Using AC Circuit Breaker in DC Circuit

You are correct.

Reliance Electric used to use MCB's with the three poles in series for DC armature protection under inverting fault conditions. The breakers where nominally 600VAC mag only with a very low trip setting (about 200% of motor NP). The breakers used had a DC rating along with the AC rating. The number of interuptions under fault conditions was very limitted, but tended to be less expensive than fuses rated for DC interruption. (Not all fuses will adeqautely clear DC inductive loads). The irony is the Fuses can be more expensive than the SCR's, but under inverting fault conditions the motor is more expensive than the entire DC bridge. Breaking the AC power delivery versus protecting the motor can be difficult, especially with high inertia loads that can drive the current for a long time across a "flashed" commutator.

The manufacturer's datasheets are the best place to find your answer.

If you are using IEC-type MCBs, these typically have a DC rating of 48V per pole, but it does vary from manufacturer to manufacturer so it is vital to check. To further complicte things, the larger manufacturers also manufacture ranges of MCBs that are either AC with a greater DC rating (~125V) or are designed for DC only, also with the higher voltage rating.

Without wishing to plug any one brand, if you were to do a search for "GE Redline Technical Catalogue" you will be able to find and download a book that I have found particularly helpful for questions such as this.

With DC if there is a resultant arc that bridges the gap across the open contacts there is no zero crossing that would normally exist with AC that would act to extinguish the arc.

There are switches, contactors and relay contacts clearly rated for AC and DC. But be ready for a rude surprise, for how much derating is there.

I have worked a lot with DC doing solar system installations. The OutBack systems provide DC breakers with thier equipment. On one installation we used 60Amp breakers and the breakers say CARLINGSWITCH, INC. on the body. One breaker failed. It did not trip it simply failed and when we removed it I could see signs of overheating. Out of curiosity I disassembled it and did a physical comparison to an AC breaker and must admit that in my opinion there wasn´t much difference on the insides. For safety reasons we use fuses up on the roof in the combiner panels. A fuse will melt under overload conditions not trip so the question of zero crossing current points for AC versus steady flow DC does not arise.

Try contacting the above INC or use fuses.

Regards

John

This has been well answered already. Some manufacturers do show some DC characteristics. Look to the automobile industry - they use fuses to protect and semiconductors to switch.

Further to #2↑, having read it, then apply it.