Peak Let Through Current

[LMAO]

As a PE, I feel really dumb for asking this. I understand the concept of interrupt rating of an OCPD and why it should be higher than total available fault current of the system but I am a little hazy about current limiting devices and their usage. Can anyone explain the reason for using current limiting breaker/fuses and how to read peak let through current curves?
thanks,

 

[mull982]

Current limiting breakers/fuses are use to limit the available fault current at a downstream location by only letting a portion of the available fault current through the device due to the fast acting nature of the device. These devices interrupt the current in less than a 1/2 cycle before the fault current can reach its peak value.

Current limiting fuses are typically used to increase the short circuit rating of equipment or interrupting devices by limiting the fault current to a value below the equipment or interrupting devices short circuit rating. Current limiting fuses can also be effective in limiting Arc Flash downstream of the fuse due to the fast acting nature of the fuse.

Keep in mind that in order for the current limiting device to be considered "current limiting" the fault current must be in the fuses current limiting range which is typically above where the fuse intersects .01s on a TCC. I've seen many cases where a current limiting fuse was mis-applied due to the fact that the available fault current did not fall in the current limiting range in which the fuse does not act to "limit" the fault current and simply interrupts the fault current based on the fuse curve characteristics.

As far at the "up over and down" method I haven't really used it much myself but my understanding is you start at the bottom of the plot based on the available fault current and go up until you hit the respective let-through curve. From that point you continue left to the main line and then continue back down to get the amount of fault current that will be let-through. Ill let others chime in with a better explanation.

I have heard these let-through curves cannot be used with breaker that may be in series with fuse due to the dynamic impedance that is introduced into circuit when breaker starts opening.

 

[templdl]

As a PE, I feel really dumb for asking this. I understand the concept of interrupt rating of an OCPD and why it should be higher than total available fault current of the system but I am a little hazy about current limiting devices and their usage. Can anyone explain the reason for using current limiting breaker/fuses and how to read peak let through current curves?
thanks,

While as a support spplication engineer for m older case circuit breakers I got the question asked a lot.
1st, as all breakers are current limiting to some degree as it is an inherent characteristic. When a trip is initiated a breaker clears in less than a 1/2 cy. Thus the let through current is limited to thst within the first 1/2 cycle. But to be considered to classified as a current limiting breaker the let though current must be limited to thst no more than the first 1/4 cycle. Such let through data is available and is often used when trying to determine how that device would affect downstream devices. Such data can only be used for feasibility studies to determine if the breaker is capable of protecting downstream devices under certain fault current conditions.
Something that is often overlooked is the a combination motor uses N MCP to interrupt but yet the MCP does not have a kaic rating. The MCP is tested in combination with a contactor and OLR and the kaic rating as actually affixed to the enclosure for that assembly.
Once it has been determined that it may be feasible the combination must be tested together to varify that protection can be infact provided, but it must be e varified by testing. This is how series rated devices are determined. Such ratings are not dependent upon calculations and trip curve evaluation.
Also, you will find that CL breakers will be those with the high kaic ratings. It is all in the contact design which determines the speed at which the contacts open. One on the original CL breaker was designed be Westinghouse which used a slot motor concept where the moving contact is propelled within a slot motor for which the magnetic field used for the slot motor is as a result of the fault current.
Another common design is where the station a ry contact on the bottom actually is in the shape of a loop which caused the mag fields of the stationary and moving contacts to opposed one another when they open when speeds up opening.
Yet another design incorporates a moving stationary contact where that contact has the ability to pivot away from the stationary contact as the contacts open qui CV leaning the speed at which CV h the contacts open.
The way arc chutes are designed to speed up extinguish the arcs also improves kaic ratings.
But no matter how one interprets the data a trip curves such data can only be used to do a feasibility study to determine if the device would function as anticipated after bring tested.

 

[LMAO]

While as a support spplication engineer for m older case circuit breakers I got the question asked a lot.
1st, as all breakers are current limiting to some degree as it is an inherent characteristic. When a trip is initiated a breaker clears in less than a 1/2 cy. Thus the let through current is limited to thst within the first 1/2 cycle. But to be considered to classified as a current limiting breaker the let though current must be limited to thst no more than the first 1/4 cycle. Such let through data is available and is often used when trying to determine how that device would affect downstream devices. Such data can only be used for feasibility studies to determine if the breaker is capable of protecting downstream devices under certain fault current conditions.
Something that is often overlooked is the a combination motor uses N MCP to interrupt but yet the MCP does not have a kaic rating. The MCP is tested in combination with a contactor and OLR and the kaic rating as actually affixed to the enclosure for that assembly.
Once it has been determined that it may be feasible the combination must be tested together to varify that protection can be infact provided, but it must be e varified by testing. This is how series rated devices are determined. Such ratings are not dependent upon calculations and trip curve evaluation.
Also, you will find that CL breakers will be those with the high kaic ratings. It is all in the contact design which determines the speed at which the contacts open. One on the original CL breaker was designed be Westinghouse which used a slot motor concept where the moving contact is propelled within a slot motor for which the magnetic field used for the slot motor is as a result of the fault current.
Another common design is where the station a ry contact on the bottom actually is in the shape of a loop which caused the mag fields of the stationary and moving contacts to opposed one another when they open when speeds up opening.
Yet another design incorporates a moving stationary contact where that contact has the ability to pivot away from the stationary contact as the contacts open qui CV leaning the speed at which CV h the contacts open.
The way arc chutes are designed to speed up extinguish the arcs also improves kaic ratings.
But no matter how one interprets the data a trip curves such data can only be used to do a feasibility study to determine if the device would function as anticipated after bring tested.

OK, so going back to the application of current limiting devices (as oppose to their construction), the reason for using a current limiting device is to increase the short circuit rating of other devices? so for example, if I use a 50kA current limiting fuse with peak let through of 35kA, I can have a 35kA rated disconnect switch downstream which is able to handle 50kA fault current, correct?

 

[mull982]

Such data can only be used for feasibility studies to determine if the breaker is capable of protecting downstream devices under certain fault current conditions.

Such ratings are not dependent upon calculations and trip curve evaluation.

This is a topic I have seen debated a lot. I have seen a lot of people throw a current limiting fuse ahead of a panel board or MCC in order to increase the short circuit rating of the downstream panel or MCC. It has always been my understanding that the fuse has to be "tested" and "listed" in combination with the devices in the downstream equipment in order to take advantage of an increased short circuit rating due to the fuse. A lot of people use the up-over-down method but I don't know that can be used accurately in a situation where there is a dynamic impedance.

There is a provision in the NEC to allow a PE using engineering judgment to evaluate combination ratings for devices in series based on let-through curves, etc... but I would be extremely hesitant to do so.

 

[templdl]

OK, so going back to the application of current limiting devices (as oppose to their construction), the reason for using a current limiting device is to increase the short circuit rating of other devices? so for example, if I use a 50kA current limiting fuse with peak let through of 35kA, I can have a 35kA rated disconnect switch downstream which is able to handle 50kA fault current, correct?

Maybe think of it this way. The object of designing a breaker to cleara fault is to assure it can do do without destroying itself. The contacts have to open fast enough to limit the energy basicaly to protect its own butt as varified by testing per UL489. The breaker is not to be considered a fuse for single use only but must be able to come back ft or some more abuse.
The contacts must draw the arc up and through the arc chute and deionize and clear the fault as it is the last line of defense in interrupting the fault.
The benefit of this product is that iit has an inherent current limiting ablitity of handling the energy of which can be used in the series rating of down stream devices. It is not a given that any down stream devices will be protected against any fault current that gets let rhrough. Curves and calculation are often assumed to be benificial to determine if the current limiting ability of these breakers could be sufficient to limit the let through current to less that which could damage down stream devices. Such calculations and studies may only be an indication if it is posibile but series combination testing still must be done to varify.