Coordination Time Intervals

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timm333

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I have a question about the protective device coordination. There should be certain coordination time interval (CTI) between the TCC's: for example 0.12 sec between relay and breaker, 0.25 sec between relay and relay, 0.3 sec between fuse and relay, and so on.

But usually in the coordination examples, these intervals are not used, they only say that the upstream TCC should be to the right and above the downstream TCC. Is it normal to not strictly use the coordination time intervals between the TCC's? Thanks
 
Other than the IEEE Buff book from almost 20 years ago, no other national consensus type document mentions coordination time intervals that I know of.

The NEC does not indicate CTI, so most Engineers will just place the curve so there is some white or light (if you were still using a light table :D) between them.
 
Thanks. What is the usual practice that how much should be the maximum time interval between two TCC’s. For example, can the time interval be 2 Sec between the TCC of upstream fuse and the TCC of downstream fuse, or is it too much?
 
timm333 said:
Thanks. What is the usual practice that how much should be the maximum time interval between two TCC’s. For example, can the time interval be 2 Sec between the TCC of upstream fuse and the TCC of downstream fuse, or is it too much?
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One of the reason for CTI is to accommodate tolerances in the protective device operation. It is pretty important when you are dealing with different types of protective devices.
LV circuit breaker curves are usually drawn with a tolerance band so the gap between devices can be negligible.
For fuses it depends on if your curve is the minimum melt or the maximum clearing time or is it drawn with both. The gap becomes more critical if the curves show only one time..
Protective relays curves almost never take into account tolerances and clearing time of the controlled device. The gap is really important for electromechanical relays, but less so for electronic ones.

For most of my coordination projects, 2 sec is an eternity.
 
CTI is still taught in coordination classes today and is important if you want to avoid cross coordination.

As Jim mentioned, the digital relays do not require the tolerance band of the EM relays. But that is just for a portion of the CTI. The other portion deals with temperature variance, CT and PT variance, etc.

There are a handful of factors that go into the CTI and I would fail if I tried to remember them all at the moment.

IIRC, there is a current white paper on it but I am not where I can look it up. Try SEL 1st then Cooper, GE, Basler. That's the search order I would probably use if I were in my office. It may be from a relaying class I've been to in the last few years but it all blends together after a while.

If you can't find it let me know and I can hunt it down later. I'm pretty sure one of the others members would be able to find it before I have a chance.
 
timm333 said:
I have a question about the protective device coordination. There should be certain coordination time interval (CTI) between the TCC's: for example 0.12 sec between relay and breaker, 0.25 sec between relay and relay, 0.3 sec between fuse and relay, and so on.

But usually in the coordination examples, these intervals are not used, they only say that the upstream TCC should be to the right and above the downstream TCC. Is it normal to not strictly use the coordination time intervals between the TCC's? Thanks
Click to expand...
You can find it in the IEEE 242:2001 table 15-3
 
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