Do you need to test, troubleshoot or voltage measure a circuit that is 120VAC, but that circuit is in a piece of equipment with a 480V arc flash label? You may ask yourself, "What is the true hazard of the 120V circuit?" "Will I have to worry about the hazard associated with the 480VAC source?", "How do I ensure I am protected?"
First, lets consider an MCC with the following sticker on each of the MCC columns (this MCC has no main breaker, each column is connected with a horizontal bus):
There are two electrical hazards indicated on the label, arc flash, and electrical shock. 6.1 cal/cm2 is a very dangerous arc flash hazard, so great that it could badly burn an unprotected person over four feet away. It's important to understand where this hazard is located when conducting your risk analysis prior to performing electrical work. This label very likely represents the worst case hazard associated with the MCC and may not necessarily represent the hazard in each MCC bucket. The worst case arc flash hazard is the MCC bus itself.
One of the most dangerous types of electrical work on an MCC is inserting or removing MCC buckets while the MCC itself is energized. Some MCCs can isolate power to a column to safely permit this type of work to be conducted but most installed MCCs do not have this capability. Additionally, the primary side of the main disconnect in each MCC bucket is directly connected to the MCC bus and should be treated the same as if it were the MCC's bus.
What is the arc flash hazard after the main disconnect though? The answer is... that value would need to be calculated by an engineer and depends on the characteristics of the overcurrent protective device (main fuse or main breaker)... However, in this example, let's consider a 15A main breaker that after calculation proves the rest of the bucket is less than 1.2 cal/cm2. If you know this for sure then you can use that information in your risk analysis to determine if interacting with the equipment increases your likelihood of being exposed to the higher arc flash hazard. Other questions to ask include: what does our local electrical policy say about this type of work? Is the 480V primary side "exposed"?
What if you LOTO the main disconnect in an individual MCC bucket, can you work in the rest of the bucket without any PPE? We have another article about this topic here:
And now, back to the original question of this post... what if you're only interested in testing, troubleshooting or voltage measuring on a basic 120VAC circuit but that circuit is in a shared space with 480VAC? The short answer is: if 480VAC is exposed and energized, then you have to treat the equipment as though you are working on 480VAC. There are ways to make equipment not exposed, see this post.
We do have another article that discusses arc flash hazards for 120VAC (do they exist?).