DC breakers use a magnet to blow the arc into the quenching channel. If the current is flowing in the wrong directly, the arc is pulled into the guts of the breaker and can cause the arc to be sustained and melt/start a fire inside the breaker (as I understand).

demo: https://youtu.be/Oaq2cvoPBRk

When we install polarized DC breakers in between the battery and the inverter, we consider the battery as the source and set the polarity accordingly.

But, when the battery is charging, the direction of the current flow is reversed and the breaker becomes reversely polarized.

The ideal solution would be using a non polarized DC breaker. My question is how safe is it to use a polarized breaker in this case?

  • 1
    \$\begingroup\$ Why would the arc be pulled into the guts of the breaker? Do you have a breaker to look at? Are you sure how it works? \$\endgroup\$
    – Solar Mike
    May 16 at 14:00
  • \$\begingroup\$ See electronics.stackexchange.com/q/619448/152903 \$\endgroup\$
    – Solar Mike
    May 16 at 14:00
  • 2
    \$\begingroup\$ The arc generates a magnetic field, just as the same current flowing in a wire would do. As OP says, circuit breakers have permanent magnets so the resulting magnetic force lengthens & cools the arc, and if the current is flowing in the 'wrong' direction, it can damage the breaker. However, unless the current is really high, it does take the energy of an inductive load to start an arc, and in the case of the battery charger, the battery inductance is minimal, so the arc shouldn't start. \$\endgroup\$
    – jayben
    May 16 at 14:28
  • \$\begingroup\$ Not professional advice, but perhaps think about which direction the current is most likely to go during a short circuit, and what the effects are. Short circuit on battery side: the charger shuts off automatically (right?), and the breaker can't do shit to save the battery anyway. Short circuit on other side: the breaker can actually shut off the power \$\endgroup\$
    – user253751
    May 16 at 15:00
  • \$\begingroup\$ One option might be a separate charging circuit with its own breaker. Then each breaker can face in the correct direction. \$\endgroup\$ May 16 at 15:27

2 Answers 2


Don't magnetic blowouts use the Lorentz force? It sounds like you think the magnet attracts or repels the arc which is not the way the directions work.

If it's an electromagnet then your concerns aren't warranted even if it did work by repelling the arc since polarities are reversed.

If it's a permanent magnet it just blows the arc to the other side, and neither of these sides is toward the magnet via the Lorentz force exterted on the arc: \$F = qv \times B\$. The direction isn't inherently towards any component in the breaker so you can design around it.

enter image description here

A Course in Electrical Engineering Vol. 1 Direct Currents, Chester L. Dawes, 1920

  • \$\begingroup\$ check this out youtu.be/Oaq2cvoPBRk \$\endgroup\$ May 16 at 16:37
  • \$\begingroup\$ I guess they don't bother designing the ionizing chamber to be on both sides. \$\endgroup\$
    – DKNguyen
    May 16 at 16:40

The charging current (even into a short circuit) will likely be a few orders of magnitude smaller then the current resulting from short circuiting the battery bank.

It is also likely that the charging arrangements have current limiting built right in, so you are unlikely to get a few kA of PSC from the charger direction, while a decent UPS battery will have no trouble producing this.

The really hazardous energy source is thus the battery.

I don't know what someone like UL would say about this but as a practical matter the charger is likely not the dangerous source here, and unless the situation was a bit weird I would not hesitate to use a polarised DC breaker in this application.


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