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Context:

I am designing an device that is placed between the main AC network (230V 50Hz) and some consumer devices (PC, TV, etc..). My device is measuring the current drawn by the consumer device, we have a voltage that is proportional to the current.

The problem comes with inrush current. Typical PC (for instance) have DC/DC converter PSU, which have capacity at first from input. When plugged, this cause inrush current to happens. This inrush current causes an over-voltage at input of my circuit and I have to protect it against that. I know that they are inrush limiting options, but this is out of my hands (I don't know how my device will be used).

Actual Question:

Are they any norms (IEC, EN, etc) that tells what is the maximum allowed inrush current by appliance class ? What value should I take to dimensionnement my circuit's protections ? Or should I rely on circuit-breaker manufacturer specification (or IEC 60898-1)?

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  • \$\begingroup\$ Since wiring capacitance is probably neglectable, starting from circuit breakers peak current specs is probably a good idea. You can size your tracks so that max current is the max allowed in EU (I believe 16A), but the rest depends on how you plan to measure such current. \$\endgroup\$ Apr 26, 2016 at 12:14

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First, there are 2 different specs on any circuit breaker: "thermal" limit and "electromagnetic" limit. The first one is typically set to 16 A. It limits continuous consumption currents. The second one is much higher (100 A or more) and it works very fast. It protects against short circuits or too high inrush currents.

So my advise is:

  1. Design your circuit for inrush peak current of 100 A during few ms.
  2. This huge current is not the one you measure - you just clamp it.
  3. The real measurement range can be 0 - 2 A or a little higher.

I guess you use current transformer for measurements, so just limit the voltage on the secondary winding load. The limiter must carry large enough current in "peak" mode (not continuous).

By the way - most low power electronic devices use thermistors to limit inrush currents. So the same device shows very different inrush currents depending on ambient temperature and the time since last switch on event.

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  • \$\begingroup\$ Master, Thanks for you answer. So, in short, your advice is to rely on Circuit-breaker specs ? \$\endgroup\$
    – Antoine
    Apr 26, 2016 at 13:10
  • \$\begingroup\$ You see, there are circuit breakers with more than 100 A... However, it is very unlikely to see more than 100 A in-rush current on a low power device. It would be burned by such current. \$\endgroup\$
    – Master
    Apr 26, 2016 at 13:19
  • \$\begingroup\$ I see no big problem to protect the measurement circuit from 100 A or 200 A. Assume you have a current transformer 1:100. Then we are talking about peak currents of order 1-2 A in a low impedance circuit. Just use fast rectifier Schottky diodes for the protection - as a first advise. Of course, the real design depends on your precision requirements etc. \$\endgroup\$
    – Master
    Apr 26, 2016 at 13:22
  • \$\begingroup\$ My question is not about how to protect, but how much (rating of TVS diode). Taken I know the type of circuit breaker (10A, type C) that will be used but I can assume that the inrush will never (in any condition, or duration) exceed a certain current (100A in that case) . Is this right ? \$\endgroup\$
    – Antoine
    Apr 26, 2016 at 15:34

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