VDC Distribution

[Aftermathematics]

I am current working on the design of 125 VDC distribution boards in a power plant. As i'm new to the DC system, I thought someone might be of help.

I want to know why you have to connect poles of breakers in series to achieve the desired current in DC application. Is the desired current not going to come from the supply cable anyways? How are the poles going to make the desired current achievable?

I have been wondering on how to achieve a good design from the schematic i created. Can you please have a look and advise how I will get around the large cables especially when I have more than one cable sized for a circuit. I'm stuck as to whether i'd use MCBs or MCCBs. The client insists on MCBs but these are not available off-the-shelf. What would be the idea breaker a DC application?

Please have a look at the attachment.

Thanks for checking

 

[rbulsara]

Multiple poles in series helps achieve desired current breaking capacity, especially during a fault. DC current (arc) are more difficult to break than AC, because there is no zero crossing. This is not done to get more current capacity to the load during normal operation.

Also many breakers are listed to DC use only when connected in such form. You do not "have" to have multiple poles in series, if listed so.

As for which type of breaker to use, you need to find one that is listed and approved for your application, both for normal current and fault duty as well as ambient conditions.

You will do well to work with you seniors at work, if you are new to the task.


Rafiq Bulsara

http://www.srengineersct.com