What causes an arc flash to be big or small?
We calculate arc flash "size" by following the equations described in IEEE 1584 2018. These equations are based on real life testing and measuring of arc flashes.
Below is a simple explanation of what can cause the arc flash size/severity to differ.
First of all, it's important to decouple arc flash hazards from shock hazards. Just because there is high voltage, does not mean there is high arc flash potential, in fact many times it is quite the opposite.
The biggest contributors to an arc flash are fault current and clearing time.
Fault current is determined from your utility, it's the amount of current they can supply in the event of a fault. This is the starting point, now move through your system, if you have a really long wire (lots of resistance) the amount of available fault current on the other end is going to be less. If power goes through a step-down transformer, voltage goes down, available current goes up. Available fault current is calculated at each point in your system.
Clearing time is the amount of time it takes for an overcurrent protective device (i.e. breaker or fuse) to open in the event of a fault. Imagine a fault occurs and an arc flash begins to grow, it will continue to grow until the power is removed from the system, the power is removed by the automatic operation of the upstream overcurrent protective device. This event happens very quickly. Each breaker, each fuse, each wire has characteristics specified by manufacturers which allow us to also calculate clearing time. This article goes into more detail regarding clearing time: https://www.70econsultants.com/how-reducing-clearing-time-reduces-arc-flash-hazards/
Other examples of variables include the size of the gap of the conductor where the fault is occurring or if the arc coming from a box or in open air (i.e. overhead power lines). A person's distance from the arc will drastically change the severity of the arc (i.e. the closer you are to the source of the arc the more intense the arc).
Sometimes a high available fault current can cause a very large arc flash, other times it can cause a breaker or fuse to clear very fast and result in a small arc flash. In other instances a low available fault current may create a small arc flash or it can cause a breaker or fuse to take a long time to clear, resulting in a large arc flash.
The learning should be that two pieces of equipment may appear to be similar or even identical, but if the fault current is different at each point or if the feeding breakers are set differently, the feeding transformers have a different impedance or if the feeding wires are different size/length, you will have a different resultant arc flash. This is why we use professional engineers and the leading engineering software to perform calculations for you.