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 Post subject: incident energy calculation
PostPosted: Wed Jan 26, 2011 12:02 pm 
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Say there is a fault down the circuit somewhere, before the breaker contacts start parting, the fault current flows through the contacts, conductors, to the fault, the total incident energy is distributed through the complete circuit (source, breaker contacts, conductor, impedience at the fault). The indicent energy on the breaker contacts is relatively small because the contacts are still closed (low impedience). When the breaker contacts start parting, the impedience on the contacts start increase which starts drawing more incident energy to the contacts.

IEEE1584 calculates incident energy based on bolted fault current, duration(time prior to contacts parting and time for contacts completely open), distance, etc.

Did I miss something? Please help.


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PostPosted: Wed Jan 26, 2011 3:21 pm 
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IEEE 1548 calculates incident energy based on the arcing fault current at the arc. The energy in the conductors and breaker contacts is not included. Calculations are based on heat measured at the working distance from the arc during tests.


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PostPosted: Wed Jan 26, 2011 6:20 pm 
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We have been using this method to calculate arc flash incident energy to label electrical equipment such as switchgear. The arc on the switchgear (breaker) only exists when the breaker contacts start parting. Within the duration from fault occurs to the time relay tells the breaker to trip, there is no arc on the breaker. However, ieee1584 counts that duration in calculating incident energy to be labelled on the switchgear.


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PostPosted: Wed Jan 26, 2011 6:44 pm 
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furay wrote:
We have been using this method to calculate arc flash incident energy to label electrical equipment such as switchgear. The arc on the switchgear (breaker) only exists when the breaker contacts start parting. Within the duration from fault occurs to the time relay tells the breaker to trip, there is no arc on the breaker. However, ieee1584 counts that duration in calculating incident energy to be labelled on the switchgear.


Incident energy is not distributed through the circuit.
Incident energy is calculated by using the amount of short circuit current at a fault and the length of time that fault exists.

If you are racking a breaker into or out of a cubicle it is possible to create a fault on the line side contacts of the breaker. This fault will exist and have incident energy until the protective device, feeding the switchgear, operates.
There are many additional ways that an arc can occur on the line side of breakers in switchgear.


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PostPosted: Wed Jan 26, 2011 7:34 pm 
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I agree that there are various situations that fault could occur at the breaker terminals. I think the same concept applies to the terminals at the end device too, for instance the cable terminals on a motor fed by a circuit breaker. When an arcing fault occurs at motor terminals, the incident energy at motor could be higher than incident energy at the breaker depending on how much fault current is and how fast the protective relay operates. The practices I have seen are that arc flash labels are applied on switchgear or MCC, but seldomly applied on end devices. The point I want to make is that arc flash calculation and labels are applied on the protective equipment (breakers). But the actual fault could be anywhere along the circuit and the incident energy at those fault locations could be higher than at the protective equipment.


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PostPosted: Wed Jan 26, 2011 7:47 pm 
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JBD wrote:
Incident energy is not distributed through the circuit.


The incident thermal energy is a result of fault current over an impedience. For instance, a circuit breaker feeds an end device through certain length of cable, if it is bolted fault, the incident energy is distributed along the circuit because there is no high impedience component in the circuit until the breaker starts operating. If the fault is resistance fault or arcing fault, because the resistance or arc impedience, the majority of incident energy will be at the fault location until the breaker starts operating, the parting contacts are actually creating a high impedience component in the electrical circuit at high speed which eventually completely isolate the circuit, during the breaker operation, the incident energy is shifting from the fault location to the breaker contacts.


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PostPosted: Thu Jan 27, 2011 6:44 am 
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There is energy dissipated throughout the circuit and in breaker contacts during a fault, but this is not an arc hazard and is not the subject of an arc hazard analysis. Incident energy calculated in an arc hazard analysis is the energy from an arcing fault at the working distance from the location of the fault. There is no arcing incident energy during a bolted fault. Bolted fault current is what is calculated in a short circuit analysis. Arc fault current is calculated from the bolted fault current.


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PostPosted: Thu Jan 27, 2011 8:24 am 
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furay wrote:
The point I want to make is that arc flash calculation and labels are applied on the protective equipment (breakers). But the actual fault could be anywhere along the circuit and the incident energy at those fault locations could be higher than at the protective equipment.


Are you putting the labels on the protective devices only, instead of where people are doing the interacting?
NFPA70E requires the labels to be on the equipment 'that is being interacted with', so that a worker knows what PPE is required.

We have been known to put labels on pull boxes and wireways, because this is where our customers take current measurements.


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PostPosted: Thu Jan 27, 2011 8:32 am 
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Just for argument, there is arcing on bolted fault when the breaker starts to open.
I totally agree that the arcing fault energy is at the location wherever arcing occurs. One thing I don't understand is that arc flash labels have mainly been put on the protective equipment which is only a part of the circuit, PPEs are also mainly required when people work on the protective equipment. But theoretically, arcing fault could occur anywhere in the circuit.


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PostPosted: Thu Jan 27, 2011 8:40 am 
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furay wrote:
The incident thermal energy is a result of fault current over an impedience. For instance, a circuit breaker feeds an end device through certain length of cable, if it is bolted fault, the incident energy is distributed along the circuit because there is no high impedience component in the circuit until the breaker starts operating. If the fault is resistance fault or arcing fault, because the resistance or arc impedience, the majority of incident energy will be at the fault location until the breaker starts operating, the parting contacts are actually creating a high impedience component in the electrical circuit at high speed which eventually completely isolate the circuit, during the breaker operation, the incident energy is shifting from the fault location to the breaker contacts.


What you are referring to is something we called "dynamic impedance" back in my circuit breakers days. You are correct that when a breaker opens, it introduces an arcing impedance during the interruption. This breaker arc impedance can actually reduce the fault current during the interruption but it is an extremely brief time period.

The dynamic impedance is one of the reasons that series ratings were developed using 2 breakers in series that were not technically current limiting (in the true sense of the definition). A fault on the load side of the downstream breaker is seen by both breakers in series, they both open together and the impedance of both lowers the current so the "protected" (under rated) downstream breaker has a safe interruption.

As far as how this fits into arc flash, it is not considered. The equations are based strictly on measuring the energy released at the equipment’s arc that reaches the worker some distance away (Incident Energy). I don’t believe the energy is that great in the protective device’s arc during interruption or it would blow up. That is what the interrupting ratings are all about.

The IEEE 1584 standard suggests taking 85% of the arcing current and review the clearing time on the time current curve as a sensitivity analysis. This accounts for some of the “who knows” (technical term :) ) issues with the actual current.

Is it possible to have one incident energy value for a fault near a breaker and then a much greater value for a fault further downstream from the same breaker. Absolutely - it depends on the short circuit current and where it falls on the time current curve. That is why all areas of the system that could be services live or where an arc flash hazard could exist should be studied.

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PostPosted: Thu Jan 27, 2011 1:01 pm 
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In the instance of a circuit breaker feeds a load via a general purpose junction box, the control compartment of the breaker is completely separated from the the breaker compartment with barriers, is it safe to say that working in control compartment has similar (but not the same) risk level as standing beside the junction box? Comparing breaker terminals vs cable terminals, breaker barriers vs cable terminal enclosure, and considering incident energy around the breaker when it interrupts a distant fault is small, the two situations seem similar to me except one senario that when the fault occurs at breaker line side and upstream breaker is coordinated with this breaker.


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PostPosted: Thu Jan 27, 2011 1:56 pm 
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As JBD said, you put the labels where a worker may interact with the equipment and cause an arc at that equipment. Labels are placed at the protective device not because of the energy in the device contacts while it is interrupting fault current, but because operating the device may cause an arc at the device location. For instance, closing a breaker to energize a faulty terminator or bus.

If no worker will ever interact with the junction box, or open it while the wiring is energized, then a label is not required at the junction box.

I an arc could occur in the control compartment where someone will be working, a label is required for the control compartment. You might want to have one label cover multiple parts of a piece of equipment, showing the worst case. This would avoid having a confusing number of labels.


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PostPosted: Thu Jan 27, 2011 3:28 pm 
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Thank you all very much for your valuable inputs.


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PostPosted: Sun Apr 13, 2014 6:31 am 
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I am not sure this is the place to post this question. Does anyone knowhow to calculate the incident energy when the bolted fault current is greater than 106KA?

Thank you so much for your help.


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PostPosted: Tue Apr 15, 2014 8:47 am 
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IEEE 1584 is an empirical (curve fit) formula based on about 300 test data points.

Below 600 V, Lee is known to give reasonably close estimates and is based on purely theoretical assumptions so I see no reason not to use it for very high kA ratings.

Above 600 V, Lee starts to fall apart, especially due to the invalid assumption that volts and amperes have identical effects (they don't).

So if you are looking at higher voltages as well, might want to consider ArcPro which models single phase vertical arc columns. It is also largely theoretical but is widely accepted for utility applications.

Either way, since as far as I know there is no test data available at that level of fault current, and since the sicence of arcing is not well settled yet (a lot of it is still very much empirical), there is no real way to check/verify/validate any kind of result that you come up with in excess of 100 kA.


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