What clearance would you need for a power line ?

[pragmatist]

The obvious answer is surely "as much as possible". However we've never been under a power line - skimmed under the odd bridge but they don't fry you. So what is the minimum clearance you would give to a power line ? We are rather interested in a mooring - we have air draught of 17 metres and there is a power line shown as 21 metres down stream. Will we fry ?

TIA
Penny

 

[pragmatist]

Whoops ! Just found http://www.ybw.com/forums/showthread.php?263651-HV-power-cables-safe-distance which gives lots of information and considerable peace of mind

Reminder to self - always use search facility before opening mouth/putting fingers to keyboard

 

[sarabande]

Weir Quay ?


4m clearance at high water sounds OK, but I would be leary of additional aerials and windvanes which may act as discharge points (triggers) especially on soggy days.

The HSE recommendations are more stringent for agricultural use. 7m for the main 275 / 400kV distribution lines !

http://www.google.co.uk/url?sa=t&rc...M3Gh6KFjD6DyaNA&bvm=bv.79142246,d.d2s&cad=rja

 

[Blue Drifter]

I went through Cuan Sound twice in the summer and I must admit it all looked very close looking up the mast from the boat, however according to the figures there was plenty of clearance (18 m). I had done the sums but in real life it looks worryingly close!

 

[NormanS]

Generally, the published heights of cables are the worst case scenario. They allow for sag due to extremely high temperatures, and also make allowances for exceptionally high tides, due to surge or low atmospheric pressure. It's still not nice going under if there is any doubt.

I had a long correspondence with SSE about a power line which crosses over a bay in Harris. The charted height of the cable is 11 metres, but of course it looks, and is, for the reasons given above, much higher than that. Further into the bay there is a slipway, which is otherwise easily accessible, and has a water supply c/w hose. Unfortunately this is an accident waiting to happen. One would expect a cable crossing a passage, channel, or river, to have warning signs with safe clearane, but this is just a power line which takes a sneaky shortcut over an inlet, so there are no warnings.

 

[Tintin]

The clearance us given for worst case at the maximum sag point of the catenary, and at HAT (highest astronomical tide -rarely acheived).

A simple solution, depth and width of channel allowing, is to sail away from the catenerary centre.

In the case you are asking about the channel is some way away from the catenerary centre so there is significantly more clearance than the 21m min figure.

 

[William_H]

Well many years ago I tried a negative 30cm clearance ona 12KV power line. It did not turn out well. Much smoke nd flames until the forestay melted and the mast fell down. Dangerous obviously but not the instant death you might imagine to the crew provided you don't touch anything.
However mast head electrics got fried. good luck olewill

 

[knuterikt]

I checked two different charts over a place we often go under a power line and a bridge.
Raster chart
Bridge 19 Power line 21.

Now look at the vector chart
Bridge "Overhead clearance 19.0" power Line "Safe clearance 21.0"

Notice the wording in the vector chart, my mast height is 17 meters, no problem going under both bridge and power line.

 

[Niander]

Used to live on Luing your bringing back memory's of that little car ferry!

 

[Ian_Edwards]

"Blue Drifter;5026548]I went through Cuan Sound twice in the summer and I must admit it all looked very close looking up the mast from the boat, however according to the figures there was plenty of clearance (18 m). I had done the sums but in real life it looks worryingly close!"

The first time we went through Cuan Sound with Eynhallow, at high water slack, I stopped and double checked the clearance. Navionics Charts don't give a clearance, but both the Admiralty chart and Reeds says 35m (in Black). We've been through multiple times this season with a mast clearance of 22.5m plus the VHF antenna, and we're still hear to tell the story. I'd been through many times with my previous boat with a mast clearance of 18m, so I didn't think to check the first time I went through with the Southery 46, but as Blue Drifter say it looks very close and I have to admit to a moment of panic, so I stopped, to double check both the paper chart and Reeds.

I also have to admit that I'd assumed that the clearance on power cables was a safe clearance, interesting to see that that is not always the case. The chart for Cuan Sound says 35m in black, which still gives us about 11m (including the VHF) at HAT. So I guess that's safe.

 

[pragmatist]

The clearance us given for worst case at the maximum sag point of the catenary, and at HAT (highest astronomical tide -rarely acheived).

A simple solution, depth and width of channel allowing, is to sail away from the catenerary centre.

In the case you are asking about the channel is some way away from the catenerary centre so there is significantly more clearance than the 21m min figure.

Thanks Comrade ! Sounds fine - 'spec we'd just run aground instead And thanks to all for your kind responses. Differs slightly from getting under the Swale bridge to Sheppey where you had to hang a bucket in the rigging to indicate your wish for the bridge to be raised !

Penny