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Old 01-07-2015, 12:37   #61
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Re: TVS Diodes for Lightning Surge Supression?

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Originally Posted by EwenT View Post
If you observe any other interesting features on your electrode surface, however subtle, I would like to hear about them. If I was involved in a forensic analysis and had your electrode on hand I would examine it with a microscope, but do not wish to influence your observations and so will not say what I would be looking for. Also, any features on your saildrive, prop shaft or propeller are relevant to the current flow through the engine.
It has been almost 4 years now since our strike, and the electrode has been in use (in the water) almost continually since. However, I just took a magnifying loupe to it and examined it very closely.

4 of the 18 edges machined into it have either round pits missing from them or ~1/4" long sections carved out of them (like someone dulled a knife edge against a stone). It takes a magnifying loupe (10X) to see this, as they are not readily apparent to (my) naked eye.

None of the flat surface shows any damage - only the edge surfaces. And the edges that have the damage all seem to be the edges closest to the main body of the electrode. This is hard to explain, but if you look at the picture of the electrode I posted, each machined ridge has 3 edge surfaces that I will call "left, top, right", as viewed on end.

In our case, 4 of the "right" edges have the damage - there is no damage to "top" or "left" edges.

I cannot get a picture of this because the camera won't focus that closely and blowing up a further picture really doesn't work well.

These marks could have always been there, or could have occurred through erosion/corrosion/impact after the strike, or could have been caused by the strike. Unfortunately, I did not examine it this closely immediately after the strike.

As for the saildrive, it is electrically isolated from the engine by teflon bushings and gasket, and the propeller is electrically isolated from the prop shaft by a rubber hub - I have observed no damage to the drive or prop. I suspect either the engine block absorbed the residual current bouncing in the system or the current was passed to the water through the cooling system.

Mark
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Old 01-07-2015, 15:48   #62
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Re: TVS Diodes for Lightning Surge Supression?

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As for the saildrive, it is electrically isolated from the engine by teflon bushings and gasket, and the propeller is electrically isolated from the prop shaft by a rubber hub... Mark
Does the spacer (thrust washer) behind the propeller span the gap between the propeller and the splined hub providing a metallic path between the propeller and the saildrive?

I've checked with a VOM meter for connections between things that I thought were isolated and been surprised more than once.
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Old 01-07-2015, 17:44   #63
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Re: TVS Diodes for Lightning Surge Supression?

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Does the spacer (thrust washer) behind the propeller span the gap between the propeller and the splined hub providing a metallic path between the propeller and the saildrive?

I've checked with a VOM meter for connections between things that I thought were isolated and been surprised more than once.
We don't have the usual spacer because we have a line cutter there. The cutter consists of a rotating blade and standing blade that are separated by a delrin spacer.

But your point is taken - there are always little electrical paths that one is unaware of.

In the Volvo saildrive, even a connection between the prop and the drive leg doesn't matter because the entire drive is electrically isolated from the engine. This isolation I check regularly.

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Old 09-07-2015, 08:48   #64
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Re: TVS Diodes for Lightning Surge Supression?

This thread has evolved from a discussion of surge suppression to an investigation into fundamental aspects of grounding. This week an experienced member of the NFPA 780 committee and I met and we agreed that more data and experiments are needed to shed some light on the relevant concepts. The members of this forum have a wealth of data and I plan to start a new thread to get a record of first-hand observations.
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Old 09-07-2015, 10:02   #65
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Re: TVS Diodes for Lightning Surge Supression?

Ewen,

There was a thread started last week soliciting first-hand observations, however I'm not sure how useful it will be as there was no standardized criteria set. Take a look at;
Have you suffered a lightning strike? - Page 4 - Cruisers & Sailing Forums

- Rusty
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Old 09-07-2015, 10:18   #66
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Re: TVS Diodes for Lightning Surge Supression?

Thanks Rusty. We will review these messages too.
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