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I found a strange phenomena when I tried to use a metal cutter that consumes about 2000 watt.
See the following illustration.

wiring diagram

If I just use one extension cable, the MCB will trip when I turn on the cutter. However, if I add one more extension cable, everything is just fine (the MCB does not trip). It looks strange to me. Why does it happen?

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  • \$\begingroup\$ You need a motor rated MCB instead of the current one (e.g. class B or C instead of class A) \$\endgroup\$
    – user16324
    Commented Dec 31, 2020 at 10:20

2 Answers 2

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The additional extension cord adds more resistance, reducing the inrush current.

The inrush current required to trip a 20A breaker instantly can be as much as 300A, though it's more likely to be about 150A. This means the resistance of the whole system is less than 0.8 ohms. An extra hundred milliohms, as would be added by a 20 foot 14 AWG extension cord, would make a difference of tens of amps of inrush current.

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  • \$\begingroup\$ IMHO, the resistance change should be negligible. Or it is related to inductance of the wound part of the extension cord? \$\endgroup\$
    – Display Name
    Commented Dec 31, 2020 at 3:18
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    \$\begingroup\$ @ArtificialStupidity The inrush current required to trip a 20A breaker instantly can be as much as 300A, though it's more likely to be about 150A. An extra hundred milliohms could make a difference of tens of amps of inrush current. \$\endgroup\$
    – τεκ
    Commented Dec 31, 2020 at 6:37
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    \$\begingroup\$ @ArtificialStupidity: You should always unroll extension cords all the way in use. \$\endgroup\$
    – JRE
    Commented Dec 31, 2020 at 10:37
  • \$\begingroup\$ @JRE: Thank you for informing this. :-) \$\endgroup\$
    – Display Name
    Commented Dec 31, 2020 at 11:12
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    \$\begingroup\$ @ArtificialStupidity, see my answer to electronics.stackexchange.com/a/232510/73158 to understand why inductance is unlikely to be a problem. \$\endgroup\$
    – Transistor
    Commented Dec 31, 2020 at 13:15
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That would happen, should there be a ground fault in the cutter and a break in the ground wire of the second extension cable.

enter image description here

A short circuit between the winding and the body of the cutter would result in the fault current, passing through the ground wire to earth, causing the MCB to trip.

The MCB would not trip, when the second extension cable is added, should the ground wire in that be broken.

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  • \$\begingroup\$ Sorry. I don't understand this answer. But +1. \$\endgroup\$
    – Display Name
    Commented Dec 31, 2020 at 11:15
  • \$\begingroup\$ My answer has been edited for clarity. \$\endgroup\$
    – vu2nan
    Commented Dec 31, 2020 at 13:04

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