AC Busbar DC Rated 1

[Humble2000]

Dear Friends,
We have bought couple bus bars 100A,AC rated to use with mini circuit breakers for a DC panel.
It shows that they are 277/480 V ac rated, It doesn’t mention anything about DC rating on the bus bars.
Is there a document or formula or anything regarding using these bus bars for DC applications?
What are the concerns?
These are ABB circuit breakers and bus bars are also from ABB ,the type that plugs into terminals for easy installations.

Thanks

 

[cuky2000]

Busbars rated for AC systems are OK to use for DC. In fat, in DC system the busbars will have slightly better performance since the charge density is almost uniform at difference than the AC system subject to skin effect.

Molded Case Circuit breaker for dc operation is usually satisfactorily compared with ac dual rated MCCB. However, the manufacturer should confirm if the breaker is suitable for dc application.

In general, the performance of a thermomagnetic MCCB it is expected to perform as follow:
- Overload range: the bimetallic mechanism respond proportional to rms current (i²). Therefore, dc does not have noticeable difference with ac.
- Short circuit range: Since the instantaneous electromagnetic mechanism will respond to the current peak (i_peak), one possible difference will be in the pickup band on that will shift slightly to the right in the time-current curve.

Other considerations to be aware in DC applications are the maximum float voltages in battery system, overload endurance and pole connections particularly for voltage levels above 250 Vdc.

 

[wschel]

The circuit breakers MUST be DC rated. DC current is much harder to interrupt than AC since there is no zero crossing.

 

[ScottyUK]

Electronic tripping units employed in many modern MCCBs are almost exclusively intended for AC only service. They may not work at all on DC, as they draw their power from CTs which rely on the current drawn by the load.

MCCBs often require multiple series poles for DC breaking, and have severely restricted capacity compared to their AC ratings. As wschel has already said, check with the manufacturer for the exact duty you are using. If you have a heavily inductive load then you will need a special - and expensive - breaker designed specifically to cope with that duty. If you try to use a general service breaker then you - or your customer - may well get an unpleasant surprise.

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