Incident Energy with CL Fuse Equations
Incident Energy with CL Fuse Equations
(OP)
Does anyone else have a problem with the IEEE 1584 incident energy equations for current limiting fuses?
Incident energy is based on the arcing fault current, not the bolted fault current. Arcing fault current is determined from the bolted fault calculation but takes several other variables into consideration, like system voltage, gap, etc. The CL fuse incident energy equations only use bolted fault values.
Why are the CL fuse equations based on bolted fault rather than arcing fault values?
For now, I am not using the equations in my analysis. Are my concerns unfounded?
Thanks.
Incident energy is based on the arcing fault current, not the bolted fault current. Arcing fault current is determined from the bolted fault calculation but takes several other variables into consideration, like system voltage, gap, etc. The CL fuse incident energy equations only use bolted fault values.
Why are the CL fuse equations based on bolted fault rather than arcing fault values?
For now, I am not using the equations in my analysis. Are my concerns unfounded?
Thanks.






RE: Incident Energy with CL Fuse Equations
You must have mixed understanding on these things. Equipment fault withstand is different from arc flash protection. We use the bolted-fault current to determine whether our equipment can withstand a full, bolted-fault short-circuit. If our equipment is underrated, we will be forced to find a higher-rated equipment. If the rating we are looking for is not available, or acquiring those could render the project not feasible, we are going to use CL fuses so that our equipment rating can be up to code.
The other item you have trouble with is compliance with arc flash protection. You can either set your breaker trip setting to the fastest possible to minimize the incident energy level, hence a lower category arc flash protection requirement or reduce the available fault current by using limiters (reactors, CL fuses, etc.). The determination of the arcing fault current (theoretically) requires the use of the bolted-fault current. that is why you need to know the bolted-fault current available at the location to be able to know the arcing fault current.
Others who are very good at CL fuses could chime in, patience!
RE: Incident Energy with CL Fuse Equations
In IEEE 1584 Section 5.2, an equation is given to determine the arcing fault level from the determined bolted fault at that location. Several variables are used, such as system voltage, gap between conductors, enclosed or not, etc. Section 5.3 gives an equation to determine incident energy based on many of the same variables. Fine.
Now, if applicable, IEEE recommends using the incident energy equations for CL fuses in Section 5.6. These equations are based only on the bolted fault values; no mention of system voltage, gap or enclosure type. How have they verified that the arcing fault value is in the current limiting region of the fuse curve?
I've made available a chart based on a 50kA bolted fault illustrating the change in arcing fault current with system voltage and conductor gap. You can see a rather drastic difference at the 13mm gap between 600V and 208V. In order to consider the CL fuse to have a current limiting effect, the arcing fault must be in the current limiting region of that fuse's curve. (See attachment.) I've also uploaded a TCC of the KRP-C CL fuse showing the highest and lowest arcing faults for the 13mm gap (dotted lines), along with the current limiting flags for the fuse. (Diagram linked in next post.) Obviously, the arcing current for a 600V system is within the current limiting region but the 208V is not.
Why, then, is it recommended to use the CL fuse equation based on bolted fault only, without any consideration for the arcing fault variables?
I haven't investigated this before because, until recently, SKM's application of CL fuse equations has been rather buggy. Recent releases have mostly fixed the problem and I've considered using the CL fuse option.
RE: Incident Energy with CL Fuse Equations
RE: Incident Energy with CL Fuse Equations
RE: Incident Energy with CL Fuse Equations
EasyPower also has an option for using the IEEE equations for current limiting fuses, but we generally do not use them.
David Castor
www.cvoes.com
RE: Incident Energy with CL Fuse Equations
Think about the difference between a (theoretical) bolted fault and an arcing fault. With a bolted fault, the energy is dissipated by the entire circuit resistance. For the 'ideal' zero resistance bolted fault, no energy is dissipated at the fault location itself. Now, start adding some real resistance to that fault (an arc, for example). While the power dissipated at that location goes up, the resistance has the effect of reducing the overall fault current. This reduction may move that point below that where a CL fuse has any effect.
RE: Incident Energy with CL Fuse Equations
dpc, what is your reason for not using the equations?
PHovnanian precisely states my concern with using the equations.
RE: Incident Energy with CL Fuse Equations
David Castor
www.cvoes.com
RE: Incident Energy with CL Fuse Equations
You can count on continuous changes over the coming years based on more testing (hopefully).
RE: Incident Energy with CL Fuse Equations
Always try to determine the lwoest available from the utility or other source.
RE: Incident Energy with CL Fuse Equations
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Incident Energy with CL Fuse Equations
I usually ask for the fault current on the primary side of the service transformer. It's easier for the utility engineer to determine. If you ask for the secondary side fault they'll assume you want to figure AIC for equipment and tell you the infinite source for the lowest impedance of the service transformer class.
RE: Incident Energy with CL Fuse Equations
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Incident Energy with CL Fuse Equations
Handing out a distribution transformer secondary figure isn't that big a deal if they assume a zero impedance at its primary. They can always add language stating that this data is only valid for this unit.
But for primary services or networked secondary, most utilities don't want the responsibility of informing a bunch of customers should they change something a ways upstream that alters their available fault current.
And should they implement such a procedure, the issue of whom that customer might be changes over time. When its new construction, there's an engineering firm attached to the project. But years down the road, the customer is just the person who pays the monthly bill. If they get a letter informing them that their fault current is going from 100 kA to 150 kA, they've got no idea what the significance of this is.
RE: Incident Energy with CL Fuse Equations
Not being a lawyer, I don't believe the utility is liable or required to notify a customer that source impedance to their facility has changed unless it is stated in the service agreement.
RE: Incident Energy with CL Fuse Equations
RE: Incident Energy with CL Fuse Equations
From the utility side we really don't care much about minimum fault levels at a given service location, just maximum so it will be difficult to ever find out what the lowest bolted fault current you might expect. But that is a very important number for arc flash analysis (often more important than the highest possible bolted fault value). What we can generally do a good job of is telling you what the highest possible fault current we can provide will be regardless of any change we might make on our system, and that is the important number for equipment sizing. As to getting more realistic numbers for other studies, your mileage may very.
RE: Incident Energy with CL Fuse Equations
An interesting solution might be the combination of CL fuses and an arc quenching device. The quenching device would practically guarantee high current and fast operation of CL fuses.
RE: Incident Energy with CL Fuse Equations
I think I've gotten a fairly good sampling of the community's feelings on the subject of CL fuse equations and arc-flash calculations in general. We do what we can with what we've got.
Thanks, everyone!
RE: Incident Energy with CL Fuse Equations
I'm not sure what you are describing as "an arc quenching device". My understanding of the term is a device that, using an air blast, gas expulsion, magnetics, etc. "blows" an arc out by dissipating its plasma. But arc quenching doesn't adapt well to arcs at random fault locations. It is used in conjunction with devices that interrupt current.
It sounds like you are referring to a crowbar. With this, once a fault condition is detected, a low impedance path is provided downstream of the circuit protection (an intentional fault) to draw sufficient current to ensure its operation. The problem here is that typical CL fuses operate by interrupting fault current within a quarter of a cycle. Crowbars need to sense the initial fault condition, initiate the crowbar action and only then will the fuses operate. Its not likely that this can be done within a quarter cycle.
For a high energy, longer clearing time arcing fault, a crowbar could divert the available energy from the site of the exposure to the contained crowbar contactor. But this is all low speed stuff compared to CL fuse operation. You may as well just trip a standard breaker.
RE: Incident Energy with CL Fuse Equations
Arc eliminators can detect (optical detection and very fast overcurrent detection) AND eliminate the arc within a few milliseconds. Manufacturers say that arc burning times of 2ms (LV) and 5ms (MV) can be reached. Thus a combination of a crowbar and a CL fuse would provide 1) very short arc burning time with minimal damage and 2) very short short-circuit current time, in case you are worried by the high current.
Light and overcurrent based arc detection is very fast. Optical sensors detect the arc light, and the overcurrent detection can be based on analog technology (I don't know the details) which does not require any algorithms for processing samples.