Incident energy is determined by several factors, but the most important are distance from the arc, magnitude of arcing current and duration of the arc.
An instantaneous overcurrent element will trip the upstream breaker with no intentional time delay whenever the current increases above its setpoint. Compared with a time overcurrent element, the instantaneous element will operate much faster, reducing the arc time and lowering the incident energy.
However, use of instantaneous elements can seriously compromise coordination with downstream protection, so there is generally a tradeoff required. Use of a "maintenance switch" can be used to activate the instantaneous trip only when maintenance work is being done on the equipment.
As Dave said, this is not something that you do as part of a system design to lower the incident energy on a permanent basis, it is something you do temporarily when someone needs to work on equipment that cannot be de-energized. Some electronic trip circuit breakers are now available with that "Maintenance Mode" input option that allows you to externally (via input contact closure) lower the instantaneous trip threshold during that work period.
But you cannot leave it that way permanently or you run the risk of a system shutdown from nuisance tripping. What some people do is provide a laser scanner in front of the switchgear that automatically enables the Maintenance Mode when someone enters the Limited Approach Boundary so that when they walk away, they don't forget to turn the Maint. Mode off.
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Instantaneous trip without jeopardizing coordination is possible by dedicated arc-flash protection, based on simultaneous sensing of arc-flash light and overcurrent. Very common in some countries, both in utility as well as industrial installations (MV and LV).
I agree with Maak as soon as we are talking for switchgear (most commonly MV). Keep focusing in switchgear let me also mention the pressure switch that trips also the breaker in case of internal fault and it is, according to manufacturers, as quick as the flash detection.
Instantaneous protection is mainly used by protection engineers in order to avoid unnecessary thermal stress to healthy equipment and helps in restricting the spreading of damage in the faulty node.
Instantaneous trip without jeopardizing coordination is possible by dedicated arc-flash protection, based on simultaneous sensing of arc-flash light and overcurrent.
