LV/MV Protection System Coordination

jim dungar

Staff member
Retired Engineer
Sometimes I start at the bottom and work my way up, other times I start at the top and work down.

It depends on the facilities acceptance of an occasional relatively easily recoverable outage (reset devices, restart production) or an occasional long duration outage (replace equipment).
The reality is usually somewhere between these two extremes.


Senior Member
Agree. Start from the bottom, then coordinate those protections upstream. Redo and change the settings below if the topmost, upstream settings are not acceptable.


Senior Member
I always coordinate between MV and LV.
My preference is almost always to reset a LV breaker than a MV breaker for a downstream event unless somehow the incident energy at the MV breaker is lower and we can have reasonable selectivity otherwise.


Curmudgeon still using printed IEEE Color Books
North of the 65 parallel
EE (Field - as little design as possible)
Disclaimer: Experience is Industrial only - mostly self generation
Always from the bottom up. The rest of this post is are just random thoughts - neither complete, nor rigorously true.

Coordination is defined as "daylight between the curves"
The MV feeder to the transformers has to be inside the transformer damage curve.
Sudden Over Pressure/Rapid Rise really aren't protecting the transformer. If these trip there is no transformer to protect, just a fire to put out.​

All the main feeders - the trips have to be inside the cable damage curves
Ratios closer than 2:1 get hard to coordinate.
HRG gets rid of a lot of murderess, slow tripping, ground faults (480Y HRG)
Coordination down to the instantaneous range gets really difficult. If that is a must do, start looking at zones.
You can coordinate anything - it is just a matter of money.

Generators are difficult. The SCC is always low. HuH? why is that bad?
2MW, 480Y gen sub-transient reactance is ~ 20%. SCC is 12KA - not too bad
1MW, 480Y gen sub-transient reactance is ~ 20%. SCC is 6KA - This is getting in the range where the CBs may not trip quickly. Not bad, just have to pay attention.​

Rest is not exactly part of the coordination:
Keep track of the Short Circuit Currents/Arc-fault. A 38 Cal/cm^2 fault while wearing a 40 cal/cm^2 suit, leaves the worker with survivable second degree burns. Maintenance switches to reduce arc flash are a good thing.

Low %IZ transformers SCC get insane really quick. For example:
1000 kva, 480Y, 5%IZ, SCC = 24kA, not bad
5MVA, 480Y, 7%IZ, SCC = 86kA, yuck
500kva, 208Y, 2%IZ. SCC = 69KA, yuck​

Tell the designers to run the MV feeders out into the plant. Put smaller transformers out there - not one big one in the middle. When they tell you that is too expensive, tell them it is a $100M job, all you are asking for is $100.5M. And the life-cycle maintenance costs will more than pay off the small capital increase.