OHL 220KV Accident 1

[FaizEll]

Dears

In order to have a technical point of view, I need your expertise to analyze an event that occurred during a construction site in progress.
Recently, during the construction phase, a crane boom hit a 220KV OHL, the power cut occurred immediately and no one was injured, the cable was not cut (a very severe cable hit). limit).

After the first view, we found that the crane's ground cable was connected to the foundation of a metal structure near the work area. There is not yet an earth network on the site so the earthing was provided by the foundations and the piles of the metal structure.
The clamps have partially melted and part of the earth cable coming from the foundation has melted (bare cable 70mm²).

The questions are: How strong was the short circuit likely? Do you have an idea of ​​the OHL shutdown sequence and the required outage time on this type of line (220 KV)? Was the metal structure dangerous at the time of the collision? A worker had a guide rope in his hand, was he in danger?

Looking forward to receiving your opinions, please accept my sincere greetings

 

[System Protection]

It's not possible to accurately gauge the strength of the short circuit. You would need to talk to the utility that owns the line.

I'm not a lineman so I won't comment too much on the required outage time but the damage doesn't sound too bad so likely less than one day.

Was the metal structure dangerous? Probably. The fault current was traveling through the boom, through the cranes ground cable, through the structure and into the ground back to its source. Any impedance that it encounters will create a voltage (potential). Do a search for step and touch potential.

For the worker on the ground there is danger regardless of the guide rope due to the step potential.

Did you contact the utility prior to the work being performed?

The crane operator should also have a safety watcher on the ground solely dedicated to keeping his boom away from the high voltage lines.

 

[bacon4life]

Typical fault currents range from 10,000 to 50,000 amps, although higher or lower values present in some locations.

Some 220 kV lines detect the sort circuit and shut down in less than 0.1 seconds. If the line lacked telecommunications aided protection or had abnormal conditions, the clearing may take as long as a second or two. Many transmission lines automatically reclose, so it is also possible the circuit could have been re-energized a couple of times. The utility should be able to provide approximate magnitudes and durations of the fault current. Since the 70mm² cable melted, the duration and/or magnitude of the current exceeded the short circuit rating of the cable.

Typically the line can be manually re-energized as soon as utility employees visit the site to verify no damage occurred to the transmission wires and that the crane has been relocated to safe distance. Could be less than an hour if someone happens to be close by. There might also be delays if the federal worker safety inspectors need to visit the scene before anything can be moved.

The structure may have had dangerous touch and step potentials during the fault. Any nearby worker could have been in danger.

 

[FaizEll]

Thx Experts.
Any other additionnal comments?

 

[stevenal]

If the construction had been properly communicated to the transmission line company, the line would or should have been placed on hold. This would involve disabling automatic reclosing.
Where in the world did this occur?

 

[wcaseyharman]

You can’t gage the fault current from the cable damage because you don’t know the clearing time, the damage will be a combination of both. The utility should know both the clearing time and the fault
magnitude either by pulling events from the relays or a short circuit study.

Typical US utility protection are distance (impedance based) relays (ANSI/IEEE 21 relay) that clear for phase faults or ground fault overcurrent (51N) or ground fault distance relays to detect ground faults. If this is a networked system relays at each end of the line would register the fault and trip the breakers. Some lines have communications as others have mentioned that may reduce tripping time, such as line current differential.
You’d again need to discuss with the utility to know what they have.