AC versus DC load breaking comparison with a knife switch
Why Electrical Engineering Stack Exchange recommends it
Electronics Stack Exchange answerers and commenters link it as the go-to visual proof that DC arcs do not self-extinguish the way AC arcs do at the zero crossing, which explains lower DC ratings on switches, relays, fuses and breakers.
Recommended by 4 people across 4 different months, 2015 to 2026.
What it is
A two-minute demonstration of opening the same resistive load with a knife switch on AC and then on rectified DC. The AC arc dies out quickly, while the DC arc persists and stretches across the gap. It uses four chrome-wire resistors (about 2600 W) at roughly 210 to 311 V.
What people said
Recommends winny · 2017-04 · Electrical Engineering Stack Exchange · on “Fit a switch to rectifier input or output?”
Don't break anything on the DC side after the rectifier! Demonstraion video here: [this video] Fit your switch on the input/AC side.
Recommends user57037 · 2021-01 · Electrical Engineering Stack Exchange · on “Why does a circuit breaker voltage rating differ between AC and DC?”
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 unle
Recommends Mister Mystère · 2022-06 · Electrical Engineering Stack Exchange · on “How are switch ratings determined?”
@Frodyne: If you liked this video you'll love this one: [this video] My jaw dropped when I first saw it. The two videos cover different currents so one can see the effect of that.
Recommends Frodyne · 2024-05 · Electrical Engineering Stack Exchange · on “Are 96V lead acid batteries a thing?”
For your last point about arching, here is a nice demonstration of how AC arcs are self-terminating when the voltage goes through the zero-point, while DC arcs are not: [this video]
Cites user57037 · 2015-11 · Electrical Engineering Stack Exchange · on “How many large format LiFePO4 cells can be connected in series?”
I have no experience with such things. But have you ever seen a 1kV DC arc? Plasma is fairly conductive. I'm going to add two video URL's. [this video] [LINK]
Cites user95482301 · 2016-09 · Electrical Engineering Stack Exchange · on “Is high temperatures breaking the circuit?”
I was watching this video, where they were demonstrating the difference between breaking AC and DC current. [this video] When they try to break the DC current, an arc forms. It continues for a while, but then breaks. Does it break because the material gets so hot, that the resistance increases and consequently reduces the voltage experienced over the gap, thus requiring the gap to be reduced to maintain the arc? I've experienced a similar effect when I was arc welding. The material would get so hot that maintaining an arc was impossible. I didn't understand why until I saw this video.
Cites user95482301 · 2017-01 · Electrical Engineering Stack Exchange · on “Switching setup to reverse B2B charging direction”
Here's an example of a custom made, low-resistance/high current switch. You could devise some mechanism that would swap the wires like a DPDT switch. You can buy copper bus bars relatively cheap and make it yourself for ~$10-20: [this video]
Cites Trevor_G · 2017-08 · Electrical Engineering Stack Exchange · on “Using AC rated relay in DC applications”
[LINK] As Manu310us said... it's all about arcing. Using the wrong relay with a high DC current can quickly set your relay on fire . Nothing demonstrates this better than this video [this video] ... So don't use an AC relay for switching DC loads. Using DC relays for switching AC loads is much less problematic, but you can usually find a cheaper AC relay which does not include the mechanical arc interrupter mechanism. NOTE: the same rules also apply for mechanical switches.
Cites The_Vintage_Collector · 2020-05 · Electrical Engineering Stack Exchange · on “Why can an AC circuit breaker not be used in DC circuits?”
@pericynthion I came across this video. Oof! I never knew the arcing can be this nasty. But probably very less at 6/12/24VDC [this video]
Cites user57037 · 2022-02 · Electrical Engineering Stack Exchange · on “Using 210 V DC to a 220 V AC heater: is it going to work?”
No. You need a DC rated switch. You can search for videos online that show what happens. [this video] AC circuit breakers may only last a few cycles if they interrupt a DC overload.