How to choose a fuse for long-term use
During continuous operation, the heating temperature of the fuse must not exceed the permissible values. In this case, the stability of the current characteristics over time of the fuse is ensured. In order to meet this requirement, it is necessary that the lamp holder and the fuse be selected for a rated current equal to or slightly higher than the rated current of the protected installation.
The fuse must not trip the device in the event of an overload that is in operation. So, starting current of an induction motor with a squirrel cage rotor can reach 7AhNo. As the vehicle accelerates, the inrush current drops to a value equal to the motor's rated current. The duration of the start depends on the nature of the load. The fuse must not blow when exposed to inrush currents and the fuses must not age under the influence of these currents.
Under normal conditions, the rated current of the insert is selected according to the inrush current of the load, using the formula: AzVT = AzStart x 0.4
In severe starting conditions, when the engine is slow or in intermittent mode, when starting occurs with a high frequency, the inserts are selected with an even larger margin: AzVT = Significance x (0.5 — 0.6)
Along with checking the insert for start-up or short-term overload conditions, it is also necessary to check for short-circuit conditions. At Azkz /Azstart> = 10-15, the burning time of the insert does not exceed 0.15-0.2 s. This path is slightly affected by the dispersion of insert characteristics. At this time the welding of contacts contactor or magnetic starter is unlikely.
However, this requirement is often not met because the range of Azkz /Azstart is determined by the power of the supply transformer and the resistance of the current-carrying wires and cables. It is allowed to use fuses in multiple Azkz /Azstart> = 3-4. With such a multiplicity, the tripping time can reach 15 s, creating a hazard for service personnel, as with this multiplicity the touch voltage can be dangerously high.
It is also necessary that the nominal voltage of the fuse is equal to the nominal voltage of the mains.