Now vs Wait: 50 Instantaneous and 51 Time-Overcurrent

8 min · difficulty 4/10

The same relay sees two faults: 9,000 A close to the bus and 800 A far down the feeder. Which should the instantaneous (50) element catch immediately, and which should wait for the timed (51) element? Decide first.

The instantaneous element catches the 9,000 A close-in fault with no intentional delay. A fault that big and that close needs to clear fast, and there is no downstream device to coordinate with at that current, so you do not wait. The 800 A far fault is handled by the time-overcurrent element on its inverse-time curve. It is lower current and farther out, where a downstream fuse or recloser may need a moment to clear first, so the 51 element gives that margin before it trips.

50 instantaneous and 51 time-overcurrent on a TCC A log-log time-current plot. A 51 time-overcurrent inverse curve in gold starts at a 600 A pickup and slopes down to the right (more current, less time). At the 50 pickup of 4,000 A an instantaneous element drops the operate time to about 0.05 s and holds it flat to the right. Current axis 100 to 10,000 A, time axis 0.01 to 10 s, both logarithmic. 50 instantaneous and 51 time-overcurrentlower on the plot = faster to trip1002005001,0002,0005,00010,0000.010.020.050.10.20.5125102050100current (A) at 12.47 kVoperate time (s)51 pickup 600 A50 pickup 4,000 A51 time-overcurrent50 instantaneoustime-dial shiftsthe whole 51 curve
On a TCC the 51 time-overcurrent curve trips faster as current rises, and above the 50 instantaneous pickup the relay trips with no intentional delay.

ANSI device 50 is instantaneous overcurrent. Once current crosses its threshold, it trips with no intentional time delay. It is built for high-current close-in faults where speed matters and there is nothing downstream to wait for.

ANSI device 51 is time-overcurrent, an inverse-time curve. The higher the current, the faster it trips. A small overcurrent times out slowly; a large one times out quickly. That inverse shape is what lets a 51 element coordinate with devices below it: a downstream fault draws less current at the relay than a close-in fault, so the relay holds back long enough for the downstream device to act first.

A 51 element has two adjustable settings. Pickup is the current threshold where the curve starts; below pickup, the element does nothing. The time-dial (also called the time multiplier) shifts the whole curve up or down in time, making every operation slower or faster without changing its shape. Set the time-dial higher and you buy coordination margin; set it lower and you trip sooner.

Pickup is the setting people get wrong first. Pickup must sit above maximum load current and below minimum fault current, so normal load never trips the relay but every real fault does. If a feeder peaks at 540 A of load and you set pickup to 600 A, normal swings stay clear of the curve. The smallest real fault still rises well past 600 A, so the relay sees it. Set pickup too low and load nuisance-trips the feeder; set it too high and a weak far-end fault never picks up. The two together, 50 for now and 51 for wait, are the foundation every coordination study is built on.

A coordination study is only as good as the device data behind it. DistroForge Insider covers protection equipment sourcing and the OEMs actually shipping relays, reclosers, and fuses on current lead times.

Question 1 of 3

Which overcurrent element trips with NO intentional time delay once current rises above its threshold?

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.