SKM Arc Flash Line Side vs. Load Side
The option of choosing the line-side or the load-side arc flash incident energy for arc flash labels is an important decision when performing an arc flash hazard analysis.
When an engineer performs an arc flash hazard analysis for any electrical equipment that includes a main overcurrent protection device (OCPD), there is a possibility of choosing between two distinct values of incident energies. They are referred to as the Line-Side or the Load-Side values. See Figures 1 and 2.
This choice could greatly affect the maintenance procedures of the electrical equipment. If the line-side incident energy - which is normally higher than the load-side incident energy - is chosen for the complete panel, the personal protective equipment (PPE) requirements would be more stringent than that required if the load-side incident energy was chosen.
When working on the incoming section of the enclosure (where the main OCPD is located), there could be an arc flash event on the line-side of this main OCPD. In this case, the electrician will be normally exposed to the higher incident energy value and he should be wearing higher rated PPE. When working on the feeder sections (on the load-side of the main OCPD), the electrician will normally be exposed to the lower incident energy value and could be wearing lower rated PPE. Nevertheless, you have to consider what would occur if the electrician is working on a feeder section adjacent to the incoming section. Normally, the arc flash is initiated by human error, but an arc flash event on the line-side could also occur while working on a feeder section (adjacent to the incoming section and without any human influence) due to insulation failure, lack of maintenance, overvoltages, animals, etc. Many switchboards and switchgears do not have strong and reliable barriers or isolation between the incoming section and the adjacent feeder sections. See Figure 3.
Therefore, if this arc flash event would occur on the line-side, the electrician working on the adjacent feeder section (protected only for lower load-side incident energy), he will probably be exposed to a higher incident energy than he is protected for. Therefore, which value of incident energy should be shown in the equipment arc flash label? There are many options for solving this issue. Three of them are the most common:
Why is the Line-Side and Load-Side Incident Energy Different?
The line-side incident energy value is normally higher than the load-side value because at the line-side the main OCPD is not involved in the clearing of the largest component of the arcing fault current. The main OCPD will only see the motor and generator contribution to the fault on the load-side of the main OCPD, which is normally much lower than the utility contribution to this fault and contributes the least amount of incident energy at that location. The OCPD that will interrupt the higher arcing current from the utility is upstream from this main OCPD and is normally is much larger. Higher ampere rated OCPD normally will result in longer interrupting time and therefore higher incident energies.
When is the Load-Side Incident Energy Higher than Line- Side Incident Energy?
There are, nevertheless, some few instances where the load-side incident energy is higher than the line-side incident energy. This might occur when there is a large motor contribution on the load-side bus. An example of this situation is shown on Figure 4, where we have several small motors (less than 50 HP) totaling 765 HP modeled as a single equivalent motor connected directly to the bus of a motor control center (MCC). Under these conditions, the load-side incident energy is more than twice as large as the lineside incident energy.
Modeling the Line-Side vs. Load-Side option in the Arc-Flash module of SKM Power*Tools for Windows software
One possible option to model the load-side vs. line-side incident energy is shown in Figure 4. Place a bus immediately upstream from the main OCPD. You could rename this bus with the name of the electrical equipment and add some indication that shows that this bus is the line-side bus. In Figure 4 the line-side bus is designated as "MCC-1 (LINE SIDE)". We want the load-side of this main OCPD to be connected directly to the bus of the electrical equipment. In Figure 4 that bus would be MCC-1. But in the Power*Tools software we can't have an OCPD connected directly between two buses without an impedance. To solve this challenge, we can insert a non-existing impedance (cable) between the load-side of the main OCPD and the equipment bus. This cable does not exist, so it should be modeled with very low impedance and it's symbol should be changed to show just a line (See Figure 4). Now you can calculate the line-side incident energy and the load-side incident energy by using the Arc Flash software module and use the options as shown in Figure 5.
After the Arc-Flash module finishes the calculations, the results table will show the load-side incident energy at bus MCC-1 (2.2 cal/cm^2) and the line-side incident energy (0.99 cal/cm^2). See Figure 6.
In this particular case the label attached to the equipment should show the higher incident energy calculated for the load-side bus.