Overhead transmission line protection failure

[Tee]

Hello everyone ,

I am still new to the protection field so i would appreciate if you can help me with analysis of the following,

A 13.8 Kv OHTL was cut and fell on ground , the cable was still live although being protected by Earth fault relays and Over current relays(Instantaneous and Time delay) , but for some reason they failed to operate.

This incident caused death of over 30 camels in Saudi Arabia.

This line was located in the desert and the marks on the sand showed that it stayed on ground for longer than one day.

What do you think could possibly cause EFR and OCR not to operate?

All opinions are much appreciated.

Thank you in advance

 

[berkeman]

The Earth ground fault will only trigger if there is a good ground fault, so if the wires landed on dry sand or dirt, the GFCI may not trip. Also, if both wires fell together, then the current might have looked differential, and again the GFCI would not trip.

 

[dlgoff]

I agree with berkeman. High resistance of the soil(sand) could cause both relays to not sense the downed lines. The gound potential rise near the lines would cause the animals to be killed. With there long steps, there would be a large potential difference between the legs.

 

[Tee]

Thanks for the replies , that was really helpful.

 

[collies_99]

Also:

Non directional ground relays may operate is the setting is below the current unbalance setting. Normally they set above to prevent inadvertent tripping during high demand period, typical in feeder circuits fro their neutral protection.

Tansmission system are trated as a balance circuit which can be set much lower in percentage terms to feeder circuit rating and often can be set to operate for broken conductor or single line to groundf ault with very low fault current on dry terrain or high ground impedance ... abnormal case for setting describe above.

 

[a7med_mos6fa]

good analysis
but
what are the suggested solutions for this problem in desert ?