20 computer chargers, say, are plugged into extension cords of 5 outlets each, and those extension cords plugged into fifth. There is no mechanism for "soft-charging". The chargers are connected to the devices they charge.

So, we have four groups, in parallel, and each group has 5 charger in parallel.

I now plug the single plug on the extension feeding everything into the electrical outlet (230V ac, 50Hz) and, no surprise, the fuse trips.

However, if I disconnect all of the chargers, plug in the "main" plug, and then connect all chargers one by one, the fuse does not trip.

That is, in steady-state operation, the voltage and current are within acceptable limits for the circuit, and below the max amps on the fuse for the electrical circuit.

My Question What are the physical effects leading the tripping of the fuse?

My Thoughts Can this situation be approximately modelled by a RLC circuit, with the chargers as the inductors, and the devices being charged as the capacitors? Would the fuse tripping be explained in the initial peak-current before the steady-state dynamics dominated, or would other, more direct effects (e.g. electrical arcing when the connection is made) be more likely?

EDIT I already know that there is a sudden surge in current when the devices are first plugged in. What I am really after is the underlying cause of this, and, further, whether this can be modelled in a way analogous to a "standard" electrical model like a RCL circuit.


1 Answer 1


when a power supply is initially plugged into the source, there is a brief surge of current that flows into it which is significantly greater than the steady-state current draw of the device. This surge occurs as the power supply's filter circuits come up to full charge, after which it dies out.

This surge is called "inrush current" and to prevent the inrush from blowing circuit breakers or fuses it is common to insert current-limiting devices into the power supply design. However, if you plug a large number of those power supplies into the same feed, the summed inrush may nonetheless trip the breaker or blow the fuse.

  • $\begingroup$ Thank you for your answer, however, I am looking for a deeper explanation. You mention that the high inrush current is due to the filter circuits coming up to full charge, and I was specifically wondering if there is any way to model this with a RLC circuit. Can you elaborate further? $\endgroup$
    – Paul
    Oct 13, 2018 at 20:16
  • $\begingroup$ Virtually all modern wall warts and power bricks are switching power supplies--a term that encompasses a range of different designs, many of which are characterized by high inrush currents. One popular circuit simulation tool originally was created specifically for simulating what happens when a switching PSU is turned on---the simulation being much easier than analyzing it mathematically. In other words, the "deeper explanation" that you seek may be very deep indeed. $\endgroup$ Oct 14, 2018 at 0:16
  • $\begingroup$ the engineer speaks: you can simulate this, but why? just measure it instead! Anyway, a capacitor in parallel with the power line has essentially zero resistance at the instant the power is turned on and infinite resistance a fraction of a second later; this can be used to model the inrush. $\endgroup$ Oct 14, 2018 at 0:51
  • $\begingroup$ @SolomonSlow So this is a characteristic of many designs for switching power supplies? Is there an archetypal model that demonstrates this behaviour, such as niels nielsen seems to indicate? What you both seem to describe is more along the lines that I expected from possible answers so I would be interested to understand more. $\endgroup$
    – Paul
    Oct 17, 2018 at 18:25
  • $\begingroup$ @nielsnielsen Thanks for the added info. However, I am not interested in "fixing the problem". That's easy - use bigger fuses, or don't plug everything in at the same time. My question is more about understanding the physical processes behind it, out of curiosity's sake. $\endgroup$
    – Paul
    Oct 17, 2018 at 18:29

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