DC fuse & circuit breaker questions 2

[kennchan]

Thanks all, My question, as banal as it might seem, involves the rating on DC breakers. while installing a 48 volt battery bank and its attendant inverter (magnum MS 4448PAE) i have come across an interesting problem, short of the items offered by the major players in the market, nobody seems to offer a dc circuit breaker that is rated for a 48 volt battery banks. Lots of 24 VDC and 150 VDC (PV types) but nobody offers a simple single circuit breaker for the 48 volt range. I am not looking for a din rail type as the system involves only one inverter and a battery bank no solar, nothing other than a 110 VAC line in to maintain the battery banks when not in use. (Simple UPS for a fridge/freezer in a cabin for the odd time when power goes out for a day or so.)

All off the easily available (to me) individual circuit breakers are rated at 12, or 24 volts. examples given: 24 volts 200 amps, 12 volts, 300 amps. My question is can these be used in a 48 volt system (DC) and if so does this derate the breaker in some fashion? these are all the simple heat and break type that heats up and then releases the circuit when x temperature is achieved over x seconds. which can then be manually reset.

It would seem in my non electrician mind, that as the voltage rises in the circuit the amount of amperage that it could handle would correspondingly drop. (battery banks keep the amperage the same as you series wire them for more voltage so it would seem that conversely a fuse or circuit breaker that is rated as 12 or 24 volts the amount of amperage that it could handle would drop if you raised it to 48 volts. IE: a 24VDC breaker rated at 400 amps could only carry 200 amps at 48VDC. a 12 VDC breaker rated at 400 amps could only carry 100 amps at 48VDC.

I was looking at carrying the full load of the inverter at the makers (magnums) recommended over current device rating of 175 amps

"NEC Current is based on the Maximum Continuous Current rating with a 125% NEC derating for sizing the overcurrent device (when not continuous duty) to prevent operation at more than 80% of rating.": from their manual.

However nobody sells on ebay or amazon (my only choices due to location) a 48 volt circuit breaker (or fuse for that matter) in that range that is not a Din rail setup which requires an additional 2~300 dollars worth of unnecessary items. Only 200 or 300 slow blow fuses that are designed to blow over 40 or 50 seconds at their rated (200 or 300) amps. what would happen if I used a 24 VDC 300 or 400 amp circuit breaker in its stead?

thoughts? sage advice? screaming fits about these morons on the interwebz?

thanks

Ken Morgan

 

[Sn00ze]

One thing to note about DC breakers is to be careful whether they are rated as adequate to be used as "Main" or only as "Branch".

Depending on your application. I am mainly talking about UPS systems within a substation.

Sorry i don't know much about 48VDC system, just ran into a project that was done using DC CB and they didn't account for that rating making the whole design wrong.

 

[cranky108]

At one time, some breaker manufacturers would rato their breakers for a DC application, but you had to look at the manufacturers data and not on the breaker.

Example: the QO breakers at one time were rated for 48VDC, but the trip current was not as on the breaker. The new ones with visual-trip I don't believe have that rating though.

There are breakers made for DC, but you need to go to the manufacturers books to see. Because it is an odd application it's hard to find.

Or you use fuses.

 

[DTR2011]

I understand you are not interested in
DIN rail, but I just pulled this out
An RTU cabinet about 5 mins ago.

It clearly has a 48VDC rating on it.

BTW this units have a hair trigger and will
Trip if you sneeze wrong.

 

[kennchan]

The concerns I have about the DIN Rail type is that you need to buy a cabinet to go with it and all the manufactures purposely make theirs propriety (for the most part) Magnums only works with magnums as the cabinet that they sell bolts to their inverter, same with all the big names. why spend several hundred dollars for a sheet metal box that is going to hold one breaker? I found a slow blow 400 amp fuse for 48VDC some Taiwanese (or possibly mainland China) company called go power, so I got one of those on order now I just need some smaller branch ones to separate each battery bank with.

i guess my real question is their a formula to calculate the amperage capability of a breaker that is for a different voltage? sounds hinky I know but there has to be a way to calculate the nominal values at different voltages. I know that AC vs DC breakers are pretty much incompatible except in a very few rare cases but i was just interested in DC to DC.

Ken

 

[xnuke]

i guess my real question is their a formula to calculate the amperage capability of a breaker that is for a different voltage?

No. The voltage rating is based on the ability to suppress the arc that happens after the operating current causes the device to begin to open. The voltage and current ratings are not directly related.

Why are you so stuck on having a 48 V DC rating? Any device with a DC rating equal to or higher than 48 V will work.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[sibeen]

I don't live in the IEEE / NEC world but there has to be millions of breakers rated to at least 48 volts installed in all the various telcos which use -48 volts as their standard distribution voltage with exchanges.

And a DIN rail breaker mounts on a piece of DIN rail, you don't need a special manufacturers cabinet at all.

 

[crshears]

Rule of thumb where I worked years ago was that provided the current trip setting was the same you could use an AC breaker rated @ =/> 2.5x the DC system voltage...

Note that although I was never able to confirm the validity of this, I knew of at least one application where 600VAC breakers were used as the main breakers in a 250VDC system; anyone else ever heard of this rule of thumb?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

crshears,

Depends massively on the nature of the load. I used to have a 10kA breaking capacity 230/400V MCB with a hole burned in the side after stupidly using it to interrupt the field of a 110V DC motor we were testing. Lost of inductance, lots of stored energy, and a beautiful blue flare coming out of the side of the breaker. The field current was only a couple of amps. With a resistive load then your rule of thumb might be viable.

 

[FreddyNurk]

There has been a lot of interest and development in terms of the capabilities of breaking DC rather than AC, in recent times.

There have obviously been applications that have dealt with it for years (e.g. rail traction, larger excitation systems) but the smaller stuff hadn't been considered as much. For instance, there used to be polarised DC breakers available, and commonly used in solar installations. That's no longer the case.

The big problems with DC are that there is no current zero, and that the directional nature of DC is problematic when things like magnets are used to assist in forcing the contacts open, as they only work when the current flows in one direction, as well as what xnuke said. Again, this is well known for bigger applications such as rail traction, but for much smaller stuff its only now becoming more prevalent with all the solar and renewable interest.

Some of these things aren't such an issue when solar level fault current is being dealt with, but are much more of a problem when battery level fault currents are being considered.

48V DC breakers are quite common (telcos mostly use -48V DC), just not necessarily in a non DIN rail format.

EDMS Australia

 

[jraef]

If you don't like DIN rail mounting, you can use a Square D "QOU" style breaker. They are lug-in, lug-out and can be mounted with clips and screws (or DIN rail).

http://download.schneider-electric...._Name=0720CT9401R105.pdf&p_Doc_Ref=0720CT9401

QOU miniature
circuit breakers are rated in the following UL 489 voltages, as shown in Table 3:
• 120/240 Vac
• 240 Vac
• 48 Vdc
• 60 Vdc

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[xnuke]

They don't go up to 175 A.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[itsmoked]

These are 150A @ 48Vdc

Otherwise, just use a fuse like everyone else does with battery supplied inverters. They're cheap and effective. You can put a box of spares in the panel for the cost of a breaker - if you could find one. Then use a standard disconnect for isolating the inverter when needed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[kennchan]

itsmoked,

Something I think that confused everyone earlier, when I say 48 volts I mean any item thats nameplate covers the basic range for a 48 VDC system. so 36~??? fine as long as it is rated at the needed range thats part of the reason I asked the questions.

Same questions apply to fuses at least in my mind. I found a main fuse that was rated for 300 amps in the 48 VDC range, great for the inverter but what about for the feeds from the panels. Have not seen anything (fuse wise) any lower than 200 amps in the 48 volt range. lots of 24 volt fuses anything from 40 amps to 150 amps but would they work correctly (blow at the same or a different amperage due to the different voltage)?

hence my original questions about derating due to voltage etc. Its a mute point though I i decided to buy some of the 24 volt breakers and test them by loading them up till they break while I record it with my ammeter.

Thanks all for chiming in.

Ken

 

[itsmoked]

Hi Ken.

If a breaker is rated for 24Vdc it could easily NOT interrupt 48Vdc at all.

If a breaker is rated for 48Vdc at a particular amperage then it will break at that same amperage give or take 1 or 2% at 24Vdc.

You can always use a breaker or fuse at a lower voltage than its rating.

What you really have to consider is the AVAILABLE fault current. If you look at all breakers they will state the maximum allowed fault current. That's because a non-resistive load can make it hard for a breaker to actually clear the fault. It is always ALWAYS a race to the clear. A breaker is guaranteed to win at the race if the available fault current is below the breaker's maximum rating. If a breaker is used that has a lower available fault current than reality, it will likely lose the race and slag down, taking everything around it with it. (see Scotty's comment above)

With solar apps the available fault current it puny, it's so low there are problems even getting breakers to respond at all. With mains applications the available fault currents can be a lot or not so much but it is usually pretty easy to find it or calculate it knowing some wire sizes and the transformer impedances supplying the power. With battery applications it can be a little harder and the available fault currents can be way higher than expected. Most automotive batteries can dish out at least a thousand amps, truck batteries 2kA, and stationary battery banks for solar or UPS service 3 to 10kA. The last Trojans I hooked to a Magnum inverter were 3,650A short circuit and I had them in 2s2p. I used one of the beakers I linked above (5kA interrupt) from each individual battery and a fuse for the whole thing combined.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[kennchan]

Thanks Keith,

Now thats an answer that I understand and gives me things to think about. I might just have to go all fuse.

 

[xnuke]

kennchan, did you not read my earlier post about voltage? You cannot use a device with a lower-rated voltage than 48 V. You do not use a device that is "covered" by the voltage range. You cannot use a 24 V or a 36 V rated breaker. You must use one rated 48 V DC or higher, and you cannot apply an AC rated breaker derated by a rule of thumb.

Ok, seriously, you do not get to decide the ratings of equipment, especially safety equipment like circuit breakers, by doing your own testing. Who taught you engineering? You are practicing outside of your competence. You're likely to start a fire. You need to be buying a listed device and applying it in accordance with the manufacturer's instructions.

An internet search of "circuit breaker 125 vdc 175 a" turned up panel-mount circuit breakers that would work in your application. Buy them. Use them. For others who have encouraged him otherwise, shame on you.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[itsmoked]

For your further edification Ken, a breaker tester supplies adjustable current (huge currents) at something like 1 Volt, breakers trip on current alone.

xnuke is correct in that you are only to use commercially approved fully rated breakers and fuses for anything.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[kennchan]

Thank you all for the input.