English
The most active element of the systems are still classic electromechanical relays with unprotected contacts. In addition to the standard resistive loads (heaters, etc.) and common inductive loads (motors, coils of contactors and relays, solenoids, wound transformers, etc.) there are increasingly more common loads with capacitive character (switching power supplies for LEDs, some ballasts for fluorescent lamps, etc.). Each of these types of loads acts in a specific way on the contacts switched by them, and on the surroundings (interference), and specific measures are required to mitigate these effects:
1. Resistive loads
They have the least detrimental impact on the switching circuits and on the surroundings. They do not require any special protection elements.
2. Inductive loads
They adversely affect the shutdown (disconnecting from the power supply voltage). At the time of the shutdown there may occur a strong voltage peak, which is proportional to the voltage, the circuit inductivity, etc. There are suppression elements that can be used to protect the switching circuits and reduce the generated interference, see Chapter 13.7.3.
3. Capacitive loads
It has a negative impact when switching on (connecting) the supply voltage.
At the time of switching (the "cold start") a high current curge can occur (in the order of dozens of amps in the switched power supplies), which can quickly cause a "sticking" of the relay contact of the switching circuit. In this case, it is not possible to simply use additional protection features. Switching these circuits requires the use of relays with special contacts (i.e. the "inrush" technology), where by doubling the contacts, the relay is equipped with a special "pre-contact" made of a material, which can withstand more than 100 A for approx. 20 ms, thus spanning the initial current surge.