DC molded case circuit breakers

[Pitt123]

Can all AC molded case breakers that have a DC voltage with interrupting rating on a breaker datasheet be used in DC circuits? Do these breakers have to have the poles in series or are these only specified DC breakers that require this.

The only reference I see to the poles in series are for a specific DC breaker but was wondering what was needed if anything to apply an AC breaker on a DC system if it is properly rated.

 

[broadgage]

A breaker listed only for AC operation should only be used on AC.
A breaker dual listed for AC/DC operation would normally be wired as for AC, but some types require 2 or 3 poles to be wired in series. Follow the instructions with the unit.

In practice most types of AC breaker are fine on DC provided that the voltage is limited to about 10% of the AC rating.
I would use standard UK type 240 volt AC breakers on 24 volts DC, and this practice is widespread, even if not strictly correct.

 

[jdsmith]

Can all AC molded case breakers that have a DC voltage with interrupting rating on a breaker datasheet be used in DC circuits? Do these breakers have to have the poles in series or are these only specified DC breakers that require this.

The only reference I see to the poles in series are for a specific DC breaker but was wondering what was needed if anything to apply an AC breaker on a DC system if it is properly rated.

Yes, if the breaker has a DC voltage with interrupting rating it is "DC rated" and can be used in DC applications. Be careful about the interrupting rating - DC interrupting ratings are much lower than AC ratings and a moderate to large sized substation battery bank or UPS battery bank will have more fault current available than the common DC breakers. We have to use Square D I-Line panels with full size MCCB's for 125 VDC battery banks in substations. There are miniature MCCBs that are DC rated but the interrupting ratings are too low for 250 Amp-hour or larger flooded lead acid batteries.

Putting the poles in series would get you a higher interrupting rating from a pure physics perspective, but we don't know how much higher unless the manufacturer has tested it and publishes an alternate DC rating with the poles in series. For example, a Square D Powerpact HG breaker has these ratings: 65 kA at 240 VAC, 35 kA at 480 VAC, 18 kA at 600 VAC, 20 kA at 250 VDC. If you have a large DC battery with more than 20 kA available you cannot use this breaker, poles in series or not, because the datasheet doesn't allow you to.

 

[Jraef]

Poles in series are used to increase the voltage rating in DC. An arc in DC is much more difficult to interrupt when opening a set of contacts, as in a circuit breaker, and the "arc length" must be higher than in AC because in AC, the current is passing through zero 120 times per second anyway. The higher the voltage, the longer that arc length must be.

So using multiple contacts in series is one way of effectively increasing the total arc length, allowing the arc chutes to do their job. But the only way to tell if it works is to test it, so regardless of putting poles in series, it is only valid if specifically tested and UL listed that way by the manufacturer. There is no valid method of gaining acceptability in the field.

So if a breaker has a DC voltage rating on it, it may also have specific instructions about putting poles in series that differ depending on the voltage involved. In other words a breaker may have a 125VDC rating w/o series poles, 250VDC with 2 poles in series and 500VDC with 3 or 4 poles in series. There is no substitute for doing your homework.

 

[Pitt123]

O.K. so if a breaker requires multiple poles to be put in series for increased voltage rating or interrupting rating it will usually say so on the breaker itself or in the attached litterature?

Also even though putting poles in series can technically increase the interrupting rating of a breaker the interrupting rating of the breaker is only good for what is published with the breaker series or non-series, etc...

And if a breaker has 3 DC voltage and interrupting ratings but says nothing about putting poles in series, then either of those 3 DC voltage ratings can be used without putting the poles in series.

Do I understand this correctly?

 

[Jraef]

Sounds good to me. But looking at an example, a Siemens VL breaker in my hand, the DC voltage rating is also dependent upon WHAT KIND of DC system you have. For example if you have a 250V or less grounded system, you must use 2 poles minimum because you must switch the grounded pole, but NOT in series. However if you want 500VDC out of the same breaker, it takes all 3 poles in series to attain that so you cannot switch the grounded leg, restricting it to being used in ungrounded DC systems, such as UPS, DC drive and some PV systems. And for them, the interrupt capacity is based on the voltage, there is nothing shown about increasing the IC on the 250V system use by adding another pole in series.

So although it may be theoretically acceptable, all that really matters is what the mfr has the device tested and listed for.

 

[LarryFine]

For example if you have a 250V or less grounded system, you must use 2 poles minimum because you must switch the grounded pole, but NOT in series. However if you want 500VDC out of the same breaker, it takes all 3 poles in series to attain that so you cannot switch the grounded leg, restricting it to being used in ungrounded DC systems, such as UPS, DC drive and some PV systems.

Regardless of which conductor the breaker contacts are in, they are all effectively in series anyway, at least for line-to-line faults.

With a grounded supply, only the contacts in the line of the ungrounded conductor are effectively in series for line-to-ground faults.

 

[Pitt123]

Sounds good to me. But looking at an example, a Siemens VL breaker in my hand, the DC voltage rating is also dependent upon WHAT KIND of DC system you have. For example if you have a 250V or less grounded system, you must use 2 poles minimum because you must switch the grounded pole, but NOT in series. However if you want 500VDC out of the same breaker, it takes all 3 poles in series to attain that so you cannot switch the grounded leg, restricting it to being used in ungrounded DC systems, such as UPS, DC drive and some PV systems. And for them, the interrupt capacity is based on the voltage, there is nothing shown about increasing the IC on the 250V system use by adding another pole in series.

So although it may be theoretically acceptable, all that really matters is what the mfr has the device tested and listed for.

Good example. Is all of that information you mentioned listed on the breaker itself, or did you have to dig up a cut sheet?

 

[BJ Conner]

Breakers and Fuses

Breakers and Fuses

I have seen several applications where breakers ( used for switching and low level fauts ) were installed in series with fuses. The reason was people didn't trust fuses to interrupt large DC faults. DC faults never have a zero crossing and can be feeding inductive loads. The arcs tend to get bigger and bigger with time.
The panels were custom made and listed.
I would peruse the data sheets on any breakers I applied to a DC system.

 

[Jraef]

Good example. Is all of that information you mentioned listed on the breaker itself, or did you have to dig up a cut sheet?

Most of it's on the breaker, although I needed some SERIOUS reading glasses to read it.