Arc Flash Calculation - where we can stop?

[anbm]

Does anyone know at what level or equipment, the arc flash calculation is not needed? I.e. only calculate arc flash for 100A and greater equipment, certain hp size for motor, certain kVA size for 480-208/120V transformers? Does NFPA 70E has any rules or limitation, exceptions on this? Thank you.

 

[xptpcrewx]

Does anyone know at what level or equipment, the arc flash calculation is not needed? I.e. only calculate arc flash for 100A and greater equipment, certain hp size for motor, certain kVA size for 480-208/120V transformers? Does NFPA 70E has any rules or limitation, exceptions on this? Thank you.

It’s not a matter of when an AF calculation is not needed. It’s more about determining the hazard with the system in various configurations. There are different calculation methods based on different conditions each of which have their limitations/practical ranges.


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[triptolemus]

The shock and arc flash hazard assessments are required when a worker is exposed (or potentially exposed) to an electrical hazard -- or stated another way, all electrical equipment not in a dwelling unit that is likely to require examination requires the hazard assessment. See 70E 2021 130.5 -- you will find no distinction is made for the requirements of an arc flash risk assessment as it relates to equipment ratings or locations in the system.

Loosely speaking, the requirements for 70E kick in at 50V.

I would be very careful making assumptions about arc flash hazards using kVA and system voltages without looking at the bigger picture. That little 112.5 kVA 120/208 transformer feeding a panel can sport an arc flash hazard of 19 cal/cm2 at the line side lugs in the panel.

 

[mayanees]

IEEE 1584-2018 cuts the arc flash calculation requirement off at 2000 amps of bolted fault current. I think that translates to about a 30 kVA transformer at 480 Volts. Prior to that, IEEE 1584-2002 cut it off at "under 125 kVA", but it was found that arcs could be established at lower transformer sizes than 112.5 kVA.
All that is to say that if there is less than 2000 amps of bolted fault current it is likely less than 1.2 calories so it could get some generic label that no arc flash PPE is required. As a study-doer, it's just as easy to do all points in the system.

 

[xptpcrewx]

IEEE 1584-2018 cuts the arc flash calculation requirement off at 2000 amps of bolted fault current. I think that translates to about a 30 kVA transformer at 480 Volts.

Please provide reference to this section you are seeing in IEEE 1584-2018. I doubt it exists.


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[mayanees]

Please provide reference to this section you are seeing in IEEE 1584-2018. I doubt it exists.


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Here's the excerpt from the standard. I miswrote when referencing 480 Volts, as it should be 240 Volts or less, but 30 kVA still holds..

Here's a 240V example:

It's not exact, and it depends on the transformer's impedance and the strength of the primary source. But it's a reasonable starting point for calling it under 1.2 calories. As I already wrote, it's easier for a study doer to include everything, but 1584-2018 does give some room for a cutoff.

 

[xptpcrewx]

Here's the excerpt from the standard. I miswrote when referencing 480 Volts, as it should be 240 Volts or less, but 30 kVA still holds..
View attachment 2556443
Here's a 240V example:
View attachment 2556445
It's not exact, and it depends on the transformer's impedance and the strength of the primary source. But it's a reasonable starting point for calling it under 1.2 calories. As I already wrote, it's easier for a study doer to include everything, but 1584-2018 does give some room for a cutoff.

I disagree 100%. What you are referring to is not a cut off. The only thing discussed in 1584-2018 is the model range and associated assumptions. These however would only be limits to the calculation method. Alternate calculations covered by other standards would need to be looked at.


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[mayanees]

I disagree 100%. What you are referring to is not a cut off. The only thing discussed in 1584-2018 is the model range and associated assumptions. These however would only be limits to the calculation method. Alternate calculations covered by other standards would need to be looked at.


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No, here's the 1584-2018 range reference:

The calculation above is well within the range of applicability of the standard. What you'd find if you're doing any of these studies is that as the bolted fault current drops off, so does the incident energy level. That's why the lower limit went form 112.5 kVA in 1584-2002 to 2000 amps bolted fault current for 1584-2018. Perhaps the lower limit is 15 kVA, as chosen by a client in their Electrical Safety Program. 1584-2018 gives a reference of 2000 amps of bolted fault current, as excerpted above.
And again for emphasis I rewrite that it's easier to perform the calcs and report the information, but there is in fact a lower limit that's suggested by the standard.

 

[xptpcrewx]

No, here's the 1584-2018 range reference:
View attachment 2556446
The calculation above is well within the range of applicability of the standard. What you'd find if you're doing any of these studies is that as the bolted fault current drops off, so does the incident energy level. That's why the lower limit went form 112.5 kVA in 1584-2002 to 2000 amps bolted fault current for 1584-2018. Perhaps the lower limit is 15 kVA, as chosen by a client in their Electrical Safety Program. 1584-2018 gives a reference of 2000 amps of bolted fault current, as excerpted above.
And again for emphasis I rewrite that it's easier to perform the calcs and report the information, but there is in fact a lower limit that's suggested by the standard.

You’re missing the point. My comment is in reference to this statement (which is misleading and incorrect).

IEEE 1584-2018 cuts the arc flash calculation requirement off at 2000 amps of bolted fault current.

I am merely commenting there is no such thing as a “cut-off requirement” in IEEE 1584-2018. There is only a model range and limitation where the calculations make sense within the applicability of the model. But again, it’s not a requirement with respect to the context of the OP. Anything beyond the model range requires calculation methods provided in other standards.


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[ron]

We perform calcs on anything three phase, unless client tells us they will not pay, then without a sticker, they "cannot" work on the equipment energized.