Line and Bus Side
Because the arcing current needs to be interrupted by a fuse or circuit breaker, it matters where the arc flash occurs. In a typical switchgear configuration there are two layers of protection: the main incoming feeder and smaller outgoing feeders. The picture above shows how these can affect the incident energy on a per-panel basis.
Line Side
In the incoming feeder panel (in red), the arc flash might occur on the terminals of the incoming cable and will not be switched off by the main incoming circuit breaker in the same panel. This is called the line side of the circuit breaker and usually has a higher incident energy associated with it.
Bus Side
Most of the switchgear will be protected by the main circuit breaker. This determines the incident energy at the bus side. Note that in this case this includes outgoing panels with protective devices installed in them, so that when the arc flash event occurs on the incoming side of the fuses, they will not interrupt it.
Load Side
Finally, the load side (in green) is the part that is protected by outgoing feeders and has the lowest incident energy associated with it. It only applies when there is adequate separation between for example a cable panel and the unit protection panel, in such a way that an arc flash event does not propagate or pose a hazard on the other side of the separation. Load sides are often neither calculated nor labeled.
