Incident Energy Increase/Decrease as you go down the line

[TM519]

I have a general question about Incident Energy levels and how they can possibly change as you work your way through the circuit. My specific situation is I have a known arc flash rating at some 480V distribution panels. The arc flash rating at those cabinets is only about 0.5 cal/cm2. These cabinets then feed various cabinets of HFPS's. The question I have is can it be safely assumed that the Incident Energy rating of the HFPS cabinet is less than or equal to 0.5 cal/cm2? Or is there a possible scenario where it could be higher than that? Appreciate all the insight here.

 

[jim dungar]

As you go downstream, the extra circuit impedance will result in a reduction in the Arc Fault Current. This reduced current level may not be sufficient to cause the protective device to operate in its instantaneous mode. If the protective device takes longer to open and clear the fault, it is not unusual, if not even likely, that the incident energy will be higher for downstream locations.

 

[JoeStillman]

SKM will tell you if the 0.5 cal/cm is a result of an assumption of maximum clearing time (usually 2 sec). If it is, then the clearing time downstream can't be any longer and your assumption is valid.

 

[wbdvt]

SKM will tell you if the 0.5 cal/cm is a result of an assumption of maximum clearing time (usually 2 sec). If it is, then the clearing time downstream can't be any longer and your assumption is valid.

I don't see how you can make that statement based on the OP's post. Nothing is mentioned about the arc clearing time with the 0.5 cal/cm^2 incident energy level nor is the software used to arrive at that value. Usually with the use of a 2 sec cutoff time the incident energy values are higher than 0.5 cal/cm^2.

As noted by Jim, it is definitely possible that the incident energy level can go up and the only way to be certain on what arc rated PPE is needed for your work location is to continue the analysis to that piece of equipment. Since you mention a definitive incident energy value, I am assuming that a study was done per IEEE 1584.

 

[NewtonLaw]

I have performed many Arc-Flash Risk Assessments over the years and I agree with Jim's comments. The Arc-Flash energy is an I^2t condition where the total energy delivered by the arc is based on both current magnitude and total burn time. Cyme, SKM, Easy-Power will all give the same results for a given system. Most times, the arc-flash energies do go down but it is never a sure thing. Also, most software programs automatically perform two calculations, one at full Arc-Flash current and a second one at about 80% of that value to see if the lower current allows enough additional clearing time that the Arc-Flash energy increases due to the lower current. Eventually the software will perform an iteration calculation to determine the maximum arc-flash energy based on differing current levels and the associated clearing times of the circuit breakers used.

This is an attempt to consider restrictive faults as opposed to a bolted fault with an assumed solid connection to ground or neutral.

 

[paulengr]

I have a general question about Incident Energy levels and how they can possibly change as you work your way through the circuit. My specific situation is I have a known arc flash rating at some 480V distribution panels. The arc flash rating at those cabinets is only about 0.5 cal/cm2. These cabinets then feed various cabinets of HFPS's. The question I have is can it be safely assumed that the Incident Energy rating of the HFPS cabinet is less than or equal to 0.5 cal/cm2? Or is there a possible scenario where it could be higher than that? Appreciate all the insight here.

It can go either way. It depends on what is going on. First although you stated 480 V I need to clarify that transformers matter. Normally incident energy goes up downstream of a step down transformer. That is except for ones with very high impedance (control power transformers, instrument transformers). 1584 gives a cutoff short circuit current for this.
This is until it gets to such a low voltage that incident energy calculations don’t apply, roughly under 208 V.

Then we need to look at whether the “trip device” is “definite time”. This is the case when the short circuit current is in the “instantaneous” trip region of a device or it is at 2 seconds or the device trips after X seconds regardless of current. In this case as we would expect as distance increases cable impedance increases so short circuit current decreases, thus arc power decreases but since opening time is fixed, incident energy decreases.

If however it is in a trip curve region regardless if it is a fuse, a breaker, or something else, as the short circuit current (hence arc power) decreases, it also extends the time needed to trip. On nearly all devices it is going to increase the opening time faster than the decrease from short circuit current so we get a net increase in incident energy. This is counterintuitive but is what happens. It does not happen with very slow devices but these are rare.