Low Voltage Coordination

[NickParker]

Upstream:
Breaker: ICCB
Rating: 800AF/400AT
Rating Plug (In): 400A (1.0 P.U)
Long time trip setting, magnitude (Ir), Adjustable: 300A (0.75 P.U)
Long time trip setting, time (tr): there are many bands. The TCC mentions trip time for all the bands at 1.5 times the pickup setting.

Downstream:
Breaker: MCCB
Rating: 250AF/250AT
Rating Plug (In): 250A
Long time trip setting, magnitude (Ir), Not Adjustable: As per the curve
Long time trip setting, time (tr): Not Adjustable: As per the curve

When I check the TCC of downstream MCCB at 1.5 times the pickup value, it trips between 90~200s
The TCC of upstream ICCB at 1.5 times the pickup value, for the band C9, the trip time is between 119~224s

Asking this, if this would cause any other problem just doing a manual check? Have attached the TCC curves of upstream and downstream

 

[dpc]

I'm not sure what you're asking. I can't get your link to work. You want the upstream breaker to be slower than the downstream breaker. If the long time trip portion of the curves overlap, that is a potential region of miscoordination - probably for a small range of current. It should be possible to adjust either (or both of the long-time delays) to eliminate any overlap. As long as there is a clear separation of the two curves they should coordinate.

Cheers,

Dave

 

[NickParker]

I have attached the TCCs again. I don't intend the upstream breaker to be slower than the downstream. Since I had no tools/software to plot it, I don't know just checking manually works or not.

My approach:

1) Check the ampere and its tripping time of the downstream breaker.
2) Check the time it takes to trip for the same ampere(as in 1) in the upstream breaker.
3) The time it takes to trip the upstream breaker should be greater than the downstream breaker.

Since the upstream TCC is so congested with curves, I cannot determine the trip time for the upstream breaker for the current which will flow at the downstream of the downstream breaker. However, the tcc mentions time at 1.5 times the pickup value.

How to approach this coordination?

 

[dpc]

If the time to trip is greater, then it will be slower. You've got the idea - compare trip times for different values of current. If you can find some log-log graph paper, you can do it the old-fashioned way.

 

[NickParker]

Hello Dave,

I would like to try the old - fashioned way. Could you please provide me the approach/guidelines/any resources to do it with the log-log graph paper.

I just have the TCCs of upstream & downstream curves with me now.

Appreciate any help!

 

[dpc]

We used this:

https://www.draftingsteals.com/485258.html

I think there are probably some available .pdf you can print. But it helps if the graph paper is vellum or semi-transparent.

You just trace each device TCC onto one log-log sheet, putting everything on a common voltage reference if necessary. If the curves overlap at any point of current, then they don't fully coordinate. Probably the old IEEE Buff Book or Red Book covers the manual process. Having a light table helps or holding everything up to a window. By sliding the published TCCs around under your graph paper, you can figure out the best settings.

It helps to plot the maximum fault current and maximum load current as boundaries. Don't forget motor starting current and transformer inrush current where applicable.

Cheers,

Dave