Understanding and Using Additional Utility Data when Performing an Arc Flash Hazard Analysis

Typically, the utility available short circuit currents (three phase and single line to ground) and its corresponding X/R ratios is the information provided by the utility. However, obtaining the primary overcurrent protective device information protecting the service transformer and conductor data from the protective device to the service transformer is also critical when preforming an Arc Flash Hazard Analysis.

The reason for obtaining this additional Utility information is to analyze the effects of the primary service transformer protection on the calculated arc flash incident energy at the service equipment.

At low voltages, this incident energy normally exceeds 40 cal/cm2 – rendering this panel unable to be maintained while energized. However, there exists a possibility of reducing this incident energy to a manageable level when including the effects of the primary protection.

Below are some examples demonstrating how this additional utility information will affect the arc flash incident energy results.

No Primary Protection:

Primary Protection:

The first two circuits have a 4.16 kV (MV) secondary voltage and the last two have a 480 V (LV) secondary voltage.

The following shows the incident energy from SE-1 to SE-4, when there is no primary protection:

Notice that for the medium-voltage (MV) service panels, the incident energy corresponds to a Category 3 level. The low-voltage (LV) service panels have a Category of Dangerous!

The following shows the effect of the primary protection is now being considered in the incident energy calculations:

The results are quite different compared to the case without primary protection.

We will now analyze each case in detail to understand the results.

First Case (With Protection):

  • Primary Protection: Fuse.
  • Secondary Side Voltage: 4.16 kV
  • The calculated incident energy is reduced by FU-UTIL-1 at SE-1 from 16
  • cal/cm2 to 1.9 cal/cm2.

Second Case (With Protection):

  • Primary Protection: Relay
  • Secondary Side Voltage: 4.16 kV
  • The calculated incident energy is not reduced by REL-UTIL-2 at SE-2. It remained at 21 cal/cm2. Lowering the time delay settings (but sacrificing downstream coordination) could achieve more mitigation.

Third Case (With Protection):

  • Primary Protection: Fuse
  • Secondary Side Voltage: 480 V
  • The calculated incident energy is not reduced by FU-UTIL-3 at SE-3. The clearing time of the fuse is above the maximum arcing time of 2 seconds.

Fourth Case (With Protection):

  • Primary Protection: Relay
  • Secondary Side Voltage: 480 V
  • The calculated incident energy is reduced by REL-UTIL-4 at SE-4 from 154 cal/cm2 to 24 cal/cm2.


In Case 1 and Case 4, the primary protection had a significant effect on mitigating the incident energy. In Case 2 and Case 3, the incident energy remained the same

We can conclude that it is not possible to know if the primary protection will have a substantial incident energy mitigation effect. The only way to know is to include the protection and perform the calculations.

Therefore, it is recommended to include the primary protection in the model, especially when the service panels have a dangerous incident energy level without its inclusion.