Would like some comments on the practice of modeling the ARMS function on main circuit breakers in a swtichboard. Gear vendors provide their system, normally in a new construction, 1200 amps or higher as required per NEC. The issued is for example, a 3000 Main Breaker Switchboard may calculate after coordinating it, 80 Calories. With ARMS, can reduce to 3 Calories. The issue remains with the line side of the main. I am not sure if the Main section of the switchboard should have a label and the distribution sections have a separate label. There still exists line side propagation. The preferred design would be the main breaker to be the upstream device, but that is not the way most of the designs are. Thanks in advance for any comments.

Overall, I think ARMS are a waste of money. As noted, they are usually installed on a main breaker in a switchgear/switchboard lineup and hence are in the same space as the feeder breakers, so credit cannot be taken for this breaker in a study. If the line up is an arc rated/tested equipment and the main breaker is in an isolated compartment or separate from the line up, then it makes sense.

To echo Barry's comments: In the case of switchboards w/ no barriers, ERMS/ARMS are pretty much useless at the switchboard location and are only useful for the equipment downstream. E.g., ERMS on switchboard feeder, feeding a panelboard. Need to work on the panelboard "hot"? Use the ERMS to lower the AFIE (if it meets NFPA 70E requirements of course for live work).

Mike

Thanks for the responses, it is unfortunate to design it this way and then require ARMS. I will keep reaching out to design engineers about this, I know they have a lot on their plate, but it is very important.

Lets say I am trying to create an electrically safe work condition or I am troubleshooting a downstream panel with a 25 cal hazard and I want to measure the voltage. The ARMS switch then becomes a valuable tool. Having an ARMS function does not validate working energized.

I agree that ARMS on the main does not lower IE at that location, however, it does lower IE at downstream locations. Perhaps for simplicity and economy, ARMS is installed at the main and not on the feeders. IE is lowered at the expense of coordination, but safety wins out.

if the ARMS is being designed in with a new install why not put it in an isolation switch on the outside of the building? That way everything inside of the building can benefit