# Thread: Time current curves- why?

1. ## History

If you want to know "why" then you need to consider how relays got their start. Review some of the history. Someone invented this magnetic device that integrates current over time to provide fault protection; do you understand how it works? They made design trade offs. Each designer and manufacturer made different relays, each with different time characteristics, because that was the state of the art. That is how it all started.

2. Originally Posted by mbrooke
General question- why do we have different time current curves? Ie Inverse, Moderately Inverse, Extremely Inverse, ect? I've always imagined that multiples of rated current and time dial settings are enough to acheive selective coordination.
Current multipliers and time dials, while providing useful curve modification, do not provide the same functionality as the inverse characteristics.

The inverse characteristics are useful for a few functions that come to mind right off. First, an extremely inverse curve is similar in shape to a distribution fuse and thus coordinates well with both fuses and traditional reclosers.

Second, a utility distribution system has fault current levels that are mostly a function of line impedance and thus a function of distance from the source. This would less true for impedance-modified systems, short systems, or systems near a relatively large generator. The speed increases as the current increases (as we get closer to the station) and so the inverse curves improve zone coordination with down-line devices.

They also provide better reaction to cold-load pickup. The sensitivity is less during the short times of temporary overload caused by cold load pickup.

3. Originally Posted by mivey
Current multipliers and time dials, while providing useful curve modification, do not provide the same functionality as the inverse characteristics.

The inverse characteristics are useful for a few functions that come to mind right off. First, an extremely inverse curve is similar in shape to a distribution fuse and thus coordinates well with both fuses and traditional reclosers.

Second, a utility distribution system has fault current levels that are mostly a function of line impedance and thus a function of distance from the source. This would less true for impedance-modified systems, short systems, or systems near a relatively large generator. The speed increases as the current increases (as we get closer to the station) and so the inverse curves improve zone coordination with down-line devices.

They also provide better reaction to cold-load pickup. The sensitivity is less during the short times of temporary overload caused by cold load pickup.

I fully understand- but why not have only one curve- extremely inverse type? What advantage does standard inverse have over extremely inverse is my question I guess. Why can't everything be extremely inverse? When would you use standard inverse?

4. Wouldn't greater inversity {is that a word?} result in less AIC ..???

~RJ~
Last edited by romex jockey; 09-15-18 at 08:12 AM. Reason: can't spell....

5. Originally Posted by romex jockey
Wouldn't greater inversity {is that a word?} result in less AIC ..???

~RJ~
Fault incident energy?

6. Originally Posted by beanland
Someone invented this magnetic device that integrates current over time to provide fault protection; do you understand how it works?.
Originally Posted by mbrooke
Fault incident energy?
I don't know how it works

I'm asking if there's some relationship between aic (or fie)? and what reacts quicker to it?

~RJ~

7. Originally Posted by mbrooke
I fully understand- but why not have only one curve- extremely inverse type? What advantage does standard inverse have over extremely inverse is my question I guess. Why can't everything be extremely inverse? When would you use standard inverse?
Depends on what else you are coordinating with and also the feeder characteristics. Not everything has the same slope in the same overcurrent range.

8. Originally Posted by romex jockey
I don't know how it works

I'm asking if there's some relationship between aic (or fie)? and what reacts quicker to it?

~RJ~
There used to be saturation elements to provide relay inversity IIRC but now it is digital math. As for inversity on the line, the fault current decreases with distance of course so we want the inversity to help with zone separation.

9. saturation elements?

even google is failing me Mivey....

~RJ~

10. Originally Posted by romex jockey
saturation elements?

even google is failing me Mivey....

~RJ~
I'm not near my references at this time and will have to look later. IIRC, and I may be completely off base, the response would change as the cores approached saturation. Take this for what its worth until I can get back and look through older texts.

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