AC MCCB in DC Circuit

When I worked for Reliance we used AC breakers in the DC circuit for the motor. The breakers we used had a little known DC interrupting rating, much lower than the AC rating. We used all three poles in series, sometimes 2 in the +ve leg and 1 in the negative leg. It requires testing to verify the effectiveness. We were only interested in minimizing the damage to the DC machine, and it also provided a convenient means to disconnect the motor for Hipot testing. Any major fault currents from the AC side were cleared on the AC side of the bridge with fuses and CB. The DC side was mag only with a low trip point about 250% to 300% of rated current. I would not recommend this technique for motor protection from a large DC bus, but rather use DC rated fuses specially designed for the purpose.

GW, as long as I know when apply an AC Breaker in DC Network, there will be other technique to wire which is depended on the type of DC Circuit either Positive-earthed, negative-earthed, or unearthed. That different type of circuit will distribute different fault current, that is why the different type of wire is used, not always two poles in series. That are my issues.

Or do you know any manufacturer that supply both AC and DC current in one package of MCCB?

Thank for the help.

Your application is somewhat vague. The application, industry, the fault capacity of your DC system, and what you are trying to protect etc all play a part in the selection. ( If it is from a charged DC capacitor bank fault levels are very high, from rotating equipment it is low, what is the impedance of your system?).

Are you using the MCCB for switching or protection? Opening under load or interrupting fault currents will possibly provide only a limitted number of operations. For switching under load a DC rated contactor would be preferred, or use solid state switches.

I would not recommend manually switching an MCCB when you are opening DC currents. Adequate operator protection would be required.

The AC/DC power conversion protecting a motor was for armatures up to 600VDC. De-rating 600VAC MCCB's generally had only 250VDC per pole, so putting them in series increases the arc length and the ability increases to extinguish the DC arc.

MCCB's are designed with electronic or thermal magnetic circuits. You need to specify what features you want - overload, over current, short circuit protection, ground fault protection, etc. These features may require external devices for detection and then use a shunt trip to open the breaker.

In the example of the AC/DC motor protection the systems were for both ungrounded AC supply, or in Canada high resistance grounded or neutral grounded. We were using the breaker for short circuit protection only to open an inverting fault. The short circuit capacity is supplied by the motor and is generally limited to about 10 or 15 times motor rated current, (200 amps x 15 = 3000 amps). The MCCB's used generally had a mag only trip unit that responds well for both AC and DC currents. The current limiting designed MCCB's (not fused MCCB's) with high interrupting ratings generally were winners.

MCCB's of mag only design only see the current flowing through the poles. If the ground fault does not generate enough current it will not open. External ground fault detection circuits opening the MCCB with a shunt trip may be required.

For serious DC short circuit fault currents only DC rated fuses are suitable. Some of them actually have an explosive charge included to extinguish the arc. Standard fuses have very low DC interrupting capacity and will form a plasma arc inside the body of the fuse and not clear. Even HRC fuses with sand filling will do this, so BELIEVE the published fuse ratings (and MCCB published ratings).

DC rated contactors are usually slow in opening, but will interrupt the rated DC current. Fault current opening is another factor and high fault levels may not clear, so the slow opening aggravates the problem since the current builds rapidly.

Do not use an MCCB for 400v AC in A 400v DC circuit, the contact size and air gap are generally smaller in AC breakers serious flash over and destruction could take place. AC breakers could be used with a considerable derating in operating voltage and current, but it will depend on the breaker construction some breakers are very similar in construction for both AC & DC Volts but the marked rating is to be taken seriously.

So far as positive & negative faults go the MCB does not care it only operates on Amps flowing through it wherever they might go

Late, but here goes..

If you really have to, you can, but then you have to derate the current rating by a factor of 5-10. This works for the lower voltage ratings I'm familiar with (for 110V DC, up to 20A DC max, we typically use 250V AC MCCBs, at 0,2 times the full load AC current if we have to, but we rather try and obtain the proper DC CBs ASAP).

I'll not use it on a 400V DC system easily though.....At the higher voltage ratings, especially with high current or motor-type loads, you'll normally see that DC MCCB's are not really miniature anymore (they have small arc chutes) - remember there's no zero point for the current to switch off, so the arc dissipates energy until extinguished (200A@400V=8kW of heat to spread around!). If the arc drawn by 200A DC is too much, it will just keep on arcing until something gives away. DC circuit breakers are available on the market (try Siemens, Moeller, Merlin Gerin?), but not necessarily "off the shelf" if you are away from the main industrial centres.

I'm not sure how the various earth faults & feeding arrangements will impact - get an engineer to calculate it!

Use a fuse instead.

Guest,

I am not talking about AC/DC motor protection. Please, perceive my very first question!

Dear Mr.Abukhasana,

As far as I know , AC Circuit breakers can be used for DC Application. It doesn`t matter because the MCCB is just a protection device to protect the circuit from the line fault whether it is connected to positive or negative terminal . But we have to take care of the connection of the device used in the out put as per DC configuration

Thanks & Best Regards

Vinod

Mob: 0097150 5270413

When you use AC MCCB in DC Circuit , the interrupting capacity of DC application of breaker is too low compared to AC Circuit .For example Ultimate breaking capacity of a breaker with 25KA@415V AC , The breaker will have the maximum breaking capacity of 14KA@250V DC. This is common to most of the manufacturer. Wiring can be done as per DC Condiguration.

+ve earth fault and -ve earth fault is doen`t matter for the tripping. Any one can be detected by the breaker. Normally MCCBs are manufactured with built in short circuit protection+Over load protection only and Earth leakage protection can be obtained by external configuration

hi i read this thread with intrest,i am just desighning my boats dc system,and am going to use mcb protection,so is it possible for me to use ac breakers in a 12v dc system?being that the breaker rating to be reduced to 0.2% of the ac rating,would these still give me over current protection?? thanks lasse

DO NOT USE AC rated breakers for this application.
There are numerous Breakers that already have DC or AC/DC ratings specifically for this application. Being on a boat you may also want to check for a salt spray rating.

Typical AC/DC rating would be 128VAC/48VDC

Breakers for your application are the same cost as AC. Potter Brumfield, and E-T-A, and many others make lines for Din rail mounting and also panel mounting. It looks really professional using the panel mounting style and then take a piece of tinned copper bus across the row of terminals at the back for the +ve bus supply (+ve is North American tradition, may be different in other countries). There are styles that are just CB, or have a bat handle or rocker switch for on/off use.

I used the E-T-A breakers in a light aircraft, both CB push button recessed reset only style and bat handle on/off type. CB style pB was for radios that had their own on/off switches, and bat handle for things like navigation, cockpit, and landing lights. Properly grouped together they made a logical layout.

Be careful with that 0.2 de-rating. A thermal/mag breaker set to trip at 15 amps will trip with an overload at about 115% with either AC or DC. That is your overload protection. The ability to interrupt the DC current usually de-rates the maximum fault capacity AND the rated voltage. NOT THE NAME PLATE THERMAL TRIP RATING OR THE MAG TRIP POINT. What we are limiting is the energy in the arc as the breaker opens.