Available fault current in a DC system

It all depends on how much battery capacity you have. The battery manufacturer can tell you how much current their product can deliver into a short circuit before the battery voltage collapses or the circuit breaker trips. Location of short circuit relative to battery and the size of connecting cable is also a significant factor.

It sounds like you might be using the Carlignswitch or Airpax Class of breakers used as service entrance breaker. These have a 10,000AIC rating.

The Blue Sea Company in Bellingham has a nice paper on fault current that you can download from their website. They also mention how the Interrupt Capacity can be extended by use of fuses in series with the circuit breaker. This kind of application may require some kind of HRC fuse or even a Class T fuse when the battery capacity is quite large.

Type of battery will also affect how much fault current can be developed. AGM will typically produce greater fault current for same rated Amp Hour capacity than Gel batteries.

Are the CB's rated for DC at 10 kAmps?

Yes , the breakers are rated 10 kIAC

I am replacing the system and adding another cabinet. I was told to replace the DC disconnect with a 42kAIC DC rated CB. It just seems like a lot of capacity to be for DC system

If the breakers are rated for AC, then they will not have the same interrupt capacity for DC. (I'm assuming that the "10 kIAC" that you wrote is mis-typed). This in itself does not rate the breaker for DC.

The reason for this, not to insult your intelligence if you already know, is that AC voltage goes to zero every 8.33 mS, worst case, when the breaker trips, thereby eliminating the arc voltage necessary to jump a gap between contacts. DC, on the other hand, does not go to zero and will continue to conduct an arc over any distance that the voltage will allow it to persist. Because of this, the distance between contacts of DC-rated breakers must be higher for the same voltage, versus AC-rated breakers.

The Proposed Main System breaker is 25kAIC @ 600 Volts DC Square D Type PAC. Each Cabinet breaker has a 600 Amp 35 kAIC rated @ 600 Volts DC CH type HLDDC. My system voltage is 480 Volts DC.

I was looking at an old battery cut sheet and it did show 1 -12 volt battery with an available fault current of over 5000 Amps. My cabinet has over 50 batteries. I am sure there is some sort of series, parallel way of adding the AIC of each battery to determining the total system available fault current.

I am trying to contact a battery mfg.

Well, to start with, paralleling 50 batteries, with 5000 Amp rating each, means that you need 250,000 AIC rating on the DC-rated breaker. The AC rating doesn't mean doo doo.

If you're running the batteries in series and you have a fault, the likelyhood that a battery won't blow up is slim, but a lower current rated breaker may be used, but it will need to be rated at at least 600 VDC and 10,000 (to be on the safe side) or more amps.

You have a potentially dangerous situation, and this forum is not the best place to seek advice without an exceptionally detailed description of your system. I would seek face-to-face advice from a local electrical engineer who has experience in this area.

Evidently the person who told you to use 42kAIC rated breakers knows something about the system you have not mentioned. Or else you are not fully understanding the nature of the spec.

The fault interrupt current rating is the instantaneous peak current which may only have a duation of less than one second.

This rating reflects more on the arc quenching ability of the breaker than the actual overload amp rating which is most likely only around 50 - 100A at typical DC system voltages of 24 or 48volts.

Standards bodies like ABYC require a 10kAIC rating on main service entrance breakers. For branch circuit protection they only requie 5kAIC.

In a typical station backup system with a 2000Amp Hour rated battery bank, the instantaneous fault current can easily exceed 10,000 amps for the time period it takes for a circuit breaker to opeate. Trouble is; with DC you never get a zero crossing point to assist in arc quenching like you do in AC circuits.

The DC fault current will continue to burn like a giant arc welder until you quench the arc itself and or open up the gap to a distance which exceeds the lenght over which the arc can sustain itself. Its impractical to use DC breakers which can open up an air gap of several inches in a millisecond period of time.

If it takes any longer, you will reduce your contact points to molten slag.

This is why most relays, switches, and contactors have a much lower current rating for DC than AC.

You will need the battery manufacturer data that will give you the internal impedance of the cells at some level of charge, temperature, etc. Then add to it the impedance of the distribution system between the battery and the CB and that should be enough to calculate a worst-case current at full charge, dead short. The shipboard circuit breakers we're using in a nominal 250VDC system are Eaton (Cutler-Hammer) breakers of the NQB-A250 variety. These particular units are rated for 250 VDC or 500 VAC and have different IC ratings depending on whether it is AC or DC. It sounds like your voltage rating may need to be higher but a catalog search should help you find what you need.

One thing that you will notice is the IC rating for DC and AC is not that different. A previous poster mentioned the zero crossing of an AC current, and that is true, but the effects of the heated, ionized gas are still there for AC currents and the current will strike again after the zero crossing. For this series of CB's, it is 20kAIC for AC and 15kAIC for DC.

Circuit Breakers for Naval Shipboard Use — Types AQB-A250, NQB-A250 Breakers

250 Volts dc, 500 Volts ac, 250 Amperes Maximum, 20,000 Amperes ac, 15,000 Amperes dc I.C.

Specification: MIL-C-17361

● 100 – 250 amperes.

● 500 volts ac and 250 volts dc.

● 2- and 3-pole.

Interrupting Ratings

● 20,000 amperes ac and 15,000 amperes dc. AQB-A250 circuit breakers are used primarily to protect feeder circuits, however, those followed by "NM" in the table have higher magnetic settings to allow for motor starting. Breakers with settings specifically for use with generators are designated "NG" in the table.

These breakers are available with interchangeable trip units in ratings of 125, 150, 175, 225 and 250 amperes. The nonautomatic NQB-A250 carries a 250 ampere rating and has copper conductors replacing the trip unit. Special generator breakers have trip units rated at 100, 160 and 250 amperes.

AQB-A250 breakers can be ordered with undervoltage release devices, auxiliary switches and shunt trip attachments complete with auxiliary switch. These attachments are also easily installed in the field.

My reply to #2 & #3 #5

CB, contactors, Switches [All dis-connectors ] are special design considerations not only in their energising coils [where they are ] but also in contacts.
Simplest is the Switch, where switches for AC are Toggle type only while AC only switches are Cam-operated type.

CBs /Contactors rated for AC "CANNOT BE USED ON DC AT ALL" due to the "BREAKING ARC"
which should be quenched by special designs in contacts & its Arc-Quenching arrangements.

For further info please read early post(s): [ Relays only]
1.
Relays?
<http://globalspec.ip09.com/rd/9z1zjh924ietefi92ob5iq8k06g0jj4c874a3h64uqo>

2.
<http://cr4.globalspec.com/thread/12827/AC-coil>

3.
<http://globalspec.ip09.com/rd/9z1zjh924ietefi92ob5iq8k06g0jj4c874a3h64uqo>
How does a DC relay work?

4.
<http://cr4.globalspec.com/thread/12767/Can-Relays-be-Energized-Using-AC-DC-Voltages
>
The physical construction of AC & DC relays differ quite a lot & they cannot be replaced interchangeably even by modifications in electrical circuit:
...