Overhead power cables and air draft

Sanderling_

Sanderling_

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We are moving up to Weir Quay this summer, the power cables crossing the Tamar above Cargreen are shown as 16m clearance. Sanderling's air draft is 15.8m (plus vhf aerial I suspect).

Can anyone please clarify, is the charted MHWS clearance on power cables to the cable itself or to a safe distance from the cable? Is there any allowance for sagging? If the height is to the cable, what is the safe distance away from a high voltage power cable?

Thanks
Tony
 
Well, heres the advice from the National Grid for

"Leisure Activites near to Overhead Lines."

[ QUOTE ]
Overhead high voltage electricity lines are normally bare (uninsulated) and if an object gets too close it is possible that a ‘flashover’ can occur, where electricity will jump over a small distance to reach earth. This can kill or cause severe shocks and burns to any person nearby.

In order to prevent this happening, there are minimum safety clearances between overhead lines and the ground, roads or objects on which a person can stand.

Please take note of the advice below.

Never fly kites or model aircraft near overhead power lines
Be careful to avoid lines when operating light aircraft, hot air balloons, gliders etc.
Fishing rods and poles made from carbon fibre and similar materials will conduct electricity. Fish at least 30 metres or more away from overhead lines
Do not light fires beneath overhead lines

[/ QUOTE ]


So, looks like you need 30 meters clearance......only if your not on fire.


Seriously... dont forget this clearance can change..... if its hot the lines will expand, and sag lower... if its cold VV...

I think you need to look more closely at the lines, find out if its a National Grid line, or if its SWEB, and call them for some advice... if these are 40 kva then you need to give them some substantial clearance...
 
16m is from Highest Astronomical Tide ( HAT ) which does not happen that often .. Seems very low .. Suppose you could try going under at 45 degrees .. Depends wether you want to get back .. /forums/images/graemlins/ooo.gif Get some good life Insurance First .. Consult with Locals First ..
 
[ QUOTE ]
what is the safe distance away from a high voltage power cable?

[/ QUOTE ]
Depends on the voltage. The safe clearance from a 66kV line is much smaller than that required from a 380kV line, obviously.

[ QUOTE ]
is the charted MHWS clearance on power cables to the cable itself or to a safe distance from the cable?

[/ QUOTE ]

Admiralty publication 5011 (Edition 5 1984 Symbols and Abbreviations) on page 10 says:

[ QUOTE ]
Safe Overhead Clearance
The safe overhead clearance above HW, as defined by the responsible authority, is given (in magenta) where known; otherwise the physical vertical clearance is shown (in black).

[/ QUOTE ]

For a power line I would assume the responsible authorities have given a safe clearance taking into account maximum sag, etc, and a good margin for safety. Kv are kV, and very dangerous!!!

On the other hand, given the very small difference between your air draft and the given clearance, I would be inclined to query the Responsible Authority before attempting the trip, and to make the trip if at all possible at LWS!!

I'm very belt and braces when it comes to playing with anything that could put more than a few micro-amps through my body. You know the story -- there are old engineers and bold engineers, but no old and bold engineers!!!
 
To be honest, I doubt that there is much risk to you personally on the boat, but the boat electronics are all likely to get fried if there is a flash over & you may get some thro' hull fittings & fixings getting hot enough to leave a hole where they used to be.
 
And you won't need shorepower for awhile /forums/images/graemlins/laugh.gif

cheers Joe
 
On my reckoning if you clear it by about 2m you will be fine. Don't touch the mast though. Safer not to risk it if its tight.
 
At the part of the Tamar that you mention, the deepest water doesn't coincide with the lowest part of the catenary of the cables so the question is largely academic. (You will always have considerably more air draft than you think.)

Interestingly I was working with Hydrographers a couple of years ago and asked this very same question. You will not be relieved to hear that I am none the wiser. They told me how they measure heights above water, and they told me how they check, but as regards the actual 'air height' that goes on the chart, I am still not sure that there is a safety margine included. My conversations with the hyrdrographers appeared to indicate not.

A friend of ours has a mooring directly under the second set of cables. His mast is about 16 metres tall and it never touches the cables or comes close. He never even worries about it.
 
I checked on this problem with respect to Swedish overwater cables.
The problem is fairly complicated.
The hieght of the boat above a fixed datum is dependent on tide, air pressure, and waves.
The cable hieght depends on the catenary but also its temperature which depends on the load it's carrying, ambient temperature and wind cooling power. The safe distance depends on voltage, humidity and, as I now realise, presence of ions such as occur in burning smoke plumes. The "experts" for the hieght of the cable are the power company men.
An expert, Ted Osborn (CA) remembers in 1976 a cow near his home being elctrocuted by a power line that drooped too much in the hot summer. Apparently the farmer held a barbeque for the neighbours.
Your air draft must include everything including burgee stick or aerial. It's best done when you have the mast down. The error estimating the hieght of the mast step above water is probably small enough to ignore.
 
[ QUOTE ]

A friend of ours has a mooring directly under the second set of cables. His mast is about 16 metres tall and it never touches the cables or comes close. He never even worries about it.

[/ QUOTE ]

Find this chap; check your mast is shorter than his and follow him through......................................................................

at a safe distance /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
 
Remember of course that the clearance given is the minimum across the span.I don't know these wires, but you can often greatly increase the margin by crossing close to one shore or the other.
 
This pic is taken from Weir Quay, looking downstream to the power lines which you can just about make out.

powerlines.jpg


The group of yachts moored far left in the distance are just beyond the power lines and this gives a reasonably accurate view of their mast height in relation to the overhead lines. The deep water channel is well over to the East (devon) side of the river, nowhere near the lowest dip of the power lines. After several years, I still mentally "duck" going under the Tamar bridge but don't even think about the power lines with my 14 metre air draft
 
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