Is arc incident energy much higher at higher voltages?

[designer82]

I modeled a 480V low voltage system in Etap and all I get are tiny incident energy levels, all requiring basic Level A PPE.

I've seen some tutorial webinars where they get huge Incident Energy levels (40 cal+) although all those webinars were at 11kv or higher voltages.

Could that be the reason or am I doing something wrong?


Thanks

 

[don_resqcapt19]

The incident energy is greatly influenced by the clearing time of the upstream OCPD. Most often you see the very high incident energies at the first OCPD on the secondary side of a transformer as the upstream OCPD is on the primary and the clearing time is long.

In an industrial plant with a transformer feeding 480 volt switchgear, the gear will often have very high incident energies, but the MCCs fed by feeder breakers in the switchgear, will often have incident energies below 8 cal.

 

[petersonra]

I suspect the webinars you watched pick specific conditions that came out very high just to Make a point.

 

[wbdvt]

Typically 480V is the worst for incident energy levels. A couple of questions:
1. Where did you get the source values from?
2. If source values from utility, did they provide actual available short circuit value or infinite bus value?
3. What is Level A? Is that site specific PPE level?
4. How many arc flash studies have you done in ETAP? Are there some settings in ETAP that need to be checked that may be default values that need to changed?

 

[Bwas]

It all depends on the system. I modeled a fairly large system last month that included 15kV distribution, 5kV gear and 480V gear. After a couple step down transformers and a bunch of wiring, the incident energy on the 480V system was pretty low - like 1 cal/cm2.

If the 480V distribution was fed directly by a utility transformer, the transformer secondary and line side of the main would be over 40cal and the remainder of the distribution would have been 8+ at least.

I use SKM, but you can always add a bus near the service point to make sure the power source in your model is providing the power you think it is. Add a bus before any overcurrent devices and run the arc flash analysis for that bus. Do the results make sense?

 

[PE (always learning)]

The incident energy is greatly influenced by the clearing time of the upstream OCPD. Most often you see the very high incident energies at the first OCPD on the secondary side of a transformer as the upstream OCPD is on the primary and the clearing time is long.

In an industrial plant with a transformer feeding 480 volt switchgear, the gear will often have very high incident energies, but the MCCs fed by feeder breakers in the switchgear, will often have incident energies below 8 cal.

Absolutely what this guy said, the incident energy is calculated based on the available bolted fault current, system voltage, and the clearing times of the associated over-current protective devices. You should be looking at the upstream overcurrent protection and observing what the time current curves and settings look like.

 

[mayanees]

I modeled a 480V low voltage system in Etap and all I get are tiny incident energy levels, all requiring basic Level A PPE.

I've seen some tutorial webinars where they get huge Incident Energy levels (40 cal+) although all those webinars were at 11kv or higher voltages.

Could that be the reason or am I doing something wrong?


Thanks

No, incident energy (IE) levels at medium voltage (MV) are actually lower than IE levels at low voltage (LV) for comparable-sized power sources. Shock hazard is certainly worse for the MV systems though.

If you take the ocpd out of the equation the IE level is proportional to the strength of the source. IEEE 1584-2018 has a low-end cutoff of 2000 amps of bolted fault current, which roughly translates to a 20-30 kVA transformer. So if you're at a panel and assessing the IE level to know what PPE to wear, you can consider 30 kVA as the low-end of the hazard, and it goes up from there. The high-end could be the secondary of a 480V, 2500 kVA transformer, which will be around 130 calories. And to support my original statement of MV vs LV IE levels, if the secondary of the 2500 kVA transformer was MV (13.2kV) the IE level would only be around 4 calories.

 

[designer82]

Typically 480V is the worst for incident energy levels. A couple of questions:
2. If source values from utility, did they provide actual available short circuit value or infinite bus value?

This is for an existing system that will have new downstream modifications. The study is only for the downstream modifications.

The available fault current is shown on approved engineering drawings for this downstream system modifications.

Do I need the actual X/R value at this location or can I just plug in an arbitrary high value to account for worst case conditions?

How reliable would you think the given fault current value on the approved engineering drawings is?

 

[wbdvt]

The X/R value is important for equipment duty ratings as it would impact whether it is bounded by the tested X/R value or a correction factor needs to be applied.

I would bet that the fault current value on the drawings are based on infinite bus values and therefore would represent worst case for equipment ratings.

 

[petersonra]

How reliable would you think the given fault current value on the approved engineering drawings is?

My guess is that whatever available short circuit current is shown on the drawings that it is likely a lot less in reality.