SEL-451 and Frequency Testing Issue

A customer was having problems timing a SEL-451 with a delay setting of 8 cycles. He was measuring a 12 cycle delay when transitioning from 59.5 to 58.5 Hz.

If you look at the spec sheet, the SEL-451 frequency element has a “Maximum Pickup/Dropout Time: 3.0 cycles”. When you add up all of the delays, you are looking at a possible 3.87 cycle delay when testing this relay. Remember to watch the specifications when performing high-speed tests!

The specification also has a “Slew Rate: 15 Hz/s maximum” which means the frequency calculation will only be accurate if the frequency changes less than 15 Hz in a second. An instantaneous transition from 59.5 – 58.5 exceeds that setting which probably caused some problems as well.