Relay Minimum Phase Trip Current

NFPA 72 or BS 71 or IS SP 30, and any local codes must be followed.

Using British Standard 7671, perhaps?

Hi Fazlay

The relay for OC overcurrent and EF earthfault needs to be chosen such that the current input to the relay matches the secondary current of the protection current transformers CTs.

The minimum phase trip current should have been established at the design phase of the system depending on how the load is fed, underground cable, aerial cable or a combination. The tripping time also needs to be set for various fault levels to account for close in faults and remote faults as does the minimum pick up level which accounts for short term motor starting loads which initially appear as short circuits on the phases if used as DOL direct on line.

The use of auto reclosing is dependant on the final load and the number of intermediate trip points between the supply and the end of line. Auto reclosing in my experience has been used on transmission lines where it is used to limit and isolate the number and causes of power outages. Auto reclossers usually have a number of instantaneous trips and time delay trips which hopefully give time for intermediate protection to operate and isolate the faulty section of line.

Regards Stef