The electrical network is an excellent flexible means of delivering energy to the desired point and using it there for solving various problems. However, it also has a serious disadvantage, which is a direct continuation of the noted merit. The fact is that electrical networks are very sensitive to short circuit, which is understood as a direct connection of the neutral and phase wires bypassing the load. In three-phase networks, a short circuit also includes the same connection of two phases. Such an event leads to various unpleasant consequences, and in severe cases is accompanied by a fire.
To protect against short circuits in modern houses, various devices are installed that monitor the network and, if necessary, de-energize the wiring. We are talking about circuit breakers that have replaced the old grandfather's plugs. The selection of the type of characteristics of the machine is carried out by the short-circuit current.
Why can the short-circuit current vary in different networks?
From a school physics course, it is known that a combination of any source of electricity (battery, car battery, electric network, etc.) and the consumer (lamp, electric kettle, refrigerator) can be represented as an equivalent circuit shown in the figure 1. To the left of the dotted line is a generator with an electromotive force E and an internal resistance Rin, to the right is a load with a resistance Rн.
The internal resistance of the source can vary over a wide range. It is significantly influenced by:
- distribution transformer power;
- section of cable cores;
- the quality of the copper from which these veins are made;
- distance from the panel to the outlet.
and much more.
Experimental determination of internal resistance
To determine the actual value of Ri, you will need:
- household multimeter;
- tee or any other splitter;
- load with a known power P = 1 - 2 kW (for example, an iron, a kettle and the like).
Direct measurement of the short-circuit current according to the diagram in Figure 2 is impossible due to the fact that:
- a multimeter that is not designed for currents of hundreds and thousands of A will burn out;
- in the absence of the machine, the wiring will ignite and melt.
Therefore, it will take some trick, the essence of which is to perform the following procedure:
- for the purity of the experiment, we will reduce the load on the network to a minimum by disconnecting the maximum possible number of consumers from the network;
- using a tee according to the diagram in Figure 3, we connect the multimeter to the network, which was previously switched to the voltmeter mode, and fix the measured voltage Uхх of the open circuit of the source (network);
- into the second socket of the tee, we connect our load with power P, as a result of which we get the diagram in Figure 4, and again we measure the voltage Uн;
- let's perform a simple calculation Rin = P * (Uхх - Un) / 220 Ohms (we take P in watts, the calculation formula itself is obtained from Ohm's law for a complete circuit).
Selection of the machine
The estimated short-circuit current in the studied network will be Isc = Uхх / Rin. The selection of the circuit breaker is carried out according to the received value.
If the calculation gives Isc of one to two thousand amperes or more, then an automatic machine with a C-characteristic is installed, at currents of the order of 200 A or less, it is necessary to use machines with a characteristic of type B.