place of the sensitive magnetoelectric element is taken by a comparative element (comparator, zener diode). For such a scheme to work, you will need a rectifier, a small filter (probably even a KREN). Because the zero-sequence current transformer is a step down (tens of times), then a signal amplification circuit is also needed, which in addition to the useful signal will also amplify the interference (or the unbalance signal present at zero leakage current) ). It is obvious from the above that the moment when the relay is triggered in this type of RCD is determined not only by the leakage current, but also by the mains voltage.
If you can't afford an electromechanical RCD, then it's still worth getting an electronic RCD because it works in most cases.
There are also cases when it makes no sense to buy an expensive electromechanical RCD. One of these cases is the use of a stabilizer or uninterruptible power supply (UPS) when powering an apartment / house. In this case, it makes no sense to take an electromechanical RCD.
I note right away that I am talking about RCD categories, their pros and cons, and not about specific models. You can buy low-quality RCDs of electromechanical and electronic types. When buying, ask for a certificate of conformity, because many electronic RCDs on our market are not certified.
Zero Sequence Current Transformer (TTNP)
Usually this is a ferrite ring through which (inside) the phase and neutral wires pass, they play the role of the primary winding. The secondary winding is uniformly wound on the surface of the ring.
Perfect:
Let the leakage current be zero.The current flowing through the phase conductor creates magnetic field equal in magnitude to the magnetic field created by the current flowing through the neutral wire and opposite in direction. Thus the total coupling flux is zero and the current induced in the secondary winding is zero.
At the moment when the leakage current flows through the conductors (zero, phase), a current imbalance occurs, as a result of the occurrence of a flux from the coupling and the induction of a current proportional to the leakage current to the secondary winding.
In practice, there is an unbalance current that flows through the secondary winding and is determined by the transformer used. The requirement for TTNP is as follows: the unbalance current must be significantly less than the leakage current reduced to the secondary winding.
Selection of RCDs
Suppose you have decided on the type of RCD (electromechanical, electronic). But what to choose from the huge list of products on offer?
You can choose an RCD with sufficient accuracy using two parameters:
Rated current and leakage current (break current).
The rated current is the maximum current that will flow through the phase conductor. It is easy to find this current knowing the maximum power consumption. Just divide the worst case power consumption (maximum power at minimum Cos (?)) by the phase voltage. There is no point in placing an RCD for a current greater than the rated current of the machine in front of the RCD. Ideally, with a margin, we take the RCD for a rated current equal to the rated current of the machine.
RCDs with rated currents of 10,16,25,40 (A) are often found.
The leakage current (trigger current) is usually 10 mA if the RCD is installed in an apartment / house to protect human life, and 100-300mA in an enterprise to prevent fires if the wires are burned.
There are other RCD parameters, but they are specific and not interesting to ordinary users.
Exit
This article covers the basics of understanding RCD principles as well as methods of constructing different types of residual current devices. Electromechanical and electronic RCDs, of course, have the right to exist, because they have their own distinct advantages and disadvantages.