AC circuit breakers on DC circuits

Phil C · Guru

AC circuit breakers on DC circuits

oldtrack123 · Guru

Hi Phil

Yes, Ac breakers & switchs can be used on DC but not necessarily ALL

Makers will usualy show the DC rating if they consider their's is suitable

It is based on both voltage & amps. but the trip current in both situations will be very similar

It is the arc rupturing abilty [which depends on design] that is the major consideration

There is no hard & fast formula for that

Which operates first will depend on the current

Thermal trip covers normal overloads but always has a time delay

The magnetic trip is fast acting ,more for quick disconnect of sudden excess currents ,such as a short circuits

All Cbs also have a MAX current interupt rating, one they can handle without damage to themselves .

The current that could occur is determined by :

The max short circuit current available fron the source

The actual circuit impedance

PeterQ

-- Edited by oldtrack123 on Friday 22nd of August 2014 12:42:24 PM

Peter_n_Margaret · Guru

Clipsal rate theirs at 48V DC.
Note however they that the time to trip will be Root2 times as long (about 1.4X).

That is an increase in trip TIME, not an increase in RATING as per the OP.

I used 240V DC circuit breakers in the OKA DC wiring, but I avoid using them as switches to break flowing current in order to reduce the frequency of arcing.
I have yet to experience a failure in 10 years of use.

Cheers,
Peter

-- Edited by Peter_n_Margaret on Friday 22nd of August 2014 04:28:22 PM

oldtrack123 · Guru

Peter_n_Margaret wrote:
Clipsal rate theirs at 48V DC.
Note however they that the time to trip will be Root2 times as long (about 1.4X).
That is an increase in trip TIME, not an increase in RATING as per the OP.

Cheers,
Peter

-- Edited by Peter_n_Margaret on Friday 22nd of August 2014 04:28:22 PM

HI Peter

Perhaps you have misunderstood some data as the thermal magnetic trip units fitted to a.c. circuit-breakers are also suitable to be used with direct current.

The thermal protection does not change with reference to its tripping characteristic as the bimetal strips of the trip units are influenced by the heating caused by the current flow, it does not matter whether AC or DC .

I think you may be confusing AC peak Current with RMS values

The peak curent is 1.4times the RMS but when we say XX amps with AC we are referring to the RMS value.

IxIx R= P[Watts] is the same in each case each case

However the MAGNETIC trip function will change under short circuit conditions, due to ferromagnetics involved with AC

Generally the reaction trip time will be faster with DC than AC.

The big reason for derating with DC is ARC suppression.

PeterQ

Peter_n_Margaret · Guru

The root2 X trip time was Clipsal's figure, not mine, unless I misread it???
Will see if I have a copy.....

Cheers,
Peter

Peter_n_Margaret · Guru

Found this updates.clipsal.com/clipsalonline/Files/Brochures/I0000097.pdf
Quotes the 48VDC, but does not describe any other DC operating characteristics.

Cheers,
Peter

Mike Harding · Guru

Hmmmmm... I'd be a bit cautious with the expectation that all MCBs will
operate in a similar manner on DC as they do on AC.

Certainly in regard to the thermal tripping the DC will equal the RMS and
all should be good but I'm not so sure about the magnetic aspect.

Page 7 of the above PDF shows the MCB design and the magnetic
tripping is performed by a solenoid the coil of which, I assume, is designed
with 240V AC in mind. Additionally the "Specifications" section of:

www.clipsal.com/Trade/Products/ProductDetail

specifies the "Voltage type" as "AC" and makes no mention of DC.

Given that the only purpose in fitting a MCB is to be as certain as
possible that it will *work* then I would seek e-mail confirmation from
the manufacturers for each model which you intend to use on DC.

MH

KFT · Guru

Think I would be looking to NHP products to be sure to get something eminently suited to the task.

frank

Phil C · Guru

Thanks folks for a lively discussion, I have had a look at some old text books and agree that the slower thermal aspect of the breaker should open the circuit. The magnetic however may not be as efficient due to not enough flux being generated (Flux = I x N) and at 12V probably not enough to stick to a fridge. Then again under a short circuit who knows? However as I have stated I only wanted to isolate the DC and if its a safety measure as well I get a bonus.

Cheers and safe travels

oldtrack123 · Guru

Mike Harding wrote:
Hmmmmm... I'd be a bit cautious with the expectation that all MCBs will
operate in a similar manner on DC as they do on AC.

Certainly in regard to the thermal tripping the DC will equal the RMS and
all should be good but I'm not so sure about the magnetic aspect.

Page 7 of the above PDF shows the MCB design and the magnetic
tripping is performed by a solenoid the coil of which, I assume, is designed
with 240V AC in mind. Additionally the "Specifications" section of:

www.clipsal.com/Trade/Products/ProductDetail

specifies the "Voltage type" as "AC" and makes no mention of DC.

Given that the only purpose in fitting a MCB is to be as certain as
possible that it will *work* then I would seek e-mail confirmation from
the manufacturers for each model which you intend to use on DC.

MH

HI Mike

Your point is valid with regard to the MAGNETIC trip, if used on a different voltage to the designed voltage

One designed for 240V would be pretty well useles on 12V, either AC or DC

It will trip under short circuit conditions but only due to the THERMAL trip

To other readers

DC circuit breakers NEED better ARC quenching than AC breakers.[for the same voltages ]

That is the primary reason why ,when used on DC, the voltage rating is so LOW compared to the A C designed rating

-- Edited by oldtrack123 on Saturday 23rd of August 2014 01:07:49 PM

03_troopy · Guru

Without wanting to derail the original topic too far, can anyone tell me why many DC thermal circuit breakers have "Batt" and "Aux" labels on their terminals. Given that they operate by heat build up from excessive current, distorting a metal (stainless?) plate which has been punched to give it a dish shape (yes i pulled a few apart ;) ), why does it matter which way the current flows? Or is it maybe that they are marked to facilitate circuit tracing once they are installed?

Mike Harding · Guru

In the case of there being only a thermal tripping mechanism then I
can see no reason why direction of current flow would matter - heat
is heat.

However many (most?) DC MCBs are combined magnetic and thermal:

bit.ly/1AIJXIe

and in these cases it is essential that current flow is in the
specified direction in order that the magnetic fields have the
required polarity.

It may simply be that cheap MCBs are produced which don't include the
magnetic aspect but use the same plastic moulding? Or, maybe, the
manufacturers tired of people asking about the polarity so specified
one anyway? :)

MH

oldtrack123 · Guru

03_troopy wrote:
Without wanting to derail the original topic too far, can anyone tell me why many DC thermal circuit breakers have "Batt" and "Aux" labels on their terminals. Given that they operate by heat build up from excessive current, distorting a metal (stainless?) plate which has been punched to give it a dish shape (yes i pulled a few apart ;) ), why does it matter which way the current flows? Or is it maybe that they are marked to facilitate circuit tracing once they are installed?

HI Troopy

They are polarised so the magnetic arc blowout system pushes the arc in the right direction to break it.

That can be very important if they are to break the arc before it does damage to the breaker itself

Qualty breakers designed for DC operation usualy do have a magnetic arc blow out

PeterQ

-- Edited by oldtrack123 on Sunday 24th of August 2014 11:11:12 AM

03_troopy · Guru

Sorry Peter, I should have been more specific. I meant the common automotive types like these.

DC CB.jpg DC CB2.jpg

03_troopy · Guru

Still no takers for this query?

oldtrack123 · Guru

03_troopy wrote:
Still no takers for this query?

HI

I have not pulled either type apart , so cannot say if either do gave a small permanent magnet to assist arc blow out.

I doubt the small elcheapo has any form just relies on luck Or the larger copper as a a heat sink

The better one could easily have a small permanent magnet or even a basic deionizing chamber [ series of ferrous metal plate which become magnetised by the current & breaks the arc up into small sections.

An other option used by some is a multi Arc gap, ie two or more sets of contacts in series

But without opening them I realy cannot say for certain.

PeterQ

-- Edited by oldtrack123 on Thursday 28th of August 2014 10:17:05 PM

03_troopy · Guru

oldtrack123 wrote:
03_troopy wrote:
Still no takers for this query?

HI
I have not pulled either type apart , so cannot say if either do gave a small permanent magnet to assist arc blow out.
I doubt the small elcheapo has any form just relies on luck Or the larger copper as a a heat sink
The better one could easily have a small permanent magnet or even a basic deionizing chamber [ series of ferrous metal plate which become magnetised by the current & breaks the arc up into small sections.
An other option used by some is a multi Arc gap, ie two or more sets of contacts in series
But without opening them I realy cannot say for certain.

PeterQ

Peter, neither of the 2 shown have any magnets and have only the metal plate that acts as the breaker. The query came up when connecting the manually re-settable one in a dual battery setup, where the limited current would be from the auxiliary battery back to the main battery when using the override function of the smart isolator. One bloke wanted to connect the main battery to the BAT terminal and the AUX (load) terminal to the smart isolator. I was thinking that because the current would be in the direction of Auxiliary battery to main battery (flat main battery) maybe it should be connected the opposite way round. But no-one can give a definitive answer on it it so far (with a reason why). To be honest, I really can't see any reason why it wouldn't work both directions.

-- Edited by oldtrack123 on Thursday 28th of August 2014 10:17:05 PM

oldtrack123 · Guru

HI Troopy

12vDC cannot draw a very long arc, so as long as the break gap is long enough & the break is fast enough,a sustained arc is unlikely

But that can change when the voltage gets above 24VDC or the load is highly inductive[ motors or coils with ferrous cores

PeterQ

03_troopy · Guru

oldtrack123 wrote:
HI Troopy
12vDC cannot draw a very long arc, so as long as the break gap is long enough & the break is fast enough,a sustained arc is unlikely
But that can change when the voltage gets above 24VDC or the load is highly inductive[ motors or coils with ferrous cores

PeterQ

Yes peter that is also true. thanks. ;)

bobsa · Senior Member

here is a link to NHP breaker at all good electrical suppliers
single pole up to 48volt dc and 2 pole 110 dc

www.nhp.com.au/files/editor_upload/File/Brochures/Business-Specific/easy-selection-guides/NHPNTUMCBAUSESG.pdf

Tony Bev · Guru

I am not an Auto or normal Electrician
I can only go by what the description says

I have recently purchased 12 VDC 30 Amp circuit breakers
From the description I will assume that there is a type of 30 amp fuse inside

I had normal inline blade fuses, but these circuit breakers will give me the ability to isolate, the following circuits, easier

Roof solar panel, positive wire before the DC/DC charger
12 volt upright fridge, positive wire, at the batteries
12 volt merit (ciggy type) socket, positive wire, (for my chest fridge/freezer if I need to use it), at the batteries end

The inline blade fuses would have done the same isolation, at a much cheaper price
The circuit breakers give me ease of isolation/re-powering

12 volt circuit breaker.png

oldtrack123 · Guru

Tony Bev wrote:
I am not an Auto or normal Electrician
I can only go by what the description says

I have recently purchased 12 VDC 30 Amp circuit breakers
From the description I will assume that there is a type of 30 amp fuse inside

I had normal inline blade fuses, but these circuit breakers will give me the ability to isolate, the following circuits, easier

Roof solar panel, positive wire before the DC/DC charger
12 volt upright fridge, positive wire, at the batteries
12 volt merit (ciggy type) socket, positive wire, (for my chest fridge/freezer if I need to use it), at the batteries end

The inline blade fuses would have done the same isolation, at a much cheaper price
The circuit breakers give me ease of isolation/re-powering

Elcheopo 12V thermal circuit are just a thermal switch .often using just the moving contact arm itself as the bimetal thermal trip

Auto rest types simply close the contacts with arm cools down & something that should not be normally used except in situations as vehicle headlights

Manual reset require manual resetting to latch in . Such should be used for most applications as you will be aware they have tripped & investigate why

Polarity again could depend on design , but with most the Pos [usually the fixed] contact, in breaker will be larger or have a heat sink to absorb the greater heat generated by arc @ the Pos contact face

Better quality Dc breakers especial for voltage above 24V will usually have some form of magnetic blow out[permanent magnet or coil]

Such will usually require mounting vertically in the direction indicated to assist the arc blowing into the extinguisher area

]

Tony Bev · Guru

Thanks for that explanation, oldtrack123, it is appreciated

T1 Terry · Guru

If you want a reliable DC circuit breaker for solar you can't go past the No Ark range. Those lever type DC circuit breakers have a nasty habit of getting hot and tripping well before they have reached their rated capacity due to poor contacts creating high resistance. There other specialty is welding the contacts together and never tripping.

T1 Terry