Good morning all,
I'm working on an arc flash study, ran into a question i was hoping someone might be able to point me in the right direction. In an SKM training I attended the instructor talked about certain switchgear or MCC sections must be rated at the same value as the main breakers regardless of any quick trip or maintenance modes you might have on the mains to help reduce incident energy. I understand that there is a chance the arc flash can propagate to the line side of the main and its upstream device has to interrupt the fault, thus being reliant on the breaker upstream of the main. Our mcc and load centers are all pretty old gear, 70's vintage, but so far the ones i have inspected the buckets or breakers are isolated from the buss-work in the back of gear. My question is, is there specs on what is considered isolated so propagation is mitigated, or is this a judgement call for the engineer working on the study?
Thanks for the help.
Mark

It requires engineering judgment. Two standards have limited language regarding this situation.

IEEE 1584 2018: IEEE Guide for Performing Arc-Flash Hazard Calculations

IEEE 1584.1 2022: IEEE Guide for the Specification of Scope and Deliverable Requirements for an Arc-Flash Hazard Calculation Study in Accordance with IEEE Std 1584â„¢

The plasma that results is conducing so one has to determine if the conducting plasma can propagate to another location such as ahead of the main. I have seem some strange things in the lab as a result. Not a back and white answer which it requires judgment. Arc resistant equipment is specifically designed to isolate the arc flash but other equipment is not.

From IEEE 1584 Clause 6.10 Step 9: Calculate the incident energy
To calculate the incident energy at a specific piece of equipment, the equations in Clause 4 are used. It is important to note that multiple arcing locations can be found within a single piece of equipment as outlined in 6.6 and Annex C and Annex G. Incident energy calculations should be performed at each of the arcing locations that are defined to determine the highest magnitude incident energy or “worst-case� condition.

When a model of the power system is developed, the equipment compartmentalization and fault location need to be considered. The arc fault could occur on the line side, bus side, and load side of protective devices located in different compartments. Refer to IEEE Std 1584.1 for more details on fault location considerations. When evaluating the incident energy at an arcing fault location in the system, the interrupting time of the protective device upstream from the point of the fault must be considered. An integral “main� overcurrent protective device may be considered in the calculation if it is adequately isolated from the bus to prevent escalation to a line-side fault. When the integral main overcurrent protective device is not adequately isolated from the bus, the upstream protective device must be considered as protecting the main and bus.

From IEEE 1584.1 Annex C.3 Overcurrent protective device considerations
Only an upstream protective device that is isolated from an enclosure can limit incident energy for that enclosure. An arc-flash in that enclosure could occur on the line side of the main overcurrent protective device (OCPD), or a load-side arcing fault could potentially escalate to a line side fault. An OCPD can only limit incident energy downstream (i.e., on the load side of device) and within a sufficiently separate or isolated compartment or enclosure from the protective device itself.

NOTE—Consult with the manufacturer to confirm if the equipment provides isolation between the main compartment and other compartments, to determine if the main internal OCPD can be used for the calculation of the incident energy at these buses.

Note: My usual disclaimer. Although I'm involved with many different standards, this response is a personal view and may or may not represent any official position of IEEE etc.

Thank you, greatly appreciate it.