Fuse replacement with MCB

[zaza123]

I am doing a project to replace control fuse of circuits with MCB/MCCB's.One of the aim is not to disturb the original design.Due to that i want to replace fuse with breaker of same breaking capacity.my question is that is there critical role for breaking capacity of control circuit fuse.How is it related with the short cicuit breaking capacity of the bus.
The bus in question has two incomers ,one buscoupler & many outgoing modules.

 

[rbulsara]

Fuses typically have much greater interrupting capacity than breakers. What is important is that the device rating is greater than the available fault current. If this is on primary side of the control transformer (or no control transformer), the device needs to be rated above the fault current at the bus.


Rafiq Bulsara

http://www.srengineersct.com

 

[zaza123]

Mr. Bulsara as i understand you are saying is that if the fuses(or MCB) are on the primary side of the control transformer or if there is no control transformer then the fuse( Or MCB) breaking capacity shall higher than the breaking capacity of the BUS.
The breaking capacity of the bus is 80 KA whereas fuse breaking capacity is 120KA.the fuse link is 80 KA.

 

[rbulsara]

Yes.
Btw, bus do not have breaking capacity, just withstand capacity.
What is the difference between the fuse and the fuse link?


Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

No offense, but since you have to ask this, I would say you seek advice of your senior engineer who knows this stuff.

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

The highest rated MCB I am aware of is 25kAIC, most are significantly less. If you are in North America you will be limited to 10kA from a couple of sources, most are only 5kAIC.

You would have to go with an MCCB, and actually a Current Limiting High Interrupting Capacity MCCB to replace your fuses. Basic MCCBs are 14-35kAIC, HIC versions are typically 65kAIC and you need to get to the highest level Current Limiting HIC MCCBs to get to anything that high. For a simple control circuit, be prepared for a 20X cost factor and 10X size factor.

Stick with the fuses.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[zaza123]

Its my mistake in writing the breaking capacity for the Bus, it is actually the short cicuit withstand capacity.The fuse link is knife type link for neutral circuits.

 

[jraef]

You need then to determine the available fault current at the bus. This has little to do with the bus bracing (what you refer to as short circuit withstand). There are various and sundry methods of calculating the AFC, I suggest you learn them before taking this on. Or better yet, hire an EE who understands all the issues involved. There are issues of coordination with other protective devices to be considered. You don't want a seemingly small fault at this level to shut down something larger up stream.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376