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AC/DC Circuit Breaker


PaulF007
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It's like relays too , they can contacts can do both AC and DC. The current and voltage ratings are diffrent for AC and DC on the circut breakers and relays.

What is the difference between a AC breaker and a AC /DC breaker I'm not sure, I imagine and the contact are larger and the physical distance they seperate might be larger .

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For all contacts, it can do both AC and DC. That's just physics, if there is continuity, the electrons will flow. What comes after that is the breaking capacity. If you consider what happens when you open a switch in slow motion. First there is just a fraction of a millimeter that is open and the air gap has a resistance too low to stop the flow of energy, it ionizes the air and jumps across. Then as your slow motion movie plays on the gap widens, but ionized air has a lower resistance so the arc grows. At one point the distance is finally so large that the arc cannot be sustained and the flow of energy stops.

All the while this little welding arc is going it heats up the air, causes pitting in your copper contacts, and so on. Which is why relays are rated for a certain number of operations, usually around a million. They actually wear down.

If the arc is bad enough, it sets things on fire.

Now with AC, there is the additional deadening effect of the voltage going through zero 100 times a second. This allows for a higher voltage, usually between 5 and 10 times more.

So in general, if it has a DC rating, it will do an AC rating of the same. And even if it only has an AC rating, it will still handle DC of a much lower value, though you don't know what that value is unless the supplier tells you.

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1 hour ago, seant said:

What is the difference between a AC breaker and a AC /DC breaker I'm not sure,

Most DC CBs and contactors have some sort of snuffing mechanism. The NOARK DC circuit breakers I have have a magnetic field which prevents/reduces the formation or the arc @plonkster describes.

One of the ways of snuffing for a contactor is to have a capacitor in parallel. It is amazing to watch the capacitor work. It is really a simple idea. I'll see if I can find a You tube video I saw a while back.

Addition: This chap is a real hobbyist. The rest of us would stop the video and  start again when we accidentally tangle the crocodile clips and get a impressive arc - not this chap he does miss a beat. I think there are several off-camera mishaps in his workshop.

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3 minutes ago, Chris Hobson said:

capacitor in parallel

A discharged capacitor has an ideal impedance of 0Ω. So essentially the arc is squelched because there is a lower impedance  path parallel to the breaker. Of course the capacitor charges very quickly and turns into (ideally, though not in reality) infinite resistance, and the idea is simply that this should take slightly longer than it takes the contacts to move far enough to stop the arc.

Capacitors are also used as snubbers in thyristor setups. Same principle.

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1 hour ago, seant said:

I imagine and the contact are larger and the physical distance they seperate might be larger

The material used for the contact might also be different, or it might be covered in a layer of something different, to help counter-act the pit-formation.

DC fuses are filled with quartz sand to absorb the energy of the arc and extinguish it faster.

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5 minutes ago, Chris Hobson said:

Not quite sure why you are not supposed to use a DC breaker in an AC application. Well, I can think of two reasons, the one is that it's a waste of money, and the second is that with DC it takes more current for an instantaneous trip on the same breaker than on AC, that is to say if you use the DC-rated breaker on AC it might cause nuisance-tripping.

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20 hours ago, Chris Hobson said:

Extracted from the site and I must say this is as good as any explanation that I have seen so far. Thanks @Chris Hobson

"

AC Breakers

In North America, the grid alternates at 60 Hz, or 60 cycles per second; hence the name “Alternating Current”.  The voltage alternates between +V and –V, 60 times a second.  That means there is a point at which the voltage is 0v, 60 times a second.  It is at this 0v point that the AC breaker will “break” the connection, extinguish the arc, and protect the wiring from too much current.

DC Breakers

In contrast, a DC circuit does NOT alternate.  It stays at a constant voltage.   Since there is no 0v point, the AC breaker design will NOT work in a DC circuit.  The DC breaker uses a magnet to attract the arc, pulling it from the air gap, and extinguishes it.  The AC breaker is NOT equiped with a magnet, and cannot extinguish a DC arc.

Moral of the strory, use AC-rated breakers for AC circuits, and DC-rated breakers for DC circuits.

--- and ----

NOTE:  It is OK to use a breaker that has a dual AC and DC rating (stated on the manufacture’s label).  It will state clearly on the breaker if it is rated for both.  WARNING: Most dual rated breakers have their DC voltage rating DIFFERENT from the AC voltage rating, for the same amps.   (such as 60A 150VDC, 120VAC)  Be sure to double-check these values.

So the answer is , yes you do get AC/DC breakers

 

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5 hours ago, PaulF007 said:

AC/DC breakers

They used to be very very expensive ... not anymore. 

Having a AC breaker with a DC magnet, best of both worlds I think?

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Late reply ,  but here is two pictures to show the difference between an AC and DC breaker. I only had a old AC to break open and had to rely on a picture from the internet to show the DC breaker. The biggest difference lies in the Speed at wich the contact part and the arc defusing mechanism. you will see that the DC breaker has a much bigger arc chamber as well as a much bigger arc divider/Defuser.

483843644_ACBreaker_LI.jpg.97dc3008139c05fa7be7469661316cf2.jpg

The AC breaker is plain and simple with only that small plates to break the arc. The contact breaks in between those plates and the arc get drawn out. The opening of the contacts are much slower than in a DC breaker. 

405599587_DCMCB_LI(2).jpg.bd5068d924ec7ab9c7d0747033b0de6f.jpg

Dc breaker differs in the following way. The bottom blue arrow points to a spring. This spring is connected to the moving part that breaks away in a trip condition, This spring is to increase the speed at witch the contacts open. The heat that is created from the spark will cause a draft into the chamber, through the Arc divider and out the top.. A Dc breaker mainly rely on the faster contact speed , a bigger contact opening and the draft to successfully extinguish the spark before damage can occur..

 

 

 

Edited by Jaco de Jongh
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  • 4 months later...
  • 4 weeks later...
  • 2 weeks later...
On 2018/08/19 at 11:57 AM, Jaco de Jongh said:

405599587_DCMCB_LI(2).jpg.bd5068d924ec7ab9c7d0747033b0de6f.jpg

Dc breaker differs in the following way. The bottom blue arrow points to a spring. This spring is connected to the moving part that breaks away in a trip condition, This spring is to increase the speed at witch the contacts open. The heat that is created from the spark will cause a draft into the chamber, through the Arc divider and out the top.. A Dc breaker mainly rely on the faster contact speed , a bigger contact opening and the draft to successfully extinguish the spark before damage can occur..

This is not a DC breaker. It is a residual current device. Or earth leakage unit. 

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For a DC breaker to work properly, it should have polarity sensitive magnets.  And size does matter in this case. Just to compare, the Noark breaker takes up the same amount of space as a normal AC DIN circuit breaker. and is rated for 220v per series wired pole. Same as ABB S280 range. But if you are looking for higher voltages, you need a bigger unit. Roughly double the physical size of a "normal" DIN breaker. The ABB S800PV range come in ratings of 800, 1000 and 1200VDC for 2, 3 and 4 pole units. Price also increases quite a bit.

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My rule of emergency thumb only works with good quality AC breakers (like Hager): (Note this is a very basic rule of thumb for small installs like 12V or 24V home/caravan)

If it doesn't specifically have a DC rating then I go 1/2 to 1/8 x the size of the AC Current (lower the better). I use the following rough calc when I can't get hold of a DC breaker, especially when traveling:

* 220VAC 10A breaker roughly can do max 12VDC 5A or 24VDC 2A
* 220VAC 20A breaker roughly can do max 12VDC 10A or 24VDC 5A
* 220VAC 63A breaker roughly can do max 12VDC 22A or 24VDC 12A

Anything bigger then don't push it, I normally use this as a temporary fix for a month or 2 max, so it definitely doesn't get close to the multiple cycles of the switch so I cannot attest to lifespan.

You also have to understand that a AC breaker will trip at MUCH lower DC current than AC. My experience has been at about 1/4 of the current. But more importantly will trip slower! ie: on AC an AC breaker will trip within milliseconds of exceeding current... An AC breaker on DC could run for nearly 2-3 seconds before tripping on DC current - so it is possibly hazardous, especially fire.

As mentioned above - they have completely different requirements. Even though it still hurts paying the huge costs of a DC breaker, it is the right thing to use.

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Just now, VisN said:

Are these Noark DC breakers able to accept 35mm+ cable easliy?

I think so yes ... electrician fitted a 25mm2 with ease.
50mm2 definitely not.

Does this help? (grabbed the first ruler I could find)

image.png.d019a1e021dc3b4cf60534d826cf5a3d.png

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43 minutes ago, The Terrible Triplett said:

I think so yes ... electrician fitted a 25mm2 with ease.
50mm2 definitely not.

Does this help? (grabbed the first ruler I could find)

TTT,

That's a huge help. I will measure against my Hagers. Thank you very much. 

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13 hours ago, anotherbrownbear said:

Wow, 25mm cable in those terminals? I would keep an eye on that if I was you.

Why is that? NoArk Breaker is rated at 40amp, and it is panel side.

But you got me to doubt myself, so I checked again:
25mm2 from MPPT to Inverter - I connected MPPT direct to battery points in Inverter.
And 16mm2 from combiner box to MPPT. Sorry @VisN mind got stuck on the 25mm2 cable I crimped last.

 

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