Safe clearance under power cables

[david36]

Can anyone point me to information regarding the safe "gap" to be left when passing under overhead power cables? I am not talking about the physical gap but the one needed to ensure safety from the electric power of the cables. I plan to go under a cable which the chart gives as 17.5m clearance with a mast height of 16.5m and do not want to become a bright blue flash. The power of the cable is not marked on the chart.
Fair winds to all.

 

[prv]

If you're concerned (and only a metre doesn't sound much), go through at low tide!

Pete

 

[Quandary]

This used to come up a lot in construction Health and Safety for diggers etc, it was to an extent dependent on voltage,humidity, and whether there was an underslung earth cable, I recall 6m. as the safe clearance for 110 kv. and I think it was about 5m. for all lower voltages. However there would be a significant safety factor in these figures; the catenary sag in long spans increases greatly on warm days and this was allowed for in the clearance figure. You were generally not given Planning Permission for any structure within 6m. of any overhead electrical cable.
You will need a very dry, cold day if you are getting closer than 3 or 4 metres.

If the risk is the same for a boat you will find plenty of info. on the HSEs website.

 

[Elessar]

Can anyone point me to information regarding the safe "gap" to be left when passing under overhead power cables? I am not talking about the physical gap but the one needed to ensure safety from the electric power of the cables. I plan to go under a cable which the chart gives as 17.5m clearance with a mast height of 16.5m and do not want to become a bright blue flash. The power of the cable is not marked on the chart.
Fair winds to all.

What a rally good question. I wonder if a call to the electricity company would do any good. I also wonder of the accuracy of the clearance measurement if cables can sag. And remember an old chart may be marked as clearance above MHWS rather than HAT as it is now.

 

[ProDave]

I'm just doing my essential navigation course, and we were told the charted clearance under a power line includes a safety factor already.

So if the chart says 17.5M and your mast is 17.5M you will be okay, the mast will not touch the power lines and there will be a safe gap.

But if that close, I would go at low tide, just to add even more safety factor.

 

[VicS]

This was discussed on here only a month or so ago. IIRC some good answers were given.

A search easily finds the previous discussion http://www.ybw.com/forums/showthread.php?t=263651

 

[Plomong]

Can anyone point me to information regarding the safe "gap" to be left when passing under overhead power cables? I am not talking about the physical gap but the one needed to ensure safety from the electric power of the cables. I plan to go under a cable which the chart gives as 17.5m clearance with a mast height of 16.5m and do not want to become a bright blue flash. The power of the cable is not marked on the chart.
Fair winds to all.

I answered this question in post number 13 of thread: http://www.ybw.com/forums/showthread.php?t=263651

I repeat my answer here:

Quote:
Originally Posted by Robih
On the Tamar cruising thread there's mention of being aware of the power cables. What is the recommended safe distance for an aluminium mast from these high tension power cables? Is there leccy engineer on these boards that can answer that?

My old and dog-eared copy of Admiralty Publication 5011 (5th edition, 1984) says on page 10:
"Power transmission line, with safe overhead clearance (see note) and pylons".

The note says:
"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, as in H18a)".

H18a is the symbology for telegraph or telephone lines.

I am quite certain that the Admiralty and the "responsible authority" woud make bl00dy sure that the value given was really safe -- HV is nothing to be played with, even 1 KV will kill, very permanently, and with sonic accompaniment.

So, all those saying subtract the length of the insulator strings + 50% or whatnot are being over cautious. The height as given in magenta is safe.

Plomong

 

[Elessar]

I answered this question in post number 13 of thread: http://www.ybw.com/forums/showthread.php?t=263651

I repeat my answer here:

Quote:
Originally Posted by Robih
On the Tamar cruising thread there's mention of being aware of the power cables. What is the recommended safe distance for an aluminium mast from these high tension power cables? Is there leccy engineer on these boards that can answer that?

My old and dog-eared copy of Admiralty Publication 5011 (5th edition, 1984) says on page 10:
"Power transmission line, with safe overhead clearance (see note) and pylons".

The note says:
"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, as in H18a)".

H18a is the symbology for telegraph or telephone lines.

I am quite certain that the Admiralty and the "responsible authority" woud make bl00dy sure that the value given was really safe -- HV is nothing to be played with, even 1 KV will kill, very permanently, and with sonic accompaniment.

So, all those saying subtract the length of the insulator strings + 50% or whatnot are being over cautious. The height as given in magenta is safe.

Plomong

Looks definitive. I like threads like this, I teach theory so should have known the answer, seems I do now

 

[david36]

Thank you all for the prompt and comprehensive answer. I am holding my head in shame for not knowing it already!
Fair winds to all.

 

[Poignard]

Not always possible. e.g. the bridge below Auray in the Golfe de Morbihan has 14m air clearance but the water depth is 0.4m LAT. This prevents access to this delightful place by large yachts.

(Maybe that's one of the factors that make it 'delightful' )