SCCR OCPD not adjusted. Why?

[mwilson87a]

All,

Glad to be a new member here. I have one taxing question. We are scratching our heads on WHY we can't use the feeder peek let through current of current limiting fuses to adjust the rating of BRANCH OCPDs the way you can the components.

We all have our "theories" here but we're looking for the U.L. reason for this? It does seem if your fuse limits the max current from the feeder to 6kA, then a 10kA branch breaker would be safe up for the rating of the feeder fuse.

So, why? And thanks in advance. These forums are a wealth of information on all the things you don't learn in college.

Marc

 

[petersonra]

You can't use the let thru numbers because they are unreliable when in series with other ocpd unless the combination has been tested together. There are a fair number of fuse/cb combinations that have been tested.

 

[jim dungar]

Glad to be a new member here. I have one taxing question. We are scratching our heads on WHY we can't use the feeder peek let through current of current limiting fuses to adjust the rating of BRANCH OCPDs the way you can the components.

Components are static devices, they just sit there and wait for the protective device to operate. This is in contrast to breakers which try to interrupt the fault current.

Do some research on the term 'dynamic impedance' and series connections. As the breaker contacts begin to separate (as fast as 1/4 cycle) the impedance of the arc cause the current through the upstream device to decrease. In many cases it is possible for the downstream breaker to clear before the upstream device.

 

[wbdvt]

Do some research on the term 'dynamic impedance' and series connections. As the breaker contacts begin to separate (as fast as 1/4 cycle) the impedance of the arc cause the current through the upstream device to decrease. In many cases it is possible for the downstream breaker to clear before the upstream device.

Exactly why it has to be a tested combination to use a series rating.