Multiples of tap value current in IDMT Curves

[charz]

Dear All,
When the relay current equals the pickup current, the relay should start picking up,i.e (when the psm is equal to or greater than 1) but why does all the curves begin at 1.5 x tap setting or 2 x tap setting in the reference curves provided by the relay manufacturer; why not at 1 (or) 1.1?

 

[davidbeach]

For an electromechanical relay the performance just above pick up is rather variable, but we don't particularly care if 1.01pu results in 5 minutes or 20 minutes. By the time the current is up to 1.5pu the performance is much more predictable and repeatable. You'd probably also have the curve going off the top of the TCC plot unless you added another order of magnitude or two to the time axis. With numeric relays the repeatability is there but if you really care about just above pickup you'll find it easier to calculate the trip time than to read it off the TCC.

 

[stevenal]

See C37.112
t(I)=A/(M^p-1)+B where M is the multiple of pickup, T(I) is the trip time, and the other constants are determined by the curve shape. You can see you have a division by zero problem at M=1, so t(I) is undefined.