arc flash - ATS, xfmr, switchboards

[malachi constant]

Two questions, somewhat related:

1. When performing an arc flash study you often have pieces of equipment that get calculated at multiple points on the equipment (for example a transformer at both primary & secondary, ATS at em/normal/load). Often the different points yield different PPE categories. I assume it is best practices to put one single label on the equipment that reflects the worst case scenario. Thoughts?

2A. The way we typically model our switchboards in SKM is: incoming feeder -> main CB -> direct to switchboard bus. This yields a calculation at the switchboard bus (on the load side of the main CB) but not on the line side of the main CB. I have tried adding in a bus in between the incoming feeder and main CB, but the program won't let you - I believe because it requires a wire be added somewhere in between the buses. In real life wouldn't opening up the first of the switchboard expose you to a potentially worse arc flash on the line side of the main? Shouldn't it be critical to get a calculation at that point? What should I do - add a one-foot section of bus or something?

2B. We typically treat our switchboards in SKM as one point. Should we be taking into account the length of the bus, and treat it as a cable busway instead of a single point? I am not aware of anyone doing this as a standard practice for arc flash calculations, but curious as to the community's thoughts.

Thanks!

 

[wbdvt]

Hello,

Here is my response based on about 10 years of performing arc flash studies:

1. I do not calculate or label transformer primary and secondary connections. Typically there is no reason to access those connections as part of normal operations. In some cases facilities will infrared the connections and my recommendation is to install IR viewports. I perform calculations for all modes of operations and discuss with the client how they would like it labeled. This usually ends up being labeled for normal operating mode. This makes sense as this is 99% of the time the facility operates and does not subject personnel to excessive PPE requirements. For example: operating a breaker or disconnect switch in normal operating mode may be a HRC 1 (1.2 to 4 cal/cm^2) but worst case may be a HRC 3 (8 to 25 cal/cm^2). I do include in the report tables showing the afh for other scenarios.

2A. I use EasyPower and when you fault the bus, you do have the choice of including or excluding the main breaker or a combination of both. Including the main gives the result on the bus being fed from that breaker, excluding gives the result ignoring that breaker and both gives you both results. I am surprised SKM does not have that option. If not, you could add in a 10 ft (or whatever the minimum feeder length is) of busduct (usually lower impedance than conductor). This will not have any major effect on the calculations.

2B. Treating switchboards, MCCs, Panelboards and Switchgear as one point is a common practice. The impedance of the buswork is negligible on the final result.

 

[mayanees]

2A. I use EasyPower and when you fault the bus, you do have the choice of including or excluding the main breaker or a combination of both. Including the main gives the result on the bus being fed from that breaker, excluding gives the result ignoring that breaker and both gives you both results. I am surprised SKM does not have that option. If not, you could add in a 10 ft (or whatever the minimum feeder length is) of busduct (usually lower impedance than conductor). This will not have any major effect on the calculations.

2B. Treating switchboards, MCCs, Panelboards and Switchgear as one point is a common practice. The impedance of the buswork is negligible on the final result.

SKM does have that feature, but I prefer to assign a node to the line side of the main breaker, add the breaker, then add a ~ 3' section of busbar at the ampacity of the gear, then land on a bus for distribution. The line side of the main breaker is often what regulates the PPE requirements for the gear unless the main breaker is completely isolated from the other sections.