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An arc flash evaluation that Rozel performed identified a high arc flash hazard. After analysis, we were able to recommend a simple setting change which ultimately saved the day and likely prevented a great amount of damage.

You can see in the one line drawing below that the main breaker and the gear itself had an arc flash value greater than 90 cal/cm^2 @ 18″ (many additional breakers on this gear are not shown). You can also see that MDP2 K1 is a power factor correction capacitor installed directly connected to the bus with no protective device mounted to MDP2.

Our recommendation was to change the Instantaneous setting on MDP2 – MAIN from “7” (28000A) to “3” (12000A). In order to make this recommendation we first looked at the TCC (time current characteristic) of the main breaker and all of it’s downstream overcurrent protective devices (breakers and fuses). This TCC does show the additional breakers on MDP2 which were not shown on the one line drawing above. Here is what we saw:

The main breaker is the purple line to the right of all the other colorful lines. We want that line to be as far to the left and as far towards the bottom as possible without overlapping other lines. More information about this topic can be found in another post here.

By dialing the Instantaneous down to a 3, we achieved the following results:

Now when a fault occurs, the breaker will trip more quickly and cause the source of the fault to disappear. This setting is important because it reduced the calculated arc flash value to less than 3 cal/cm^2. See the resultant arc flash values on this drawing:

The factory followed our recommendation, which was very simple. A turn of a screwdriver implemented this.

Approximately one year after implementing the change, the MDP2 K1 power factor correction capacitor was replaced during a shutdown. Safe work practices were utilized including de-energizing and performing appropriate Lockout Tagout.

Once work was complete, the power was reapplied… and the main breaker had an instantaneous trip.

The install went as planned however the new capacitor was faulty. This bad unit caused a bolted fault directly on the MDP2 bus, this in turn caused the main breaker to trip.

The damage was minimal and isolated to the faulty capacitor. The people in the room while the gear was energized were not injured. Prior to the Instantaneous setting change however, this same fault could have caused injuries to people over 20ft away. Additionally a fault at the previous arc flash level would have most definitely destroyed electrical equipment and brought long downtime to the factory.

It is impossible to tell if your equipment is at it’s safest arc flash hazard without an evaluation. Your employees must be trained to recognize these hazards and understand how to work around them safely. Rozel can help with both. Contact us today.

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