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There are some obvious areas that typically result with high arc flash hazards.  When we walk through a facility for the first time, we will notice these areas and immediately realize that we may soon be applying a sticker indicating a high arc flash value.

Where are these areas?

PLEASE NOTE: this list is not all encompassing and nothing should be assumed without a full incident energy analysis

Main electrical rooms will almost always have at least one high arc flash hazard.  The reason is because the main breaker is tied directly to the main transformer.  If your main breaker doesn’t have an instantaneous setting or the setting is turned off, it’s likely we will find out that all of your sub breakers are also a high arc flash hazard.  Main fuses instead of main breakers may yield higher arc flash hazard for connected equipment but this is not always the case.

As we walk through your facility we will anticipate arc flash hazards above 1.2 cal/cm^2 on panels fed directly from a transformer.  This is true for almost all transformers that are 50kVA or bigger (this is sometimes true for even smaller transformers!).

Do you see the trend?  Transformers.  Why are transformers such a troublemaker?

Two main reasons

  1. When a voltage steps down in voltage the current steps up.  Divide this value by the impedance and you have your maximum available fault current (aka infinite bus).  When there is a high amount of current and a low impedance then you will end up with a high fault current which is one of the driving factors for arc flash.
  2. When you are far enough downstream in an electrical system you will have high amounts of resistance between the source (i.e. utility) and your device.  This will reduce the amount of available fault current.  Low fault current can cause a long trip time on an OCPD (overcurrent protective device, i.e. fuse or breaker).  In the event of being downstream and on the secondary side of a transformer, not only will an OCPD on the primary side of a transformer take longer to trip but it will also see a fraction of the current on the secondary side of the transformer.  Additionally, transformers themselves are impedance, thus lowering the fault current at the upstream overcurrent protective device.

Both of these reasons are the same for almost all arc flash hazard, high fault current and/or long trip time.  We don’t get into what causes the high hazards in this article, but here’s another post if you’re interested:

Arc Flash Size


Additionally, more information about long clearing time and how this affects arc flash hazard size can be found in this post:

How reducing clearing time reduces arc flash hazards

Very long conductor runs (300’+) are also a red flag for us as they usually cause a long trip time as well.

Typically for high arc flash hazard floor-level equipment we will make a recommendation to install a breaker between the transformer and the device it feeds to. This will not eliminate the hazard but instead move it to a new location (the breaker) which can then trip quickly enough to lower the hazard at your area of concern.  A similar strategy is applied in your main electrical rooms however there are usually breaker adjustments that can help to lower your hazards.

We itemize all areas of concern in a final report then remain available to support you after the report is issued. We will also verify settings on your breakers. After ensuring proper coordination with downstream devices it might be possible to lower many of your high arc flash hazards.

AGAIN, this list is not all encompassing and nothing should be assumed without a full incident energy analysis.  Here is a good article relating to making assumptions for arc flash values:

“Arc Flash Fuses”


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