It is pretty well known that setting protective devices for selective coordination often conflicts with settings for reducing incident energy. Assume there are no maintenance switches or other similar devices available. This week's question of the week is from WBD

Given the choice between setting devices for improved coordination or to reduce incident energy, which criteria is/should be more important?

Coordination
Reducing Incident Energy
It depends

I imagine this might stir up a bit of discussion or debate.

I voted for coordination. I believe code even requires that you properly coordinate your system. Having a system that allows for nuisance and unnecessary trips is not going to do anyone any good, even if your incident energy is lower in places.

When possible, if I can maintain proper coordination I will adjust settings to reduce incident energy. Especially at times when I am near a threshold (such as 8 cal/cm^2). But I'm not going to mis-coordinate an upstream device just to achieve that.

It's better to have a properly coordinated system and require the workers to use a higher level of PPE and care than to have an improperly coordinated system.

Well, this certainly does inspire some discussion. I did vote reduce Incident energy. The reason I did, is because in an arc flash event, at the point of the event, you are relying on your upstream device to open and clear the fault within specified times. So even if your upstream device is coordinated properly it should open and clear the downstream event. It all depends on the type of upstream OCPD you have,and wether you have breakers and fuses, fuses and fuses or breakers and breakers, so in other words depending on the LT/ST and IT settings, it may cause that device to operate just a bit slower if the settings are what are required for coordination. Hope that is not too confusing. It is a fine balancing act.

Absolute, ideal coordination is not always possible, especially when you mix fuses and circuit breakers, as anyone who has done coordination studies can attest to. I would add to this that even equipment combinations are not ideal outside of any question of coordination. For instance if there is just one large motor load on a transformer where the transformer is sized reasonably close to the size of the motor, rather than sizing the primary overcurrent protection device to protect the transformer, one has to instead size it to avoid inrush from the motor. The primary protection device thus only provides short circuit protection and no overload protection. Overcurrent protection must be provided by protection on the secondary side only. I can't tell you how many times when I typically have one or two 1500 HP motors running on a 2500 kVA or 3750 kVA transformer, the line fuse sizes will come in grossly undersized because the design engineer looked at the transformer in isolation to the loads.

I voted for "It Depends".

If its legally mandated that we coordinate such as a NEC 517 or NEC 700 situation, then coordination rules.

If its a commercial facility that doesn't have in-house electricians, then I would probably opt to lean towards coordination. The owners want to minimize nuisance trips. Typically, the problems you have tend to be the 20A branch circuit to a light or something equally small tripping the building main off line. So having a GF function for the main is important, then making sure the GF is set to coordinate against the 20A branch breakers. That will take care of 95% of all nuisance trips of the main.

If its an industrial facility with in-house electricians, I would tend to set adjustable protective devices down to get fast trips & get as much coordination as I can. I've found that sometimes its possible to look at the arc fault on the TCC's and get a fast trip for an arc and still get coordination for a short-circuit. Not always, but sometimes. But usually, you end up sacrificing coordination in order to get the PPE level down. If you have 2 or 3 levels of GF relays in your system, you can get them to coordinate, and again, you've probably taken care of 90% of your nuisance trips of upstream devices. In reality, most faults or failures will be ground faults. 3-phase or LL faults are most likely to occur when someone is working on an energized system and in that case, you want the fast trip & don't care as much about coordination.