Selection of the rating of the circuit breaker

When assembling an electrical panel or connecting new large household appliances, a home craftsman will surely face such a problem as the need to select circuit breakers. They provide electrical and fire safety, so choosing the right machine is a guarantee of the safety of you, your family and property.

What is the machine for

In the power supply circuit, the machine is installed to prevent overheating of the wiring. Any wiring is designed to carry a certain current. If the passed current exceeds this value, the conductor begins 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.

A bag or circuit breaker is required to prevent overheating of the conductors and shutdown in the event of a short circuit

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 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 parameters. There are not so many of them - only three, but each must be dealt with.

What are the automatic circuit breakers

To protect the conductors of a single-phase 220 V network, there are single-pole and double-pole disconnecting devices. Only one conductor is connected to single-pole ones - phase, to two-pole ones and phase and zero. Single-pole machines are installed on a 220 V circuit of internal lighting, on socket groups in rooms with normal operating conditions. They are also placed on some types of loads in three-phase networks, connecting one of the phases.

For three-phase networks (380 V), there are three and four-pole. These circuit breakers (the correct name is a circuit breaker) are put on a three-phase load (ovens, hobs and other equipment that operates from a 380 V network).

In rooms with high humidity (bathroom, bath, pool, etc.), two-pole circuit breakers are installed. They are also recommended to be installed on powerful appliances - on washing machines, dishwashers, boilers, ovens, etc.

It's just that in emergency situations - with a short circuit or insulation breakdown - phase voltage can get on the neutral wire. If a single-pole device is installed on the power line, it will disconnect the phase wire, and a zero with a dangerous voltage will remain connected. This means that 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 double-pole on some. The exact amount depends on the condition of the network.

Single-phase circuit breakers

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 supplied - an electric stove, a three-phase hob, an oven, etc. For other consumers, two-pole circuit breakers are installed. They must disconnect both phase and neutral without fail.

Example of wiring a three-phase network - types of circuit breakers

The choice of the rating of the circuit breaker does not depend on the number of wires connected to it.

Determining the 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.

For each line it is required to select the correct circuit breaker

Based on this, the algorithm for choosing a circuit breaker is simple:

• Calculate the cross-section of the wiring for a specific site.
• See what maximum current this cable can withstand (see the table).
• Further, from all the ratings of the 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 (available in the table). The list of denominations looks like this: 16 A, 25 A, 32 A, 40 A, 63 A. From this list, choose the right one. There are ratings 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 shows the maximum permissible current for conductors that are used when laying wiring in the 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.

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 routing 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 as usual - 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 power pack 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 load power? If only one device is connected to the power line (usually these are large household appliances with high power consumption), then it is permissible to calculate 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 nominal value of the input machine, it is necessary to add up the power of all devices that will be connected to the house network. Then the found total power is substituted into the formula, the operating current for this load is found.

The formula for calculating the current from the total power

After we have found the current, we select the value. It can be either slightly more or slightly less than the found value. The main thing is that its shutdown current does not exceed the maximum permissible 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 draw attention to the fact 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 the circuit breaker be correct.

Choosing the breaking capacity

The above describes the selection of the packet for the maximum permissible load current. But the circuit breaker must also turn off when a short circuit occurs in the network. This characteristic is called breaking capacity. It is displayed in thousands of amperes - this is the order of currents that can reach during a short circuit. The choice of a circuit breaker for breaking capacity is not very difficult.

This characteristic shows at what maximum value of the short-circuit current the automatic machine retains its operability, that is, it can not only turn off, but will also work after it is 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 done very rarely, but are guided by the distance from the transformer substation.

Breaking capacity of circuit breakers

If the substation is located near the entrance to your house / apartment, they take an automatic device with a disconnecting capacity of 10,000 A, for all other city apartments, 6,000 A is enough. and a breaking capacity of 4,500 A. The networks here are usually old and short-circuit currents are not large. And since the price rises significantly with increasing breaking capacity, the principle of reasonable economy can be applied.

Is it possible to put 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 it will be inoperable. In the worst case, the contacts will melt and the machine will not have time to shut down. Then the wiring will melt and a fire may occur.

Electromagnetic release type

The machine should operate when the current rises above a certain mark. But the network periodically experiences short-term overloads. They are usually associated with inrush currents. For example, such overloads can be observed when turning on the compressor of the refrigerator, the motor of the washing machine, etc. The circuit breaker with such temporary and short-term overloads should not be tripped, therefore they have a certain response delay.

But if the current increased not because of overload but because of a short circuit, then during the time that the circuit breaker "waits", its contacts will melt. This is why there is an electromagnetic automatic release. It operates at a certain amount of current, 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 operating current can be different and is displayed in letters that stand in front of the numbers indicating the rating of the machine.

There are three most common types:

• B - triggers when the rated current is 3-5 times higher;
• C - if it is exceeded 5-10 times;
• D - if it is 10-20 times more.
  

  Machine class or cut-off current

With what characteristics to choose a bag? 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 grids, the choice of the circuit breaker is carried out using simple rules:

• With the letter "B" on the body, they are suitable for summer cottages, houses, villages and townships that receive power supply through air vents. They can also be installed in the apartments of old houses, in which the reconstruction of the in-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 can be delivered on request.
• Pouches with "C" on the body are the most widely used option. They are installed in networks with normal conditions, suitable for apartments in new buildings or after overhaul, in private houses near the substation.
• Class D is set in enterprises, in workshops with equipment with high inrush 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 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 which company the circuit breakers you will buy. You should definitely not take unknown companies - electricians are not an area where you can experiment. More details about choosing a manufacturer in the video.