Relay

When a relay has only cables as downstream equipment, the Tap is set to the specified value times the ampacity of cable.

When a relay does not have dependencies, its time dial is set such that the relay trips close to the specified time at the maximum short circuit current available at the CT for the relay. You can also have the time dial set at the specified value instead of basing it on the short circuit current.

The instantaneous is set to the specified multiple of maximum short circuit current.

The desired minimum time difference between two relays is the sum of Over Travel time, Safety Margin, Auxiliary relay time, and circuit breaker opening time. The time dial settings available in a relay may not provide the exact desired minimum time difference. This Minimum Gap Tolerance enables you to set the time delay of the relay up to the specified percentage higher than the desired minimum time. For example, the desired time difference between two relays is 0.283 seconds. If the Minimum Gap Tolerance is 20%, the program lets the upstream relay time dial to be set such that the time difference is not greater than 0.33 seconds (0.283 + 20%).

This is the factor used in setting instantaneous pickup. A factor of 1.1 corresponds to a 10% tolerance. For example, if the downstream relay has instantaneous pickup of 3000 Amps, then the upstream relay pickup needs to be higher than 3300 Amps (3000*1.1).

Solid state relays typically have dc offset filters so that trip characteristics are based only on the AC component of currents. The relay library has data indicating whether the relay has a dc offset filter. The DC component may not be completely filtered.

When this check box is selected, the program uses the asymmetrical currents (with dc component) it calculates. If the check box is not selected, the offset specified in the field below is used.

If the Use Calculated SC Asym Offset check box is selected, then the specified value will not be used.