Assume we have a radial system with two adjacent line segments as shown in Figure 4.9. The
segment farthest from the source is set as shown in Example 4.4. The relay protecting the next line
segment closest to the source must protect its own line and, if possible, back up the relays protecting
the next line. The pickup should therefore be for the same primary current as the downstream relay
and the time setting must coordinate with it. This time setting should be made with maximum
current conditions, i.e. assuming a three-phase fault, with maximum generation behind the relay.
In a radial system, all relays for which coordination is required must be examined for operation
at this same primary current. (Example 4.6 shows how to coordinate relays in a looped system.)
When coordination is achieved at maximum current, the shape of the inverse curves, provided they
are all of the same family of inverseness, will ensure coordination at all lesser current values.