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When assembling a switchboard or connecting new large household appliances, a home master will definitely face such a problem as the need to select circuit breakers. They provide electrical and fire safety, so the right choice of machine is the key to the safety of you, family and property.
What is the machine for?
In the power supply circuit, an automatic machine is installed to prevent overheating of the wiring. Any wiring is designed for the passage of a certain current. If the passed current exceeds this value, the conductor starts to heat up too much. If this situation persists for a sufficient period of time, the wiring begins to melt, which leads to a short circuit. A circuit breaker is installed to prevent this situation.
The second task of the circuit breaker is to turn off the power when a short circuit current (SC) occurs. When closing, the currents in the circuit increase many times and can reach thousands of amperes. So that they do not destroy the wiring and do not damage the equipment included in the line, the circuit breaker must turn off the power as soon as possible – as soon as the current exceeds a certain limit.
In order for the protective circuit breaker to perform its functions properly, it is necessary to correctly select the machine in all respects. There are not so many of them – only three, but each one must be dealt with.
What are automatic protection
To protect the conductors of a single-phase 220 V network, there are single-pole and two-pole disconnecting devices. Only one conductor is connected to single-pole – phase, to two-pole and phase and zero. Single-pole machines are installed on 220 V internal lighting circuits, on socket groups in rooms with normal operating conditions. They are also put on some types of load in three-phase networks, connecting one of the phases.
For three-phase networks (380 V) there are three and four poles. These circuit breakers (the correct name is a circuit breaker) are put on a three-phase load (ovens, hobs and other equipment that runs on 380 V).
In rooms with high humidity (bathroom, sauna, swimming pool, etc.), bipolar circuit breakers are installed. They are also recommended to be installed on powerful appliances – on washing machines and dishwashers, boilers, ovens, etc.
It’s just that in emergency situations – during a short circuit or insulation breakdown – a phase voltage can get to the neutral wire. If a single-pole device is installed on the power line, it will disconnect the phase wire, and zero with a dangerous voltage will remain connected. So, there is a possibility of electric shock when touched. That is, the choice of the machine is simple – single-pole switches are placed on some of the lines, and two-pole switches on some. The exact amount depends on the network condition.
For a three-phase network, there are three-pole circuit breakers. Such an automatic machine is installed at the entrance and at consumers, to which all three phases are connected – an electric stove, a three-phase hob, an oven, etc. The remaining consumers are equipped with bipolar circuit breakers. They must necessarily disconnect both phase and neutral.
The choice of the circuit breaker rating does not depend on the number of wires connected to it.
Deciding on a denomination
Actually, from the functions of the circuit breaker, the rule for determining the rating of the circuit breaker follows: it must operate until the current exceeds the wiring capabilities. And this means that the current rating of the machine must be less than the maximum current that the wiring can withstand.
Based on this, the algorithm for choosing a circuit breaker is simple:
- Calculate the cross section of the wiring for a specific area.
- See what maximum current this cable can withstand (there is in the table).
- Further, from all the denominations of circuit breakers, we select the nearest smaller one. The ratings of the machines are tied to the permissible continuous load currents for a particular cable – they have a slightly lower rating (there is in the table). The list of ratings looks like this: 16 A, 25 A, 32 A, 40 A, 63 A. From this list, choose the right one. There are denominations and less, but they are practically not used anymore – we have too many electrical appliances and they have considerable power.
Example
The algorithm is very simple, but it works flawlessly. To make it clearer, let’s look at an example. Below is a table that indicates the maximum allowable current for conductors that are used when laying wiring in a house and apartment. There are also recommendations regarding the use of machines. They are given in the column “Rated current of the circuit breaker”. It is there that we are looking for denominations – it is slightly less than the maximum allowable, so that the wiring works in normal mode.
| Cross section of copper wires | Permissible continuous load current | Maximum load power for a single-phase network 220 V | Rated current of the circuit breaker | Circuit breaker current limit | Approximate load for a single-phase circuit |
|---|---|---|---|---|---|
| 1,5 sq. mm | A 19 | 4,1 кВт | A 10 | A 16 | lighting and signaling |
| 2,5 sq. mm | A 27 | 5,9 кВт | A 16 | A 25 | socket groups and electric underfloor heating |
| 4 sq. Mm | A 38 | 8,3 кВт | A 25 | A 32 | air conditioners and water heaters |
| 6 sq. Mm | A 46 | 10,1 кВт | A 32 | A 40 | electric stoves and ovens |
| 10 sq. mm | A 70 | 15,4 кВт | A 50 | A 63 | introductory lines |
In the table we find the selected wire section for this line. Suppose we need to lay a cable with a cross section of 2,5 mm2 (most common when laying to medium power appliances). A conductor with such a cross section can withstand a current of 27 A, and the recommended rating of the machine is 16 A.
How will the chain work then? As long as the current does not exceed 25 A, the machine does not turn off, everything works in the normal mode – the conductor heats up, but not to critical values. When the load current begins to increase and exceeds 25 A, the machine does not turn off for some time – perhaps these are starting currents and they are short-lived. It turns off if the current exceeds 25 A by 13% for a sufficiently long time. In this case, if it reaches 28,25 A. Then the electric bag will work, de-energize the branch, since this current already poses a threat to the conductor and its insulation.
Power calculation
Is it possible to choose an automatic machine according to the load power? If only one device is connected to the power line (usually it is a large household appliance with a large power consumption), then it is permissible to make a calculation based on the power of this equipment. Also in terms of power, you can choose an introductory machine, which is installed at the entrance to a house or apartment.
If we are looking for the value of the introductory machine, it is necessary to add up the power of all devices that will be connected to the home network. Then the found total power is substituted into the formula, the operating current for this load is found.
After we have found the current, select the value. It can be either a little more or a little less than the found value. The main thing is that its tripping current does not exceed the maximum allowable current for this wiring.
When can this method be used? If the wiring is laid with a large margin (this is not bad, by the way). Then, in order to save money, you can install automatic switches corresponding to the load, and not to the cross section of the conductors. But once again we pay attention that the long-term permissible current for the load must be greater than the limiting current of the circuit breaker. Only then will the choice of circuit breaker be correct.
Choosing a breaking capacity
The selection of a packet box for the maximum allowable load current is described above. But the circuit breaker should also turn off when a short circuit (short circuit) occurs from the network. This characteristic is called the breaking capacity. It is displayed in thousands of amperes – this order can be reached by currents during a short circuit. The choice of a machine for breaking capacity is not very difficult.
This characteristic shows at what maximum value of the short-circuit current the machine remains operational, that is, it can not only turn off, but will also work after being turned on again. This characteristic depends on many factors and for accurate selection it is necessary to determine the short-circuit currents. But for wiring in a house or apartment, such calculations are made very rarely, but are guided by the distance from the transformer substation.
If the substation is located near the entrance to your house / apartment, they take a machine with a breaking capacity of 10 A, for all other city apartments, 000 A is enough. and a breaking capacity of 6 A. The networks here are usually old and short-circuit currents are not large. And since the price increases significantly with increasing breaking capacity, the principle of reasonable economy can be applied.
Is it possible to install bags with a lower breaking capacity in city apartments. In principle, it is possible, but no one guarantees that after the first short circuit you will not have to change it. He may have time to turn off the network, but will be inoperable at the same time. In the worst case, the contacts will melt and the machine will not have time to turn off. Then the wiring will melt and a fire may occur.
Type of electromagnetic release
The machine should operate when the current rises above a certain mark. But the network periodically experiences short-term congestion. They are usually associated with inrush currents. For example, such overloads can be observed when turning on the refrigerator compressor, washing machine motor, etc. The circuit breaker should not trip during such temporary and short-term overloads, because they have a certain delay in operation.
But if the current has increased not because of an overload, but because of a short circuit, then during the time that the circuit breaker “waits”, its contacts will melt. That’s why there is an electromagnetic automatic release. It works at a certain current value, which can no longer be an overload. This indicator is also called the cut-off current, since in this case the circuit breaker cuts off the line from the power supply. The magnitude of the tripping current can be different and is displayed by the letters that precede the numbers indicating the rating of the machine.
There are three most common types:
- B – works when the rated current is exceeded by 3-5 times;
- C – if it is exceeded by 5-10 times;
- D – if more than 10-20 times.
Automatic or cut-off current class
With what characteristic to choose a package? In this case, the choice of a circuit breaker is also based on the remoteness of your household from the substation and the state of the power grid, the choice of a circuit breaker is carried out using simple rules:
- With the letter “B” on the case, they are suitable for summer cottages, houses in villages and towns that receive power through air ducts. They can also be installed in apartments of old houses in which the reconstruction of the house electrical network has not been carried out. These circuit breakers are not always on sale, they cost a little more than category C, but they can be delivered to order.
- Bags with a “C” on the body are the most widely used variant. They are installed in networks with a normal state, suitable for apartments in new buildings or after major repairs, in private houses near the substation.
- Class D is put in enterprises, in workshops with equipment that has high starting currents.
That is, in fact, the choice of a circuit breaker in this case is simple – type C is suitable for most cases. It is available in stores in a large assortment.
Which Manufacturers Should You Trust?
And finally, let’s pay attention to manufacturers. The choice of a machine cannot be considered complete if you have not thought about what brand of circuit breakers you will buy. You definitely shouldn’t take unknown firms – an electrician is not an area where you can experiment. Details about choosing a manufacturer in the video.