Selection of protection circuit breaker rating

When assembling an electrical panel or connecting a new large household appliances, a home handyman will certainly face such a problem as the need to select circuit breakers. They provide electrical and fire safety, so the right choice of circuit breaker – the key to the safety of you, family and property.

What is the purpose of automatic circuit breaker

In the chain of power supply automatic machine is put to prevent overheating of wiring. Any wiring is designed to pass a certain current. If the passing 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 used to prevent this situation.

The second task of the circuit breaker is to cut off the power supply in the event of a short-circuit current (short-circuit). When a short circuit occurs, the currents in the circuit increase many times and can reach thousands of amperes. To prevent them from destroying the wiring and damaging the equipment included in the line, the circuit breaker must cut off the power as soon as possible – as soon as the current exceeds a certain limit.

In order for the protective circuit breaker to properly perform its functions, it is necessary to correctly make the choice of automatic machine for all parameters. They are not so many – only three, but each must be dealt with.

What are the automatic protection circuit breakers

To protect the conductors of a single-phase 220 V network, there are disconnecting devices single-pole and double-pole. Only one conductor is connected to the single-pole – phase, to the two-pole and phase and zero. Single-pole automatic machines put on the circuit 220 V internal lighting, on socket groups in rooms with normal operating conditions. They are also put 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 are these automatic protection devices (the correct name of the circuit breaker) put on the three-phase load (ovens, cooktops and other equipment that operates from the network 380 V).

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

Simply in emergency situations – in case of a short circuit or insulation breakdown – the phase voltage can fall on the neutral wire. If a single-pole device is installed on the supply line, it will disconnect the phase wire, but the zero with dangerous voltage will remain connected. This means that there is still a possibility of electrocution by touch. That is, the choice of automatic machine is simple – on part of the lines are put single-pole switches, on part – two-pole. The specific number depends on the state of the network.

For three-phase network there are three-pole circuit breakers. Such a circuit breaker is placed at the input and on consumers to which all three phases are supplied – electric stove, three-phase cooktop, oven, etc. On the rest of the consumers put two-pole automatic protection devices. They must necessarily disconnect both phase and neutral.

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

Determine the nominal value

Actually, from the functions of the circuit breaker and follows the rule of determining the rating of the automatic protection device: it must be triggered until the moment when the current exceeds the capabilities of the wiring. This means that the current rating of the automatic circuit breaker must be less than the maximum current that can withstand the wiring.

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

• Calculate the cross-section of the wiring for a particular area.
• See what the maximum current withstands this cable (there is a table).
• Then, from all the ratings of the circuit breakers choose the nearest smaller. The circuit breaker ratings are linked to the permissible continuous load currents for a particular cable – they have a slightly smaller rating (see the table). The list of ratings looks as follows: 16 A, 25 A, 32 A, 40 A, 63 A. From this list you choose the appropriate one. There are nominal values and less, but they are practically not used – too many electrical appliances we have appeared and they have a lot of power.

Example

The algorithm is very simple, but it works without errors. To make it clearer, let’s look at an example. Below is a table that shows the maximum allowable current for conductors that are used when laying wiring in the house and apartment. There are also recommendations regarding the use of automatic machines. They are given in the column “Rated current of the automatic protection device”. It is there that we look for ratings – it is slightly less than the maximum allowable, so that the wiring works in normal mode.

In the table we find the selected wire cross-section for this line. Let us assume that we need to lay a cable with a cross-section of 2.5 ^mm2 (the most common when laying to medium power appliances). Conductor with this cross-section can withstand a current of 27 A, and the recommended rating of the automatic circuit breaker – 16 A.

How will the circuit work then? As long as the current does not exceed 25 A the circuit breaker is not disconnected, everything works in normal mode – the conductor heats up, but not to critical values. When the load current starts to increase and exceeds 25 A, the circuit breaker does not switch off for some time – perhaps it is the starting currents and they are short-term. It switches 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 packetizer will trip and de-energize the branch, as this current already poses a threat to the conductor and its insulation.

Calculation by power

Is it possible to choose an automatic machine according to the power of the load? If only one device will be connected to the power supply line (usually a large household appliance with high power consumption), then it is acceptable to make a calculation on the power of this equipment. Also according to the power can be selected input circuit breaker, which is installed at the entrance to the house or apartment.

If we are looking for the nominal value of the input circuit breaker, 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, is the operating current for this load.

After finding the current, we choose the rating . He can be either a little more or a little less than the value found. The main thing is that its tripping 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 reserve (this is not bad, by the way). Then, in order to save money, it is possible to install automatic circuit breakers corresponding to the load and not to the cross-section of conductors. But once again we draw attention to the fact that the long-term allowable current for the load must be greater than the current limit of the circuit breaker. Only then the selection of the circuit breaker will be correct.

Selecting the breaking capacity

Above described the selection of the packer on the maximum permissible load current. But the automatic circuit breaker must also trip when a short-circuit (short-circuit) occurs in the network. This characteristic is called tripping capacity. It is displayed in thousands of amperes – this is the order of currents can reach at short circuit. The choice of automatic circuit breaker on the breaking capacity is not very difficult.

This characteristic shows at what maximum value of short-circuit current automatic machine retains its operability, that is, it will not only be able to disconnect, but will work after reclosing. This characteristic depends on many factors and for accurate selection must determine the short-circuit currents. But for wiring in a house or apartment such calculations are very rarely made, and are focused on the remoteness from the transformer substation.

If the substation is located near the entrance to your house / apartment, take a circuit breaker with a breaking capacity of 10 000 A, for all other urban apartments is enough 6 000 A. If the house is located in the countryside or you choose a circuit breaker for the protection of the electrical network for the cottage, it may well be enough and the breaking capacity of 4 500 A. Networks here are usually old and short-circuit currents are not large. And since with increasing breaking capacity of the price increases significantly, you can apply the principle of reasonable savings.

Is it possible to install sachets with lower breaking capacity in city apartments? In principle, you can, but no one guarantees that after the first short circuit you will not have to change it. It can manage to disconnect the network, but will be inoperable. In the worst case, the contacts will melt and the automatic machine will not have time to disconnect. Then the wiring will melt and there may be a fire.

Type of electromagnetic release

The automatic unit should trip when the current rises above a certain point. But in the network there are periodically short-term overloads. They are usually associated with inrush currents. For example, such overloads can be observed when turning on the compressor of a refrigerator, the motor of a washing machine, etc. The circuit breaker at such temporary and short-term overloads should not trip, because they have a certain delay for tripping.

But if the current has increased not because of an overload but because of a short circuit, then for the time that “waits” circuit breaker, its contacts will melt. That’s why there is an electromagnetic circuit breaker. It trips at a certain value of current, which can no longer be an overload. This value is also called the trip current, because in this case the circuit breaker cuts off the line from the power supply. The value of the tripping current can be different and is displayed by the letters that stand before the numbers that denote the rating of the automatic circuit breaker.

There are three most popular types:

With what characteristic to choose a packet circuit breaker? In this case, the choice of automatic circuit breaker is also based on the distance of your home from the substation and the state of the power grid selection of automatic circuit breaker protection is carried out using simple rules:

• With the letter “B” on the body are suitable for summer houses, houses in villages and settlements, which receive power through the air. Also they can be put in the apartments of old houses, in which the reconstruction of the internal electrical network has not been made. These circuit breakers are not always on sale, cost a little more than category C, but can be delivered to order.
• Packetizers with “C” on the body – this is the most widespread variant. They are put in networks with normal condition, suitable for apartments in new buildings or after overhaul, in private houses near the substation.
• Class D put in enterprises, in workshops with equipment with high inrush currents.

That is, in fact, the choice of automatic protection in this case is simple – for most cases is suitable type C. It is in stores in a large assortment.

Which manufacturers are worth trusting

And lastly, let’s pay attention to the manufacturers. The choice of automatic circuit breaker can not be considered complete if you have not thought about what firm of circuit breakers you will buy. Definitely do not take unknown firms – electricity is not an area where you can put experiments. Details about the choice of manufacturer in the video.