I have performed and arc flash study for a small pump station with a "plastic" building. This pump station has a single phase of 120/240V - coming directly into a small load center with a 100A MCB. The results of the study show Hazard of > CAT 4 at the MCB. I am trying to find a means of lessening the hazard. Placing a fused disconnect before the load center does not help (tried many different types of fuses) - the switch and the MCB both are rated at >CAT 4.

If I insert the fused disconnect and change out the load center to one without an MCB, the fused disconnect remains at > CAT4, but the load center drops to CAT 0. Is this type of result expected - having to remove an integral MCB in order to lessen the hazard in the panel? By the way, the "new" panel would have less than six circuit breakers.

What is the size (kVA) of the transformer that feeds the 240V?

The transformer is a 50 kVA (12kv primary)

According IEEE 1584, if the source voltage is 240v or less and the single transformer feeding it is 125kVA or less, then there is no need to calculate the incident energy. A CAT 0 label can be placed.
This may change in the future.

Less than 240 V. Not 240 V and less.

NFPA 70E misquoted IEEE 1584 on this one.

Seems high, what was your clearing time? Did you use the 2 second cutoff?

Fault Clearing Time was 2 seconds (maximum configured into ETAP).

How did you calculate the incident energy on a single phase circuit?

Yes, it is common to relocate a high incident energy to another location as you are contemplating.
Have you considered a remote breaker instead of a fused disconnect?

Why do you think you need to remove the existing 100A main MCB?

Calculations via ETAP - I realize that the base equations are for 3-phase and the results will be conservative, but no other method available until I.E.E.E. committee comes up with single phase equations.

Hadn't considered a remote breaker, the load center is inside the tiny plastic building and the meter is directly outside of it. Would need to insert a weatherproof fused disconnect or circuit breaker. I just used a Bussman NON 100A fuse because the County uses a lot of them.

The model is showing that even with a 60A fuse in front of the 100A MCB, the MCB still has a rating of >CAT 4. If the load center has the MCB it must be labeled at >CAT 4. If I take out the MCB, the fused disconnect on the outside of the building is rated >CAT4, but the load center can be labeled CAT 0.

This is your opinion. Some people consider this to be a mis-application of the IEEE standard, and instead use 70E table 130.7(C)(10) with its applicable footnotes, for single phase circuits.


I would question your model results, if either a remote fuse or breaker does not lower the incident energy on the line side of the MCB.

So since every house has a 120/240 volt service, they all need an arc flash study???

Have you considered lowering the 2 sec cutoff time? Read Chet Davis' paper, "Calculating Arc Flash Energies and PPE for Systems <250V", ESA, Inc.

Have you looked at a different fuse than a NON? Like a A6D?

I will read the referenced paper - Thanks

I tried many different types of fuses, including "Fast" and "Very Fast". Fuse type had little or no effect on the results.

NFPA 70 (NEC) does not require AF labels on dwelling occupancies. (110.16)

Chet's paper is great work and IMHO makes more sense BUT it's not an adopted IEEE 1582a standard therefore the 2 second cutoff applies.

I've run into this problem of 240V panels with main breaker option being fed from 125KVA xfmrs with primary side PD only located considerable distance for the CB panel. Technically it meets NEC. But due to the cable run and xfmr impedance the CB panel line side fault clearing time can be pretty long.

I recommend installing a fuse disconnect at the xfmr secondary with LPS Low Peak fuses sized to 100% FLA. NEC allows up to 125% over current protection but you can go lower.

I've also witnessed many arc flash events in 240v and 120/208v CB panels where the T&M MCCB main breaker didn't trip. Yet the panel was cooked. My theory (and have no scientific proof) is T&M MCCB's don't respond to L-G or L-L-G arc faults the same way as bolted faults, The arcing current is lower and the fault is burned open before the MCCB trip point is reached. By then the CB panel is destroyed.

Jim... care to respond?

First the arcing fault current is always less than the bolted fault current.
UL489 listed circuit breakers are tested for single phase as well as poly-phase circuit interruption.
It makes no difference if the device is a breaker or a fuse, if the fault current is below the operating curve, panel destruction is possible.

Have you compared the operating curves of breakers and fuses? From 100A to 400A they overlap and cross each other occasionally.

This sounds suspect. Did I understand correctly?

A fuse protects a load center with no main to Category 0, but if a main is added to the same load center and the same fuse is used, the panel becomes Category 4?

If this is correct, it sounds more like a software or modeling glitch. It seems the fuse should provide the same reduction in incident energy whether there is a main or not in the panel.

Great discussion! JBD’s point hits it. It all comes down to there is enough current for a device to operate or not. This is still at the heart of the 125 kVA exception. Smaller transformers result in a lower short circuit current. Often so low the next upstream device takes too long to operate (according to the time current curve). The real issue is whether or not the short circuit will sustain or self extinguish so that it is a short duration and results in a low Ei.

I just received some new test data about 6 weeks ago. Although I am prohibited from passing it along (for now) I can tell you we are learning more about what it takes to sustain a low voltage low current arc. It has a lot to do with the equipment, enclosure size, bus orientation and distance to barriers or insulation.

We’ll be reviewing all of this over the summer and discussing the future direction of the 125 kVA exception in September at our next IEEE 1584 meeting. My thought continues to be that we will likely have a low current cut off at some point - perhaps 5 kA or something similar. I hope to do away with the kVA cut off.

A few of us were also kicking around having a default Ei level in the absence of calculations. Perhaps 4 cal/cm^2. This is only from side discussions and nothing formal or official has developed. A default level will take the guess work out of “we did not perform calculations - now what?�

I'll update everyone in the fall after we take another step forward with this.

Though your calculations seem high for the application, YES, removing an integral MCB is a good way to lessen the hazard in the panel. (I did not review all the preceding discussion, just the basic question).

Some transformers have a secondary main attached feeding a remote MCC and that is a fine way to limit arc flash exposure.

EDIT: didn't read your question carefully enough. The downstream device should have similar arc flash exposure based on supply side fuse with or without a MCB.

Not sure I am understanding correctly either, but it seems basic that removing the fuse from the enclosure would reduce the enclosure exposure by the amount of the exposed, unprotected load side terminals.

EDIT: I reread the OP and agree it seems like a software glitch. The calculated exposure for downstream panel would seem the same if protected by the same fuse, whether or not there is a main device in that panel. I wonder if the software is picking up the wrong device for a clearing event(?). Could there be some phenomena such as series rating where both devices "share" the fault energy and thus delay the fuse clearing time? Seems unlikely that a software program would pick that up.

That is discouraging to read. I am sincerely hoping this onerous safety standard can be simplified by consolidating the existing 14 parameters that presently govern arc flash calculations.



I hope your team understand folks are depending on the results to govern their daily work and the sign of a true expert is to make these matters SIMPLE.