Distribution Line Protection Question

[coloradog1]

Hello,

It seems that the pickup setting for time-overcurrent relays protecting overhead distribution lines is typically well above the ampacity of the conductor. Can someone explain why this is?

Is the thermal capacity of the line greater than the ampacity given in tables?

Thanks for the help.

 

[jghrist]

The relays are set to trip for faults, not to protect against overloads. Time-overcurrent pickup must allow for temporary overloads.

 

[jtester]

Is there a requirement in the NESC or elsewhere that requires utilities to protect against overcurrents?
I'm not aware of one.

Jim T

 

[davidbeach]

Dealing with overloads is the responsibility of the operators and dispatchers. Dealing with faults is the responsibility of the protection system. When you get to transmission systems, the FERC/NERC rules are quite clear, thou shalt not trip on load (including some pretty severe overloads).

 

[rbulsara]

Medium voltage (and higher) system O/C protection is usually for short circuit currents only. Typical "overload" is addressed by design.

Rafiq Bulsara

http://www.srengineersct.com

 

[bacon4life]

Lines are typically rated on the "worst case" combination of high ambient temperature, maximum solar heating and lowest wind speed, however those don't alway exist during the overload event. Sometimes the thermal capacity is greater, once in a while it is actually less. For example see page 5 of the link.

With a thermal time constant on the order of 15 minutes, operators often have an opportunity to move load around before the wires exceed the allowable temperatures.

http://www.naspi.org/meetings/workg...ons/stelmak_valley_emerging_apps_20091008.pdf