The power-on start-up current refers to the phenomenon that industrial electronic instruments will be much larger than the normal working current at the moment of power-on. There are two performance indicators for power-on start-up current:

(1) Maximum power-on current (2) Duration

There are capacitors or inductive capacitors inside industrial electronic instruments. At the moment of power-on, the capacitor needs to be charged. The capacitor exhibits an instantaneous short-circuit phenomenon, which makes the starting current instantly increase. Therefore, when designing industrial electronic instruments, engineers must reduce the starting current as much as possible to reduce the adverse effects on the system. However, because the type experiment of industrial electronic instruments is often conducted for one instrument, or the experimental power supply is ideal, the hazard is often ignored, so that in the field, multiple instruments used together will cause the system to work unreliably.

At the industrial site, the hazard of excessive starting current is that multiple instruments often share a set of power supplies in the industrial site. If the starting current is too large, the peak current may exceed the specified value of the power supply at the moment of power-on, making the load capacity of the power supply The power of industrial electronic instruments is incomplete, and the entire system cannot work reliably.

 Therefore, industrial electronic instruments must reduce the starting current as much as possible to reduce the adverse effects on the system.

In safety instrumented systems (SIS systems), the indicator of power-on start current is particularly important.

Any circuit of the safety instrument system has its own diagnostics. For example, the field instrument power supply circuit has short circuit diagnosis and overload diagnosis. The current of the field instrument power supply circuit will be detected to determine whether the circuit is short circuited or overloaded. If it is diagnosed that the current is greater than the internal set value, it sends a fault signal to the system, enters the fault state, disconnects the power supply circuit, and ensures the safety of the entire system.

If the startup current is too large after power-on, the system may not work properly.