Selective Coordination Clarification

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xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Selective Coordination should be clarified to include coordination between phase and ground elements. Too often, TCC's are evaluated separately on a phase only or ground only basis. Also, whenever selective coordination is required between two OCPD's, any other OCPD in parallel with the lower of the two OCPD's (which may compromise coordination with the upstream OCPD), should be required to be selectively coordinated as well. Thoughts?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
The engineering group I worked for had a policy of including a minimum of a 30A thermal magnetic breaker on their Ground TCCs. The thought was this would cover the circuits most likely to be cause ground faults. Our experience was that GF selective coordination was rarely possible above a 70A thermal magnetic.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
The engineering group I worked for had a policy of including a minimum of a 30A thermal magnetic breaker on their Ground TCCs. The thought was this would cover the circuits most likely to be cause ground faults. Our experience was that GF selective coordination was rarely possible above a 70A thermal magnetic.

I hear you. Selective Coordination is exceedingly difficult - if not impossible with standard designs. The cost adds up quick with special devices/equipment, time going back and back and forth with designers/engineers, application engineering/consulting with manufacturers, etc. however, if a fully selective system is the requirement, I think it should be clearly articulated that phase and ground elements must be evaluated together rather than independently.

I’m going to go further and say it also needs to be performed by a licensed professional engineer due to it requiring a different standard duty of care and specialized skill.

I’m kinda sick of the “Eaton’s” of the world low balling a hospital job, doing a garbage study and not having accountability.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
I hear you. Selective Coordination is exceedingly difficult - if not impossible with standard designs. The cost adds up quick with special devices/equipment, time going back and back and forth with designers/engineers, application engineering/consulting with manufacturers, etc. however, if a fully selective system is the requirement, I think it should be clearly articulated that phase and ground elements must be evaluated together rather than independently.

I’m going to go further and say it also needs to be performed by a licensed professional engineer due to it requiring a different standard duty of care and specialized skill.

I’m kinda sick of the “Eaton’s” of the world low balling a hospital job, doing a garbage study and not having accountability.
Modeling software has made it much easier over the years...

You have a thick book of onion paper left over that no one uses anymore?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I’m kinda sick of the “Eaton’s” of the world low balling a hospital job, doing a garbage study and not having accountability.

Not much in 517 requires selective coordination....only coordination down to 0.1 seconds which is much easier and cheaper to accomplish than down to time 0 as required for selective coordination.
517.31(G) Coordination.
Overcurrent protective devices serving the essential electrical system shall be coordinated for the period of time that a fault’s duration extends beyond 0.1 second.

If a supplier is doing the study as part of selling the gear package, they will only do a "coordination study" and not a "selective coordination study" unless you specifically tell them what you want. There can be substantial differences in the gear package pricing between the two studies.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Not much in 517 requires selective coordination....only coordination down to 0.1 seconds which is much easier and cheaper to accomplish than down to time 0 as required for selective coordination.
Agreed. I was speaking generally in the second comment despite the thread being titled "Selective Coordination". Note: 517.17(C) makes matters difficult enough. This was my initial point. You can't evaluate phase and ground individually...

If a supplier is doing the study as part of selling the gear package, they will only do a "coordination study" and not a "selective coordination study" unless you specifically tell them what you want. There can be substantial differences in the gear package pricing between the two studies.
The problem is (as a PE) you are still required to comply with the law, irrespective of what the customer specified. Fully selective or even just coordination down to 0.1s may be impossible with the gear provided, so the fight turns to the equipment supplier and application engineer...
 
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