PA-0616000C SCHNEIDER

Ring Main Arrangement 

A particularly common arrangement within distribution networks is the ring main circuit. The 

primary reason for its use is to maintain supplies to consumers in the event of fault 

conditions occurring on the interconnecting feeders. 

A typical ring main with associated overcurrent protection is shown in the following figure.

As with the previously described parallel feeder arrangement, it can be seen here that 

current may flow in either direction through the various relay locations. 

Therefore, directional overcurrent relays are again required in order to provide a 

discriminative protection system. 

The normal grading procedure for overcurrent relays protecting a ring main circuit is to open 

the ring at the supply point and to grade the relays first clockwise and then counterclockwise. The arrows shown at the various relay locations in above figure depict the 

direction for forward operation of the respective relays, i.e. in the same way as for parallel 

feeders, the directional relays are set to 'look into' the feeder that they are protecting. The 

above figure shows typical relay time settings (if definite delay time co-ordination was set), 

from which it can be seen that faults on the interconnections between stations are cleared 

without any discrimination by the relays at each end of the feeder. 

Again, any of the three overcurrent stages may be configured to be directional and coordinated as per the previously outlined grading procedure, noting that IDMT characteristics 

are selectable on the first and second stage. 

Note that the above requirements outlined for the parallel transformer arrangements are 

equally applicable for plain feeders, which are operating in parallel.