This system consists of a 5KV feeder to a load break disconnect ahead of a 1500KVA (4160/480) xfmr to a main breaker. Calculations were made using the SKM software & the IEEE standard.

The rating of the main breaker line side(81) is calculated at a Dangerous category. The load side bus is calculated at a Category 3 rating. The feeder breakers off of this bus are rated Category 2 & one device is rated as a Dangerous category. The substation bus is contained with in barriers within the gear. Removing the rear covers exposes only the load side terminals of the breakers.

The normal tasks performed in this facility are typically rear cover removal for IR scanning, voltage & amperage tests. The entire system follows a stringent PM cycle at which time the substation is de-energized and the breakers are tested & calibrated. Due to the high incident energy of device 83 as well as other locations with in the facility the customer has ordered a remote racking sled in case the breakers needs to be racked out when the sub is energized.

How would you label this, specifically Sub 5? I think this could play out a few ways.
Do you label all breaker cubicles at a HRC 3 level since they are connected to the bus? Would you label the rear covers with the HRC 2 level since that is all that is exposed (load side of breakers) when the covers are removed?

Your thoughts?

I am confused about your 82.5 cal/cm2 on device #83. 82-89 should all be about the same




Where do you get HRC 3 from? Having different labels on the front and back is fairly common. Have you considered mitigating these? Easy fix for sub 5 (Besides the main) is retrofit the main with a trip unit that has arc flash reduction capability. We do those every day. The main breaker and the transition section on the 480V side of the transformer is tougher but I have mitigated that down to HRC 1 at a few plants.

Sounds like they have the racking figured out, what about operating the breaker before racking? Have a solution for that one yet?

Since you cannot control who will have access or how they will interpret the labels, I would label the entire lineup as Dangerous since that is the exposure available on the line side of the main breaker. This makes a compelling case for detached main breaker located on the transformer secondary.

It is debateable whether to label the rear access panels at the lower Risk Category 2 IF that is the only exposure there, and it is very obvious the reduced hazard is only for those compartments with higher risk category labels on the front compartments.