Circuit Breaker Question

[Boondocker]

Can an AC circuit breaker that is available in the local hardware store be used on DC?

If so, would it trip out at the same amp rating?

[fungus]

It all depends on the circuit breaker and how it is rated, some are rated for DC, some arent. As long as the circuit breakers are rated for RMS then yes they will trip at the same current on DC. One important thing as well if being used on battery systems is the very large short circuit current available from batteries; in cases over 1000A, make sure the breakers are rated (AIR) to break this current, otherwise they may not disconnect succesfully in the case of a short circuit. AC is much easier to trip than DC as it goes through a zero crossing on every AC cycle, 50 times a second, which extinguishes arcs more easily than DC so DC breakers usually have to be heavier.

[ghurd]

Good answer Fungus.

No data to back it up, but I have a feeling a DC breaker trips faster than a hardware store AC/DC rated breaker.

I usually use fuses, but when I use a breaker I still use a fuse with a bit larger rating than the breaker.  Just in case.

G-

[Lumberjack]

Generally speaking - breakers are rated for two things.

Maximum voltage that can be safely interrupted.

Maximum sustained current.

Some breakers also have a peak instantaneous current rating as well.

There is usually no real differance between ac and dc for a normal mechanical breaker. There are some breakers that work only on ac or dc and others that can change ratings depending on the exact input. many of these breakers are intended for exotic applications and should only be used as a last resort.

To determine the breaker you want first consider what it is you want to protect. Wire and cables are a lot more tolerant of overloads then an inverter will be. You must also consider if you need to allow for inrush currents. Motors can draw up to 6 times their listed current while starting but only do so for a split second. The breaker must be sized to protect the most sensitive part of the circuit and the lowest current bearing part of the circuit at the same time without tripping during normal operations.

Determining the breaker voltage rating is simply a matter of figuring out the absolute maximum voltage possible in the circuit and make sure the breaker is equal or greater.

All breakers have a data sheet that shows all of its characteristics including resistance to water and vibration as well as it electrical properties. Final selection should be based on how well the breaker will stand up to its overall enviroment. Consideration should also be made for future replacement as well. Exotic devices tend to disappear shortly before you actually need to replace them. Common breakers tend to be much cheaper when standard sizes are used and have a very long shelf life.

Much of this also applies to fuses as well. I hope this gives you a better idea of what to look for. In direct answer to the question, if you are refering to the normal household power panel breakers then yes if not GFCI and yes it should trip near to its normal rating. BUT. you may find a small disconnect box with internal fuses to be a better choice.

 

[TomW]

To whom it may concern;

Somewhat curious that nobody has mentioned the differences in breaking an AC circuit and a DC circuit especially under a high current load. In particular extinguishing the arc on breaking the circuit.

I am not experienced in this area so I will refrain from offering advice beyond mentioning the arcing bit.

Cheers.

TomW

[Flux]

Probably at 12 or 24v it is reasonable to use an ac breaker on dc.

I am not familiar with the devices available in N America, but generally I would be suspicious of using an ac breaker on anything much over 30v dc. At 110v no common ac breaker will manage dc satisfactorily on normal load. On a short circuit it will be a spectacular failure.

The dc breakers that we used to produce were very costly devices with magnetic blow out, arc chutes and other expensive features. In comparison ac devices were simple and cheap.

Even at 48v I would expect an ac breaker to be in trouble and I would indeed back it up with a HRC fuse as Ghurd suggests.

At much over 24v you have to be careful with fuses as well so make sure you have something such as HRC ceramic cased fuse. If you use wired fuses in your part of the world they are no use on high voltage dc.

Flux

[partsman]

What size breaker are you looking for.

Go to a tractor dealer and ask for one.

In the 90s it was popular to use breakers instead of fuses.

I work at a CASE IH dealer so I am spoiled.

We carry 5,10,20,30,50 amp.

[TomW]

partsman;

Great idea. Motorcycles used them too but only up to like 15 amps if I recall correctly.

One problem if using them to get "up to code" is they probably are not UL approved for household installation.

Just a thought.

Cheers.

TomW

[Boondocker]

Thanks all for the information.

I'm considering puting 60amp protection between the alternator and the 24v battery bank.

Now, think I'll be better off ordering a double pole fuse block instead of picking up a circuit breaker at the local hardware store.  Will look into finding a small box with a disconnect and fuse as an option as well.

Boondocker

[kurt]

some square D breakers are rated for dc up to 24v you need to check the spec sheets to see what ones but that is it for all other aplacations you need a dedacated dc breaker or a fuse

[oztules]

If you wish to do such a thing, take into account that 24-30v will not be a high enough rating.

The failure will have the full forcing voltage of the alternator. Depending on the design this may be quite high eg 60-70v unloaded.

As the breaker tries to open, the load will decrease and the voltage will shoot up from batt v to open v and will try and extend the spark length well beyond what you may have anticipated. So a higher rating may well be in order.

........oztules

[Lumberjack]

Might want to reconsider that idea for a moment... When the circuit gets interrupted it will become open and the generator will free spin with no load on it....  

[dyslexicbloke]

All valid stuff folks ...

However the comments about AC and DC disconnection are probably the most important.

Firstly a breaker rated at 'some' amps AC probably wont even cope with the same current DC without welding closed, in normal operation let alone a fault condition. Obviously there will be odd ones with massively oversized contacts that will but I wouldn't rely on anything that wasn't certified for a DC load.

Secondly, and equally importantly, the time in which a breaker trips at a given fault current is a fundamental component of the circuit design. Designers will talk about 'Energy Let Through' in practice what this means is:- will the current be interrupted before the cable / installation is damaged by thermal or mechanical shock, thermal usually being the bigger issue.

NO reputable dealer would ever recommend the use of a breaker for which time/current disconnection graph could not be supplied, given the proposed application, this would include DC operation. The reason is that without such data it is not possible to design the circuit in the first place.

Magnetic breakers, designed for AC operation, probably wont trip at all and thermal ones are likely to work relatively slowly. Measure your PSC (Prospective short circuit current) and then look how long it will take for your breaker to disconnect. You may be surprised just how many watts will be dumped into your cables before it dose!

By the way, the same is true of fuses, any fuses, but in most cases DC data will be available.

Treat it with respect folks, AC or DC, keep it your friend, if you have identified a need for circuit protection you will want to make sure the protection works as expected, there isn't much point in fitting it otherwise.

One final point, up to about 60A breakers can often be found in commercial vehicles, try a breakers yard.

Al

[partsman]

PS the breakers I am talking about can be stacked for more amps.

One aplication I am thinking of uses 3 50s to make 150a.

[Boondocker]

The dilemma is circuit protection vs over speed protection.  However, if there is a short I don't want the battery bank to get damaged either.  I'm using this diagram for reference.  If the fuse blows the diversion controller could still clamp the blade speed; if it is still functioning.  

[Lumberjack]

so long as the fuse is between the controller and battery your good. In this usage I would tend to go with a delayed fuse at about 20 percent under the max current the controller can handle.(rough estimate) Battery banks are not all that sensitive to overload and will stand a short for much longer then the controller will. Also be aware that there will be power on both sides of the fuse when it is blown

BTW in this position you can wire a relay parallel(across the fuse) to it and short the genny whenever the fuse blows. (It acts like a short on the coil and keeps the relay from working.)

[Flux]

This raises an interesting question and my interpretation of it may not agree with others.

Personally I would use a fuse between mill and battery big enough that it can not blow under any possible wind condition. It will protect your wiring from a short circuit at the machine end but if it blows under any working conditions it will cause more trouble than it is worth.

I would not expect a charge controller to survive long and it may totally fail to work without the low impedance supply of the battery. It would be better to blow it up rather than have the mill run away ( assuming the mosfets will fail to short circuit).

If the rated current of the fuse need to carry the mill current under all conditions ( storm and otherwise) is greater than is reasonable to protect the wiring to the charge controller then I would fuse that separately.

Sometimes with long cables it becomes quite difficult to protect the cable run against a short and still have a fuse rating that the mill can never blow.

You will most likely need HRC fuses to get sufficiently precise fusing characteristics and they are the best to use anyway as conventional wired fuses get tired and eventually fail below their nominal rating.

Flux

[RUFUS]

What size do you need?

                    Rufus

[Boondocker]

For the turbine/alternator size and anticipated loads, 60 amps

[Lumberjack]

Your on the right page. In the end it is all about what is being protected. Preventing a fire and keeping a controller intact are two very different levels of protection.Other then the controller and rectifiers most of the components are not sensitive to overloads per say. A second or two in an overloaded state wont neccesarily kill the genny or the cables or a batt bank. The controller itself wont last a split second past its limits however so it present a different set of requirements.Aside from fusing there is also the over engineer approach in that the item in question is sized large enough to handle a worst case situation.

One consideration to look at is the amount of instantaneous current a battery bank can provide. Unlike a power supply which is limited somewhat to its operating capacity a battery bank can output several hundred amps for a short burst and totally fry almost any device attached to it.

On the ac/dc front It occurs to me that magnetic breakers might not work very well with the wild AC that the gennys output...

 

[RUFUS]

I have a pair of double 100A

thier new you can have them

if you can use them that heavy

thier adjustable too.

                     Rufus

[Jimmer]

Very true Fungus and this is why the NEC says that ONLY DC rated breakers be used for DC.  An AC breaker will not standup to the on/off switching(ARCing) and will leave deposits due to the heavy ARCing caused by DC, and eventually short circuit. They might work for a while, but I wouldn't use AC breakers in my house for DC.