We have discussed where labels should be placed (see this link). Now its time to discuss our methodology behind applying appropriate labels as a result of calculations and best practices.
Typically a main breaker in a piece of equipment will lower the arc flash hazard for downstream loads (due to the breaker's clearing time, see link here for more information on how clearing time affects arc flash)
This means that the line side (primary) could have a different arc flash value than the load side (secondary) and that a panel with a main breaker could actually have two different ratings, one for the line side of the main breaker and one for the load side or internal bus of that panel. So how do we label this piece of equipment?
See the picture below for the first example. The feeder wires come in on the bottom left and into the main breaker (section A). From there the main breaker connects to the internal bus of the cabinet (section B), each sub-breaker connects to the main bus then distributes to downstream equipment.
In our example we will consider that the line side of the main breaker (section A) has a 2.1 cal/cm^2 arc flash hazard @ 18" and the load side of the main breaker (section B) is <1.2 cal/cm^2 @ 18".
This is also represented in the engineering software output as seen below. The main breaker has a higher arc flash rating than the rest of the panel.
For this panel we will apply one label which will indicate 2.1 cal/cm^2 @18".
The reason we do this is to indicate worst case behind the panel cover. If a qualified employee were to remove the cover to perform energized work such as testing, troubleshooting or voltage measuring then that employee could be exposed to the higher hazard 2.1 cal/cm^2 and must wear PPE to that level.
However, the type of work must also be considered. If an employee is performing a non-normal operation such as turning a sub-breaker on after work downstream was performed on the circuit then the hazard to worry about is <1.2 cal/cm^2 and not 2.1 cal/cm^2. This is why an engineered report is so important as it allows personnel to understand the actual hazards they are being exposed to. This is also why it's important to follow a recommendation in our report to change settings on main breakers even though that setting change might not change the value indicated on the one label applied to the equipment. Lowering the arc flash value on an internal bus will lower the likelihood of being injured in an arc flash when performing work on those sub-breakers. If repeated work is to be performed on a lower arc flash hazard area then an SOP can be generated to help clarify the actual hazard that employee is being exposed to (as opposed to relying on the label which indicates the higher hazard).
Let's look at another example of an MCC. See the picture below. The main breaker is at the left side (section A) and all of the buckets fed from the main breaker are on the right (section B).
The main breaker (section A) will receive a label indicating a higher arc flash label. All of the buckets to the right (section B) will receive the same label indicating a lower arc flash hazard.
Some of our customers ask that we label each bucket, others ask that we only place one label on each column.
Each bucket/column will receive the same label because they are all connected to the same bus (the load side to the main breaker), this is the worst case for each bucket. It is likely that the load side of the overcurrent protective device (i.e. breaker or fuse) in each bucket is actually a lower arc flash value than indicated on the sticker. However, the most dangerous task (not recommended) is racking a bucket into an energized MCC, so we will always want to label for this worst case to protect workers.
If an MCC has a transformer built in then that bucket and it's load (i.e. lighting panel bucket) will receive a different label from the rest.
Below is a switchgear with a very similar situation as the MCC above. The main breaker is a higher arc flash value (section A) and the downstream sub-breakers (section B) have lower arc flash hazards. in this case we applied a label indicating a higher arc flash label on the main and each of the sub-breakers received the same label indicating a lower arc flash value.
It is possible based on the internal configuration of the cabinet that we will choose to label the sub-breakers of a switchgear with a label of the same arc flash value as the main if we feel that there is no protection from propagation of arc flash between the sub-breakers and the main breaker. In the above switchgear example we would likely label the back of the cabinet this way, the same label on each column, as that area has no column separation.
Overhead busbars can be very long and because of this will likely have a different arc flash value at one end of the bus vs the other end of the bus. We will label the entire bus bar as whichever value is worst case. Our standard practice for labeling bus bars is to not label bus taps, bus taps can and do get moved, from one bus to another and if the tap is labeled, then the label must be removed before moving which is likely to not happen. We like to get at least one label on/near the main connection point and then every 25’ if possible.
What about transformers, how are they labeled? That information can be found here: https://www.70econsultants.com/should-transformers-be-labeled/