Definite Time Relay Time Selection

[WiringBoy]

Hello,

I came across this document published by CEATI international, Distribution Overcurrent Protection.
I went through the whole 240 page document and found some interesting information about coordination between fuses, reclosers and station breakers. One of interesting arguments was to have a time delay element instantenous overcurrent element with .067 secs delay.

What I want to know is the impact of a time delayed 50 element on the equipment damage curve. It cannot be visually imagined. Could you please shed some light on this concept?

Thanks

 

[burnt2x]

It's just what it is. When you don't want to black things out when a fault down the line occurs, you coordinate all the fault protection devices in that line. It means the fuses nearest the fault should drop-out first (no time delay) while those upstream will be designed to act just in time, but not above the equipment damage curve of the line or other equipment carrying the fault current. Remember, equipment damage is function of the amount of fault current and the time the fault current is allowed to persist in your system (I²t).

 

[dpc]

It cannot be visually imagined.

Take a log-log TCC graph. Plot the pickup of the instantaneous element. Go up the page until you find 0.067 sec and draw a horizontal line extending from the pickup point to the maximum available fault current. This "L" shaped curve is the response characteristic of the 50 element with a time delay. It's really pretty common.

We probably need a new term (in the US anyway) to replace "instantaneous". Even with no extra time delay the 50 element is never truly instantaneous. Then when we start talking about the "instantaneous element time delay", it can get confusing.

 

[racobb]

DPC you are correct. I usually discuss the instantaneous function as "no intentional time delay".

Alan

 

[DiscoP]

We are the same here, instantaneous means non intentional delay.

Therefore an instantaneous element with a time delay setting is a bit of a contradiction.

However most protection relays will allow a time delay on their "50" elements, and we often use a small delay for various reasons.

To help a little with this confusion, we use the term high-set for the 50 elements.

You can then use the terms instantaneous high-set or a time delayed high-set without contradicting this definition.

 

[burntcoil]

dpc: well known but excellent answer.