Hi Everyone. I just got into this conversation with a few people yesterday and have a question.

I know you can model arc flash with neutral grounding resistors and they are suppose to eliminate single phase arc flash. However does anyone have any practical experience with these and how well they work, i.e. actual arc flash experiences (non experiences if I understand how they work) and maintenance issues etc?

Thanks everyone!

They do not eliminate single phase arc flash, they simply reduce the amount of current that can flow on a single L-N/G fault. Unless the fault is cleared in a timely manner the next single phase fault could actually be a L-L fault based on the full available fault current.

So two big issues with them are:
No line to neutral loads allowed. This makes it hard to implement in many existing facilities.
First faults must be cleared in a timely fashion, any benefits are non-existent until this occurs.

First faults must be eliminated...true, which is why alarm-only is not practical for most plants. Need to trip.

In my experience we had legendary problems with damage from failures BEFORE installing them. After going nearly 100% resistance grounding, we rarely have a lot of damage from equipment failures. When we do, the result is usually extremely spectacular because it is always either L-L or L-G-L. The "single phase" issue is generally a non-issue. You can't have 277 V lighting but you can still have 240/120 V transformers and they don't care if you don't have balanced current draw on the secondary side. Everything else (motors, transformers) are always balanced 3 phase anyways. Very small loads such as CPT's mounted in 480 V starter buckets might be single phase but they're not large enough to actually worry about them.

With high resistance grounding at least, you basically don't get arc flash faults with L-G faults of any appreciable amount. Since this accounts for an average of 90% of all faults, you are basically reducing the likelihood of an arc flash by a factor of 10. You don't get to reduce the arc flash incident energy (it actually increases slightly) but you can all but eliminate most of the events in the first place.

480/277 single phase loads are easily accomodated with 480-480/277V xfmrs.