Chemical current sources (batteries and accumulators) have so firmly entered our lives, becoming something familiar and natural that many of their owners do not even read what is written on their cases and packaging. But it’s completely in vain, because there is a very curious warning - “Do not lock”. Anyone who is even a little familiar with the basics of electrical engineering knows well what a short circuit current is and why it is not possible to close contacts.
Any current source always has two poles: positive and negative (+ and -) for direct current, and also phase and earth (zero, mass, sometimes another phase) for alternating. The movement of charged particles is possible only if these poles are connected by a conductive material. If an electrical device is connected to the gap of a conductor, then a certain work will be performed by the current passing through the circuits of the device. The main feature is that the circuits of the specified electrical appliance do not allow electrons to pass through them unhindered, so you can always assume that there is a resistance (load) that limits the maximum particle flux (short circuit current).
To simplify the understanding, we give an example with water flowing through a channel (conductor). The beginning of the flow is one pole, and its end is the second pole. In the middle of the channel are the blades of a water mill (load), rotated by the movement of water (current). Obviously, the flow rate is always limited by the resistance of the blades. But what happens if the mill disappears? Water that does not experience resistance will rush along the channel at the maximum speed that the source can provide (short circuit). In this case, the probability of damage by a rapid stream of both the source and the channel is very high. In electricity, flow intensity is the strength of a short circuit current .
The same thing happens in electricity. If for any reason the resistance of the circuit begins to tend to zero, then a short circuit current occurs. The conductor then heats up, and the current source is damaged. This operating mode of the electrical installation is emergency and requires immediate removal. Typically, a circuit breaker is installed in the circuit to break the circuit if it detects a short circuit current. By analogy with water: the channel is blocked and the flow disappears. With the power source turned off, it is quite simple to determine the fact of a short circuit in an arbitrary section of the electrical circuit . For this, a special device (multimeter) measures the resistance value between the points of application of the probes. The indirect method is based on the use of an indicator or dial-ups: if it is known that there should not be any direct contact of poles in the area being checked, then this condition is checked. In power devices, this is more rational than measuring the exact resistance with a multimeter.
By the way, that is why it is impossible to close the battery poles. Although their power is not enough to significantly damage the conductor, but the chemical components inside are quickly degraded by a short circuit, reducing the total life of the source.
When studying electrical safety, the concept of "short circuit shock current" is often used. It determines the highest value of the current resulting from a short circuit. In AC circuits, its maximum value will be reached after a half-cycle (0.01 s for a frequency of 50 Hz).
It is worth noting that there are special electrical devices for which the short circuit mode is not emergency, but normal. A vivid example is welding machines in which the visible arc is precisely the short-circuit current.