Accident in the apartment electrical network: causes and consequences. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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Everyone knows that the voltage in our apartments is ~220 V, and they also know that in the evening the voltage decreases, as indicated by the light bulbs (they don’t shine so brightly). However, perhaps only electricians know that the voltage in the same electrical network can increase sharply, up to ~380 V. This is already an emergency situation, and its consequences are severe: light bulbs in apartments instantly burn out and all household appliances connected to the network at that time fail.

What to do in such a situation, why does it occur and how to prevent it? This is what this article will discuss.

Our multi-storey buildings are powered by a three-phase network. A three-phase high-voltage transformer, placed near houses, lowers the high voltage, usually from ~10 kV to ~220 V. The three secondary windings of the supply transformer are connected in a so-called star (Fig. 1), resulting in three phases, designated in the diagrams by the Latin letters A , B, C, and a common wire N, which is called zero.

The voltage ~220 V between the neutral wire and any phase is called phase voltage, and this is what is intended to be supplied to our apartments. The voltage between phases is ~380 V and is called line voltage. A four-core cable supplies three phases and a neutral from the transformer to the electrical panel of the house, usually located in the basement, and from there it is routed through the electrical panels of the staircases (Fig. 2, a and Fig. 3, a). This power supply circuit is called 4-wire. From the electrical panels of the staircases, 2-wire wires branch off the neutral and phase, i.e. ~220 V, to each apartment (Fig. 2, a).

(click to enlarge)

Electricians strive to load each phase evenly, as this creates favorable conditions for the operation of a three-phase transformer (magnetic equilibrium is established in it and it does not overheat), and in the neutral wire, with absolutely equal phase loads, the current is zero. For this reason, in 4-core power cables the neutral wire is made of a smaller cross-section [1]. Electricians turn on the same number of apartments in each phase in the hope that they will consume the same power. However, this cannot be achieved, so some current always flows in the neutral wire, but this does not matter, the three-phase network works normally. Its work can be represented by three vectors A, B, C (Fig. 2,b), the length of each of them, on a scale, is equal to ~220 V, and each phase lags behind the neighboring one by 120°.

In what cases can tension in apartments increase sharply? An emergency situation occurs when the neutral wire in a three-phase network breaks (in Fig. 2, a - loss of contact in the “clamp”). In this case, the voltages in apartments 4, 5, 6 will change dramatically. In some apartments the voltage will increase, in others it will decrease, it all depends on the ratio of loads at the time of the accident. In phase B (Fig. 2a, quarter 4), where the load is minimal, the voltage will increase, and in phase C (quarter 6), where the load is maximum, the voltage will decrease. In phase A (quarter 5), where the load is intermediate, changes may be insignificant. These changes are depicted in the vector diagram shown in Fig. 2c: vector BN1 has lengthened, which means the voltage has increased; the CN1 vector has decreased - the voltage has decreased; vector AN] shows a slight increase in voltage. Finally, a new vector NN1 has appeared (the length of which indicates that voltage has appeared between the neutral wire of the consumer and the source. In this case, they say a zero shift has occurred, which is unacceptable [1].

In terms of time, this accident can be described as follows. After the neutral wire breaks, the voltage in the phase with the minimum load increases sharply, and light bulbs along with household appliances included in this phase in apartments burn out, the load on this phase instantly decreases, from this decrease the voltage distribution between the phases sharply changes for the worse (Fig. .2,d). Now the voltage on phase A (vector AN]) has increased significantly, and light bulbs and all household appliances burn out there, and the described version of the accident ends with the voltage being set on phases B and A close to 380 V, and on phase C - about 0 V (Fig. 2, d). This all happens within a few seconds.

And now a few clarifications. Firstly, the accident will continue until contact in the neutral wire is restored. Secondly, if the neutral wire, indicated in Fig. 2,a, breaks, the accident will occur only in apartments 4,5,6; the owners of apartments 1,2,3 will not be affected by this accident. Thirdly, do not confuse a break in the neutral wire of a 3-phase network when the above-described accident occurs and a break in the neutral wire supplying voltage to your apartment. In the latter case, only the light in your apartment will be lost (Fig. 2, a).

Is it possible to prevent such an accident? To some extent yes. The first symptoms of an approaching accident are sharp flashes of brightness of light bulbs in your apartment or rural house. This is a sign that there is poor contact in the neutral wire of the three-phase network that powers your multi-storey building, and in rural areas a street or several streets. It's time for you to call an electrician. By the way, in rural areas, light bulbs can also blink due to the work of a neighbor’s electric welding, which is not associated with a poor contact in the neutral wire, but they do not flash: first the brightness decreases, and then returns to normal. Although, as practice shows, electric welding can provoke loss of contact, and if it (the contact) was previously unreliable and was located in the neutral wire of a three-phase network, then an accident is guaranteed.

If you inspect the electrical panel located on the staircase of your multi-story building, you will be convinced that there are preconditions for an accident there too. In Fig. 3,6, the author drew almost from life the state of the contacts in the electrical panels on the staircases in the house where he lives. Aluminum wires from the neutral wire and phases on each floor are cut and screwed to the iron plates. The plates rusted over time, and the aluminum wires, having plasticity, were flattened under the screw - the contact resistance increased. The current heats these contacts and the insulation on the wires becomes charred. All this creates the precondition for an accident. To ensure reliable contact in these places, the clamping screws must be periodically tightened, and the plates must be lubricated with grease to prevent rust. All this should be done by the housing office electricians servicing your home.

In rural areas and in summer cottages, in short, where overhead power lines are used, a break in the neutral wire in a three-phase network can occur in windy weather and in a thunderstorm. This is when the wires become overwhelmed. The branches of trees growing near the wires play an important role in this. With such overflow, the neutral wire most often burns out or breaks.

How does a specific household appliance react to such an accident?

Consider this on the example of one accident that occurred in Troyeschina in Kyiv.

1. The computer system unit was turned off. As already described in [2], the computer owner thinks that by pressing a button on the front panel of the system unit he turns off his computer. In fact, it puts it into standby mode, and part of the power supply unit (PSU) continues to work. Due to the increased voltage in the power supply unit of the LC-235ATX model, the auxiliary converter ~220 V/=5 V burned out and shorted out the network. Because of this, the bridge diodes broke through, and the quenching thermistor and fuse in the network filter burned out. The computer's motherboard remains intact.

2. The LG Studio Works monitor and the SONY KV-G14Q1 TV were in standby mode. The power supply chip burned out and shorted out the mains rectifier, causing the bridge diodes to break through and the quenching resistor and fuse to burn out.

3. The Broksonic CTVG-5472 TV was in standby mode. In this mode, the TV is powered not from a switching power supply, but from a separate rectifier assembled on the basis of a ~220/=12 V transformer. The primary winding of the transformer burned out, the fuse did not burn out.

4. The AKAI VCR was in standby mode. The switching power supply has burned out.

5. HP inkjet printer. The printer itself was turned off, but its separate power supply unit (located outside the printer) LUCENT 3502V ~220 V/=30 V, 400 mA worked (was plugged into the network), since it does not have a network switch. The transformer burned out.

6. Refrigerator. As you know, it works periodically. Its compressor is turned on by a thermostat and has protection against overheating of the compressor motor, which is what saved the refrigerator.

7. Radiotelephone Panasonic KX-SPP-58. The basic power supply from a separate power supply ~220 V/=9 V, is made on a transformer. The primary winding of the transformer burned out, the fuse in the power supply did not blow.

All the household appliances listed here were damaged in only one apartment in which an emergency occurred, and there are many such apartments.

What conclusion can be drawn by analyzing the damage to household appliances?

The weakest point was the power supply. Conventional fuses, which, it would seem, were supposed to protect equipment from large currents arising from power surges, turned out to be powerless. Possessing inertia, fuses burn out only after the pulse power supply microcircuit is damaged (broken) and this short-circuits the mains rectifier. Varistors installed in the computer’s power supply did not protect either, since, according to their parameters and purpose, they smooth out only short-term large voltage surges [3].

As you can see, all household appliances are very afraid of a significant increase in voltage and this causes them to fail. But how does it behave at low voltages? Undervoltage is less dangerous for it, since switching power supplies installed in TVs, computers, monitors, VCRs, etc. are simply turned off. It remains dangerous, perhaps, for electric motors of compressors installed in refrigerators, since their windings can burn out.

Is there a technical means of protection against voltage surges?

Yes, they do exist. Nowadays, many different devices for protecting consumers from sudden voltage changes have appeared in electrical goods stores. One of them is the AZA-5 accident protection circuit breaker. It is installed between a power outlet and a power consumer (for example, a computer with peripherals). AZA-5 is capable of automatically switching off a load of up to 1,1 kW in 0,04 s when the supply voltage changes abruptly beyond the established limits of ~160...~250 V. The machine can operate in automatic or manual modes. Power consumption in standby mode is 1 W. In one apartment you can install several such machines, one for each group of consumers.

You can make such devices yourself. Their schemes are published, for example, in [4].

The simplest and cheapest device is offered by the author of this article. True, it will only protect your equipment from excess voltage (above V). To manufacture it, you need to purchase a power extension cord and install a fuse holder and a double-sided suppressor type 1,5KE350CA into it. This suppressor is sealed inside the extension cord according to the diagram shown in Fig. 4.

The maximum permissible current through the suppressor is 5 A, so the fuse must be less than this value, for example 4 or 3 A. Its operating principle is simple: as soon as the amplitude voltage of the network exceeds ~350 V, which corresponds to the effective value of ~250 V, the suppressor opens and the fuse burns out.

To protect computers, you can also purchase UPS uninterruptible power supplies. They not only protect computers from voltage surges that exceed the established standards, but also power them for some time in the event of a power outage; for this purpose, batteries are built into them.

What to do if, nevertheless, an accident happened and all household appliances burned down?

First of all, you need to turn off the lights and unplug all household appliances. Then, if you intend to defend your rights in court, you need to call an emergency energy team, which will document the accident. It is not advisable to call an electrician from the housing office: he is an interested party - most likely, he will try to cover up the traces of his shortcomings, and after that it will be difficult for you to prove anything. Then you need to submit an application to the Housing Office in writing, indicating all the equipment that is out of order, so that a commission from the Housing Office can attest to this list. The commission, interested in covering up the traces of the housing office's shortcomings, may begin to ask you questions that are not substantive, for example: who allowed you to install a washing machine and computer and why do you have metal doors? In any case, they are obliged to record your damaged equipment (name, model) and give a written response to your application, but their response may be strange and unpredictable. Do not collude with the commission, as this will not lead to anything good.

Their fault is that they were unable to provide you with quality service, i.e. provide high-quality electricity. Therefore, you can also contact your local branch of the consumer protection society. They will advise you on how to act in the current situation and provide practical help. It is advisable that they draw up a report indicating which equipment has failed. To create such an act, you can also involve your neighbors, who, by putting their signatures (several people), will record a list of damaged equipment.

Next, contact household appliance repair shops (but not private owners who do not have licenses) with a request to repair it, provide an invoice and a written report (certified by a signature and a wet seal) about the reasons for its damage. After repairing all the equipment, you will collect the above-mentioned documents, namely: a certificate from the emergency power engineering team about the causes of the accident; the above-mentioned acts of damage to specific household appliances in your apartment; housing office's response to your application; invoices for the repair of equipment in the workshop and the conclusions of the specialists who repaired this equipment; It is advisable to attach a certificate from the local weather service that there was no thunderstorm at that time; you are filing a lawsuit against the housing office for damages. Damages may include not only the cost of repairs, but also other costs associated with preparing for trial. The consumer protection society or lawyers will help you draw up a statement of claim. Coordinate your actions with neighbors who have suffered the same as you, and immediately file a claim in court. File a claim in court on your own behalf, and not as a group.

In regional centers (rural areas) there are no housing offices, so you file a lawsuit against the oblenergo, i.e. to local power engineers who are responsible for the three-phase networks that supply electricity to your homes.

A few important points: areas of responsibility of power engineers and housing offices. Power engineers are responsible for supplying power to the switchboard of your home (Fig. 2, a). The housing office is responsible for the area from the switch to the meter inclusive - this is where the above-described accidents most often occur, but a break in the neutral wire can also occur in the three-phase section of power engineers. The owner of the apartment is responsible for the area from the meter to the apartment.

During a thunderstorm or lightning strikes, large voltage surges (6000 V or more) can also occur in the network. Naturally, household appliances that are turned on at this time, throughout the house where the load is turned on, fail. During a thunderstorm, you should disconnect all household appliances from the power supply, including refrigerators (remove the plugs from the sockets). Unnecessarily, do not leave household appliances plugged in; it is better to purchase an extension cord with a power switch and turn it off, especially when you leave home, at night and during a thunderstorm.

References:

1. Evdokimov F.E. General electrical engineering. - M.: Higher school, 1990.
2. Vlasyuk N.P. Do you turn off your PC with the "POWER" button? //Radioamator. - 2003. - No. 6. - P.30.
3. Kucherov D.P. Uninterruptible power supplies for PCs and peripherals. - St. Petersburg: Science and Technology, 2005.
4. Balinsky R.N. Protection of electronic equipment from voltage surges//Radioamator. 2003. - No. 6. - P.20.

Author: N.P. Vlasyuk

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