Recently I switched off several molded case circuit breaker (MCCB) and saw a flash at the back of the breaker for an instant. This happened to all the MCCB on-site (around 14). The MCCB are all rated correctly - 200A capacity with a feed in peak load of 145A.

Is this perfectly normal? My knowledge tells me that the flash is a normal occurrence because I am breaking the circuit at a high current (probably around 100A), hence, the breaker had a flash as it is breaking the circuit contact (which is intended I suppose).

Thank you.

  • \$\begingroup\$ Do they still work properly? How big was the flash? \$\endgroup\$
    – Andy aka
    Oct 1, 2019 at 15:30

1 Answer 1


Yes, it's normal.

But keep in mind that circuit breakers are not intended to be used as power switches. They should only be operated when the load current has been stopped by other means. They are designed to reliably interrupt on an overload, but every time they do so, the arc causes erosion of the contact surfaces that increases their resistance.

  • \$\begingroup\$ I see! Well, I am in the field of renewables (solar) and we typically use the MCCB to cut off the grid voltage to the inverter. Will this be an issue still considering most solar inverter do not come with an AC switch? (Sorry, might be irrelevant here!) \$\endgroup\$
    – SunnyBoiz
    Oct 1, 2019 at 15:52
  • \$\begingroup\$ You probably know more about the details of grid-tied inverters than I do. Are you saying that they typcally have no other means of shutting down provided through their user interface? \$\endgroup\$
    – Dave Tweed
    Oct 1, 2019 at 16:07
  • 2
    \$\begingroup\$ There should be an isolator switch as well as the breaker. \$\endgroup\$ Oct 1, 2019 at 18:37
  • \$\begingroup\$ Thanks everyone! Isn't an isolator meant for off load on/off and a circuit breaker meant for on-load on/off kind of thing? \$\endgroup\$
    – SunnyBoiz
    Oct 2, 2019 at 5:23

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