Current Limitation to expand SCCR of panel - OCPD opening sequence?

[MWh_Pro]

Howdy,

I am wondering how exactly it is that a current limiting fuse of greater value than a branch circuit breaker can be used to protect that device, and be used to to increase the short circuit rating of the overall panel. For instance, panelboards often use fuses to take a panelboard full of 14 kA breakers to a much higher rating. In my case, the fuse is larger than the breaker: say 400A J-type CL fuse with 200kA interrupt rating and some generic 40A MCCB, rated for 35 kA.

Here's my issue: since the fuse is much larger than the CB, and presume they are "selectively coordinated," then will we not have an undesirable sequence of operations? I.e. the CB will open before the fuse (desireable for coordination), but will consequently be forced to break a fault above its interrupt rating, say if the bolted fault current is 50kA.

Then again, perhaps I am thinking about it all wrong... let's assume that for the faulted circuit, the fuse and CB are effectively in series. Then that means that the current limiting fuse will act to clamp down on the current passing through this circuit, and the peak let-thru current is what the branch CB will "see." In that case, everything seems fine. But I've plotted the TCC curves on Etap, and indeed the instantaneous trip curve of the CB is about 10X 40A, or 400A, while the fuse will not even start current limiting before 8kA. Using the Etap Star Sequence of Operations function to simulate, indeed the branch CB opens before the main fuse under fault conditions.

To get to the core concept here: is the whole premise that the current limiting fuse keeps the let-thru current safely below the interrupt rating of the branch CB, but that the branch CB will still open before the fuse, even under high fault conditions?

Thanks for the help!

Dave

 

[ron]

The current limiting fuse keeps the let-thru current safely below the interrupt rating of the branch CB, but that the branch CB will still open before the fuse, when the fault current is not too high. When it is high, both will "trip".

These are hard to see on regular TCC's because it happens faster than what the TCC time shows, which is why it needs to be a tested combination for it to be series listed.

 

[jim dungar]

If there is not enough current flowing to cause the fuse to operate, then it cannot limit the current. Period. End of story.

Series combination ratings between fuses and circuit breakers cannot be determined by looking at any published literature. These ratings can only be found by actual testing of the devices which takes into account the dynamic impedance of the circuit breaker contacts opening and drawing an arc prior to the fuse link melting fully open. There is no requirement for selective coordination as part of a series rating. It is not uncommon for the 'lower AIC' downstream breaker to clear the fault while the upstream device remains closed.

 

[MWh_Pro]

Both great replies. Yes the question is really about series rating a main with its branch device. I'll see if the manufacturer has some literature on listed series pairs I can use. Then it's just up to testing the withstand of the completed panel. Thanks!

 

[templdl]

Howdy,

I am wondering how exactly it is that a current limiting fuse of greater value than a branch circuit breaker can be used to protect that device, and be used to to increase the short circuit rating of the overall panel. For instance, panelboards often use fuses to take a panelboard full of 14 kA breakers to a much higher rating. In my case, the fuse is larger than the breaker: say 400A J-type CL fuse with 200kA interrupt rating and some generic 40A MCCB, rated for 35 kA.

Here's my issue: since the fuse is much larger than the CB, and presume they are "selectively coordinated," then will we not have an undesirable sequence of operations? I.e. the CB will open before the fuse (desireable for coordination), but will consequently be forced to break a fault above its interrupt rating, say if the bolted fault current is 50kA.

Then again, perhaps I am thinking about it all wrong... let's assume that for the faulted circuit, the fuse and CB are effectively in series. Then that means that the current limiting fuse will act to clamp down on the current passing through this circuit, and the peak let-thru current is what the branch CB will "see." In that case, everything seems fine. But I've plotted the TCC curves on Etap, and indeed the instantaneous trip curve of the CB is about 10X 40A, or 400A, while the fuse will not even start current limiting before 8kA. Using the Etap Star Sequence of Operations function to simulate, indeed the branch CB opens before the main fuse under fault conditions.

To get to the core concept here: is the whole premise that the current limiting fuse keeps the let-thru current safely below the interrupt rating of the branch CB, but that the branch CB will still open before the fuse, even under high fault conditions?

Thanks for the help!

Dave

If I asm to interpret the question cvorrectly is this a question of being series rated where an upstream OCPD protects down stream devices? If so then there would be series testings conducted resulting is a series combination rating for the devices. One simply can not look at a set of TCCs and the conclude that a specific up stream OCPD will in fact limit current and protect down stream devices. There asctuaslly must be series combination testing that must be conducted to varify that the desired protection is actually being provided. The UL listed as such.
Is this the

 

[MWh_Pro]

templdl, the question was about the theory of operation with a current limiting device in the circuit: physically how it is possible that a CL fuse or CB could protect a smaller breaker with a lower instantaneous trip setting, and lower interrupt rating. The simple answer was that using TCCs to examine this question was useless, because the data provided by manufacturers does not have that kind of resolution. You are right that if someone were to use a series combination, then it should be a tested and listed set of OCPDs. Given that many manufacturers already have an array of series-listed pairs to choose from, the solution is to grab one off-the-shelf and be done with it. Although the OCPDs have a documented interrupt rating, our cabinet/panel will still need to be tested for its short circuit current rating. That has to do with demonstrating that the assembly can withstand magnetic forces on the busbars, etc.