# Why does a circuit breaker voltage rating differ between AC and DC?

I'm looking at circuit breakers from Allen Bradley's 1492 series: (https://configurator.rockwellautomation.com/api/Doc/1492-SPM%20Specifications.pdf)

For each circuit breaker, it lists a AC and DC voltage rating. Why would the voltage rating differ between DC and AC applications?

• Because AC crosses zero 100 (or 120) times a second.
– user16324
Commented Jan 31, 2021 at 21:01
• @BrianDrummond or ~33 times at the workplace of my mother... Commented Feb 2, 2021 at 11:28

An AC waveform consists of a (ideally, mostly) sinusoidal waveform that crosses zero voltage and current periodically.

All wiring has stray inductance, and when contacts are opened the inductance will cause a rise in voltage until there's an arc across the contacts.

There's plasma in the arc that makes the impedance between the contacts much lower than air, meaning the arc will tend to continue as the contacts move further apart.

With the AC waveform, the current crossing zero will extinguish the arc.

With DC, there's no zero crossing, so the contacts are limited to a much lower voltage so that the there's not enough electric field to keep the arc "lit" while the contacts pull apart.

Excess arcing is very damaging to the contacts, and will destroy the breaker if not controlled by observing the ratings.

There are special breakers and relays for higher voltage DC that use various techniques to interrupt the arc like magnetic coils to blow out the arc.

Interrupting DC is harder because there's no zero crossing to extinguish the arc. Therefore there's a lower electric field rating across the contacts for the same contact configuration.

• @Vinzent Right, I thought about adding that but I also thought it might confuse the issue, and the DC and AC ratings for the particular breaker the OP picked are dramatically different enough that it's clearly not due to the AC peak voltage vs DC voltage. Commented Jan 31, 2021 at 20:18
• OP seems to only be asking about why the numerical value is different, not current or the difference between the DC vs AC label itself. Might want to delineate between OP's direct question and additional stuff he needs to know anyways which is way more important than OP's actual question. Commented Jan 31, 2021 at 20:19
• @JohnD Sorry for the deletion, my comment was; AC voltage also has higher peak than rms, compared to DC, but that might just be confusing to the OP..
– user173292
Commented Jan 31, 2021 at 20:19
• Arc horns and special platings (cadmium iirc) are also used to minimise arc damage.
– K H
Commented Feb 1, 2021 at 3:11
• "With DC, […] the contacts are limited to a much lower voltage" – It took me a moment to understand what you meant by this. At first, I thought you meant that with DC, there's some physical process which causes the voltage to be lower. I had to go back and re-read in order to figure out that you meant that with DC, the acceptable voltage is lower. Commented Feb 2, 2021 at 1:36

When you open a switch, typically you get a spark. It may be a small spark you don't see, but it is still there. If there is enough current flowing at the time you open the switch, and if there are long wires or inductors connected, then that spark will turn into a full-on arc that bridges the gap and allows current to flow even though the switch is open or opening.

With AC, this arc will be interrupted when the current passes through zero. It will go out. And after that, the switch will be truly open. The arc cannot re-form. AC current stops and reverses twice per cycle.

DC is different. Once the arc forms, current flows through the arc and it can become self-sustaining. It will remain unless and until the contacts have moved far apart from each other, or some other technique is used to extinguish the arc. The arc can cause a lot of damage, melting metal, starting a fire, or violent destruction of a small part, causing little pieces to fly out everywhere. The higher the voltage, the worse it is and the farther apart the contacts need to be to extinguish the arc.

Switches and breakers and fuses generally have different ratings for DC for this reason.

Watch this video. Seeing is believing.