Greetings,
When a feeder tap is present in the feed to disconnects supplying power to a number of industrial control panels and machines, what factors should I consider? Should the possible energy levels be figured at the tap and that value be used for the control panels downstream? Also, if a control panel has a feeder that ends at a set of fuses and that is distributed to motor control starters/overloads, is the worst case scenario at the end of the feeder?
Thanks in advance.

Hello,
Not entirely sure about what you are describing but my take is this:
I assume the feeder tap you describe is a feeder going to something like a wire trough that has several disconnects tapped off of it within 10ft. I would see what the incident energy is a the wire trough and see how much farther you need to take the analysis.

For the other situation, are you describing a feeder that goes to a fused disconnect switch and then to a control panel? It is likely that the AFH would be different at the fused disc switch than at the control panel.
Or is a feeder coming out from the control panel to a fused disconnect switch which serves several loads? Does the feeder leaving the control panel have a breaker?

The general rule that I use is to look for the highest available incident energy without a given enclosure. This means that for instance if I have an industrial control panel which is fed by a cable and contains a number of circuit breakers, motor starters, etc., then the incident energy for the panel would be taken as assuming an arcing fault on the cable feeding the panel. Once the cable exits this enclosure then the analysis would be repeated for "downstream" devices.

Case in point that recently came up. A set of fused cutouts feeds a pole mounted 500 kVA, 208 V/3 phase transformer. It comes down the pole and enters a panel board with 5 breakers sized 600 A, 400 A, and three 30 A breakers. The 600 A and 400 A breakers in turn feed panel boards further downstream where the largest breaker is 100 A. Short circuit current is only around 2.5 kA and arcing currents are down around 2 kA. The low fault current turns out to be a major problem. As is typical for many MCCB's the settings for instantaneous are 6-10X frame size so even the 400 A breaker will not sense and trip instantaneously in the event of an arcing fault. They will still trip, just that it will take over 2 seconds for this to occur. The rating in this panel calculated by IEEE 1584 is 10 cal/cm^2. The ratings in the panels downstream of the 400 and 600 A breakers are slightly lower. The ratings downstream of all the 30 and 100A breakers is <1.2 cal/cm^2 since they trip during an arcing fault. Note that the incident energy levels are highly likely to be much less than predicted by IEEE 1584 since low voltage arcs and low current arcs tend to be overestimated. The fault currents are also below the recommended ranges for other calculation methods.

The inherent difficulty (and arguments) come in when looking at propagation from one enclosure to the next. With switchgear, consensus is that this is unlikely. With panelboards, obviously it is. The tricky one is MCC's. MCC buckets are all open at the back where the bus bars are at and there are openings along the horizontal bus bars connecting sections. The jury is out whether propagation from one section to the next is realistically possible. Propagation DOWNWARD is not only possible but routinely happens when a bus fault occurs and is magnetically propelled away from the power source to the bottom of the bus bars.

In my case most of the studies are for industrial plants. In the past year I have seen many plant maintenance staffs strongly pursue mitigation trying to reduce the incident energy to 1.2 cal/sq cm or less. In most cases these are large control panels and/or drive panels. I would like to lay out my system for reducing the incident energy on drive cabinets or industrial control panels.

1. If the feeder is fusible and 400 amperes or less, by changing the feeder fuses to a fast-acting RK1 fuse, this typically drops you to under 1.2 cal/sq cm. The part number would be a Mersen A6K + (amp rating) - R.


2. If the feeder is a circuit breaker and the control panel is 400 amperes or less the following is the scheme:

A. An external fuse switch with aux contacts is located adjacent or attached to the control panel.

B. If the the control panel current rating is 400A or less we utilize a UL fast acting Class J fuse. The Part # is a Mersen A4J + (amp rating).

C. You also need an electrical/mechanical interlock. We use the Hoffman door solenoid.


3. If the control panel feeder is 600, 800 or 1200 amperes the method is as follows:

A. As we all know, as the overcurrent device size gets large the incident energy increases. The Class RK1 and Class J fuses are available in a range from fractional all the way to 600 amperes. Above 600 amperes the only UL listed fuse would be a Class L, those go all the say to 6000 amperes. In summary, the larger (600A) Class J and RK1 and the 800/1200A UL Class L fuses are not fast enough to reduce the incident energy to less than 1.2 cal/sq cm in the majority of cases.

There are faster fuses that can be utilized, however, those fuses do not fit into a conventional safety switch. We have worked closely with the Boltswitch Company and they now offer 400, 600, 800 and 1200 amp enclosed switches that can accept the Mersen A50QS fast acting fuse. This fuses is typically used for the protection of semiconductors.

B. In your arc flash system software, insert a Mersen A50QS + (amp rating) fuse in front of a control panel and watch the incident energy drop. In almost all cases with a A50QS fuse at 400, 600 and 800 amperes the incident energy will fall under 1.2 cal/sq cm. It will be similar with the A50QS1200 with high available fault currents, however, at lower fault current it may not drop below 1.2 cal/sq cm, it will drop significantly though. In many cases the plant staff may only want to drop under 12 cal/sq cm, this allows a less obtrusive form of PPE.

The caveat is that these fuses do not fit a standard safety switch, at this time only Boltswitch has a switch that excepts these fuses. The fuses must be outside the cabinet being protected and the switch should have a set of aux contacts that lock the main control cabinet until the switch is opened. In addition, these switches are always down stream in protected feeder circuit.

One last item, If any of these control panels contain a single across-the-line start motor that is 50% of the panel size or larger, the inrush capability of the fuse must be reviewed. There would be no inrush issue with drive cabinets.

In my experience, very few circuits, <400A and >1600A, benefit from using fuses instead of molded case circuit breakers.
Until there is sufficient arcing fault fault current to cause the fuse to enter its current limiting range, the instantaneous trip of a molded case breaker breaker, is most often quite adequate.