We continue the theme from last week. This week it is about performing arc flash calculations and labeling METAL-CLAD switchgear (i.e. compartmentalized).

Some will perform only one calculation for the entire switchgear lineup with the arc flash duration based on the clearing time of the next device upstream from the entire switchgear. Label(s) have only this value.

Others perform two calculations – one for the main section based on the next upstream device and one for the feeder sections based on the switchgear main device. 2 different labels are used – main and branches sections.

When performing arc flash calculations for switchgear, do you perform:
> One calculation - for entire switchgear
> Two calculations - main section and feeder sections
> It depends
> I don’t perform studies

I selected the "Two Labels" because the majority of our projects have one main breaker. However, if this is a double ended substation with (2) Mains and a Bus Tie breaker, then we produce (5) labels. This would be (2) for each of the Mains, (2) for the Main Buses protected by the Main Relays, and (1) for the Bus Tie Compartment. We produce a label for the bus tie compartments because there are two sources of fault current within the cubicle. We run a special Bus Tie Arc Flash scenario to determine what this energy level is. This energy level will be higher than the main buses.

Robert Fuhr wrote "...(1) for the Bus Tie Compartment. We produce a label for the bus tie compartments because there are two sources of fault current within the cubicle. We run a special Bus Tie Arc Flash scenario to determine what this energy level is."

Robert: I don't understand what the special bus tie arc flash scenario is. In order to get the left bus and right bus arc flash values, you need to run three scenarios for each: 1) fed from left and right with tie breaker open, 2) tie breaker closed and fed from the left and 3) tie breaker closed and fed from the right, then use the higher of the three scenarios. Why wouldn't the tie breaker be the same? Please explain what the special tie breaker scenario is.

Going back to Jim's original question. I think insulated busbars versus bare busbars has an influence on arc flashes propagating to adjacent vertical sections. Most if not all medium voltage switchgear uses insulated busbars, so propagation of an arc flash between sections is less likely. They may see some effects of the initial blast in adjacent sections, but that's not where you're working.

A bus tie compartment can have two sources of arc flash energy. Both the load and line sides of the breaker stabs inside the cubicle can be energized. An arc flash on the load stabs could easily propagate to the line side. The result is also most twice the AF energy and at the line side of either mains.

The same can be said for the situation where one main breaker is open and the bus tie is closed. If the line side of the stabs of the open breker are energized, then you have main line and load side stabs energized...and a whole lot of AF energy.

Another example of multiple sources in the same enclosure is an ATS. During a ATS/Generator Exercise test, both the normal and emergency sources are energized. The test switch on the ATS simulates a loss of utility power. This means that if an arc flash occurs inside the ATS, you will have two sources of AF Energy. One from the utility and the other from the Emergency Generator.

This ATS multiple source situation also occurs after a normal power outage, the generator starts, the ATS transfers to the generator, and then the utility power comes back on. Again you have two sources of energy within the cubicle at the same time until the ATS switches back to normal power and the emergency generator turns off.

That's an intriguing concept. However, if we're going to use that logic, then inherent in that would be the short-circuit ratings of the switchgear itself and each main breaker.

I guess I haven't found in IEEE C37.010 or IEEE 1015 that you're supposed to consider the combined available short-circuit from both sources simultaneously. Following that logic, it would apply to not only the tie breaker but to both main breakers also. Maybe I missed that requirement in there, but I don't know anybody that rates circuit breakers that way.

For an ATS, this wouldn't apply to ones where the two source terminals are opposite of each other and the load terminals are on the side, only the ones with the load and emergency terminals stacked front to back. I'm also fairly sure, ATS's aren't rated that way. ATS's are tested and labeled based on the protective devices on each source side independent of each other, not the combined short-circuit of both sources. I'm not sure I could make SKM Power*Tools give me the available short-circuit from both sources combined. Since I don't normally use closed transition ATS's, I've never tried.

I agree with the concept but it seems it would be a bit confusing to the worker, why not just put worst case?