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Janet Hartman of Beaufort, North Carolina, asks:"Recently I contacted the National Ocean Survey (NOS) to ask whether the authorized clearances for overhead cables shown on their charts include the extra distance needed to avoid arcing. I received an email from nautical.charting@noaa.gov stating 'The ‘authorized clearance’ for an overhead power cable does not include the

Janet Hartman of Beaufort, North Carolina, asks:

"Recently I contacted the National Ocean Survey (NOS) to ask whether the authorized clearances for overhead cables shown on their charts include the extra distance needed to avoid arcing. I received an email from nautical.charting@noaa.gov stating 'The ‘authorized clearance’ for an overhead power cable does not include the clearance necessary to avoid possible arcing. The clearance indicates the distance between the water level and the cable.'

An electrician friend of mine tells me the distance needed to avoid arcing varies depending on a cable’s voltage. So how can you know how much clearance you need when sailing under a cable? Local politicians tell me that, according to OSHA, a mast must not come within 10 feet of an overhead cable. I wonder whether local bridge tenders might refuse to open up for a sailboat whose mast might come within 10 feet of a cable on the other side of the bridge? I also thought the OSHA restriction applied only to construction and electrical equipment, not to sailboats that are underway. Can you shed any more light on what seems to be a problem with no apparent answer?"

Nigel Calder replies:

Although I hadn’t thought about the problem before you raised it, there’s no question this could be a matter of life or death in certain situations. In practice, of course, most of us don’t push our mast clearances to the limit. As for your local bridge operators, I strongly suspect cables near bridges are insulated, which means they are not an issue.

But high-voltage cables on major pylons in this country can carry up to 750,000 volts and they definitely are not insulated. They are potentially lethal, and I for one will make sure I give them a generous clearance from now on.

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