What are they doing?

[viewer]

http://www.sciencedump.com/content/...ail&utm_term=0_ab3c58e0d1-023f5f0598-57226533

I understand the principle of a Faraday cage, but that doesn't seem to fit the pictures. On the other hand, they can't really be risking high voltage discharge to earth through the crane can they? Can anybody explain please?

 

[westie101]

Live line repair team, using a fully insulated platform. In the a lot of countries they operate live at these voltages, sometimes using helicopters to access the cables.

http://www.youtube.com/watch?v=GxDKVAYJ_t4

We have teams in the UK doing this at 11kV.

 

[viewer]

Ah, so insulated platform. Many thanks.

 

[SimonH2]

And key point is that those white suits they wear are conductive - wires woven into the fabric. Part of the pre-work safety checks is to ensure full connectivity between different sections (there are (for example) press studs linking gloves to sleeves). That gives the operator a full faraday cage to work inside.

According to some goggle box programmes I've watched in the past.

 

[Porque223]

The helicopter has a tiny capacitance to ground and so there will be AC current flowing in an arc between the heli and the line. The current may not be enough to be fatal but from the look of the arc that I've seen drawn it looks to be pretty nasty.
To figure the capacitance you need to know the area of the helicopter footprint and the height above ground, then the 50 Hz gives you the reactance, then the line megavolts gives you the current in the arc.

 

[JohnW2]

Live line repair team, using a fully insulated platform.

If the platform is fully insulated, is the current in those arcs to their gloves getting to ground solely via the capacitance to ground?

Kind Regards, John

 

[westie101]

is the current in those arcs to their gloves getting to ground solely via the capacitance to ground?

I expect so.

On the lower voltages as clearances are smaller the conductive suits are not worn, rather they use insulated gloves & garments

 

[bernardgreen]

Most of the current flow from wire to helicopter is due to corona discharge from points and sharp edges on the helicopter discharging into the air.. To prevent this flowing through the worker's protective clothing the first task is to connect the helicopter to the wire using a large crocodile clip.

http://en.wikipedia.org/wiki/Ion_wind

http://membres.multimania.fr/plasmapropulsion/Historical_&_Basic/Electric_Wind.htm

 

[JohnW2]

is the current in those arcs to their gloves getting to ground solely via the capacitance to ground?

I expect so.

That would certainly seem the only explanation.

On the lower voltages as clearances are smaller the conductive suits are not worn, rather they use insulated gloves & garments

If the platform really is 'fully insulated', do I take it that the capacitance from the workers to ground sufficiently high that they need protection (of any sort)? ... I think I would probably have guessed not.

Kind Regards, John

 

[westie101]

If the platform really is 'fully insulated', do I take it that the capacitance from the workers to ground sufficiently high that they need protection (of any sort)? ... I think I would probably have guessed not.

At 11kV the phase to phase clearance can be about 0.8m so the risk of contact between two phases has to be taken into account, so a metallised suit would be a "risk" hence the use of insulated gear

 

[JohnW2]

At 11kV the phase to phase clearance can be about 0.8m so the risk of contact between two phases has to be taken into account, so a metallised suit would be a "risk" hence the use of insulated gear

Right. That makes sense. Thanks.

Kind Regards, John