Elec­tri­cal wiring

David Berry dis­cusses the pros and cons of us­ing var­i­ous meth­ods of con­nect­ing elec­tri­cal wiring on board a boat

Practical Boat Owner - - Contents -

There are var­i­ous meth­ods of con­nect­ing elec­tri­cal wiring on board, but which is best?

When I in­stalled the an­chor wind­lass on Aderyn Glas (PBO Jan­uary 2012) I used the built-in mag­netic sen­sor to make my­self a chain counter at the same time.

This, I know, is a lux­ury. My wife, Ann, has painted marks on the chain like ev­ery­one else and stead­fastly uses her marks when­ever she is on the fore­deck drop­ping the an­chor. In fact the only time the counter dis­play in the cock­pit is rou­tinely used is when we are re­vers­ing up to a quay­side be­cause then Ann is in the cock­pit han­dling ropes. So when it failed it was not a catas­tro­phe, just a nui­sance. But I had a fault to trace.

Hav­ing checked the ob­vi­ous I fi­nally de­cided to take the unit out for a closer look and as I un­screwed the screw con­nec­tor the ca­ble dis­in­te­grated be­fore my eyes.

I should have known bet­ter. Af­ter all I had worked for a ca­ble man­u­fac­turer that de­signed and supplied ca­ble to war­ships. I know how good those ca­bles are: the typ­i­cal mil­i­tary con­nect­ing ca­ble, for ex­am­ple, that sim­ply joins equip­ment to­gether is sil­ver plated cop­per strand in­side a PTFE sheath. It’s ex­otic stuff the likes of you and I can’t eas­ily get. And it’s de­signed that way be­cause the most cor­ro­sive nat­u­ral en­vi­ron­ment in the de­signer’s bible is salt laden air. And that’s what we spend most of our lives sail­ing around in. This episode led me to mis­trust much of the ca­bling on our boat.

So is there any­thing I could have done to pre­vent the fail­ure of my an­chor counter ca­ble? Well yes there was. The first mis­take I made was us­ing a cheap co-ax­ial ca­ble de­signed to carry TV sig­nals through the house. I thought this was a PVC sheathed ca­ble which, con­sid­er­ing the many mil­lions of miles of PVC ca­ble the tele­phone com­pa­nies use, should have been up to the job. But I ob­vi­ously got that wrong. I should have cho­sen RG58 or some­thing equally rugged. It’s in­ter­est­ing to see that the cop­per braid shield layer has com­pletely dis­in­te­grated, not just at the ends, where you might ex­pect mois­ture to en­ter, but all through the ca­ble. This is ev­i­dence for a fairly por­ous sheath­ing ma­te­rial.

The sec­ond mis­take was to use a chocolate block. Screw con­nec­tors are OK for most things but they don’t seal the

core of the ca­ble so the at­mos­phere can get in and, over time, cor­rode the con­duc­tors.

Pre­ven­tion

So can we pre­vent this? Yes. The MoD gave up sol­der­ing about 30 years ago pre­fer­ring in­stead to use cal­i­brated crimps in cal­i­brated tools. This pro­duces a cold weld when used with care­fully di­am­e­ter­con­trolled ca­ble cores. But I am ad­vo­cat­ing a re­turn to sol­dered con­nec­tions. Sol­der­ing can fill the in­ter­sti­tial spa­ces in a stranded ca­ble and seal it off to the at­mos­phere, lo­cally mak­ing a stranded core into a solid one.

To pro­tect the joint made in this way the best prac­tice was a layer of flex­i­ble epoxy fol­lowed by a heat shrink sleeve then, to make ab­so­lutely sure, an­other layer over that. Joints made like this are no-kid­ding wa­ter­proof and are per­fect for any joint we need to make in the bilge or an­chor locker or in any other wet en­vi­ron­ment.

You can buy lined heat shrink on­line, which will do for straight­for­ward sin­gle wire joints in non-tax­ing en­vi­ron­ments but if you want to join com­plex wires or wires that will live in the bilge then al­ways use un­lined heat shrink over an epoxy layer. I use cheap, fast set­ting epoxy in the (untested) be­lief that it may be slightly more flex­i­ble than the bet­ter stuff. When I made joints in ser­vice equip­ment we were obliged to use an epoxy called Ray­chem S1005 which re­tained its flex­i­bil­ity for­ever. You can buy it, but it’s very ex­pen­sive stuff.

We should also seal wires wher­ever the cores are ex­posed so if you at­tach a wire to a switch, a crimp, or the dreaded chocolate block then we should at least make sure any stranded wire core is blocked with sol­der.

Lim­i­ta­tions

Solid core wires are bet­ter than stranded wires be­cause they com­pletely fill the plas­tic sheath. But they are stiff. Stranded cores have that in­ter­sti­tial space be­tween each of the in­di­vid­ual strands and this al­lows them to be flex­i­ble but also al­lows the at­mos­phere to en­ter the core and over time to de­grade or de­stroy it.

Don't join an­tenna ca­bles like this. You may have no­ticed that an­tenna ca­bles are de­fined by some­thing called char­ac­ter­is­tic im­ped­ance. For ex­am­ple the RG58 ca­ble on your VHF an­tenna has a char­ac­ter­is­tic im­ped­ance of 50 ohm while the ca­ble that plugs into the Nav­tex or the TV in your lounge may be 75 ohm. Ev­ery joint should be made with a con­nec­tor with the same im­ped­ance. So a BNC con­nec­tor of 50 ohm must be used to join a 50 ohm co-ax­ial ca­ble and so on.

The var­i­ous con­nec­tion meth­ods have pros and cons and I’ve tried to sum­marise them in the ta­ble be­low. Chocolate blocks and crimps do have their place but if you want the best pos­si­ble seal­ing then epoxy and heat­shrink is your only choice.

Ul­tra Vi­o­let

Then there’s the prob­lem of UV. If you have ca­bles ex­posed to sunlight, such as those run­ning from so­lar pan­els, you’re go­ing to find out that UV is pretty de­struc­tive. Ca­ble man­u­fac­tur­ers in­clude UV block­ing pig­ments in ca­ble sheaths that are des­tined to be used out­doors but again whether th­ese are avail­able to the likes of us is rather untested.

I take the prag­matic ap­proach and put all my ex­ter­nal ca­bles in flex­i­ble split con­duit. This pro­tects the ca­ble at the cost of sac­ri­fic­ing the con­duit which tends to crum­ble af­ter a year, two at most. But con­duit is far cheaper than the ca­ble and eas­ier to fix.

One other point to watch is the ca­ble ties that fix the con­duit or wiring. I be­lieved that black ca­ble ties would con­tain car­bon black as a UV block­ing pig­ment but when they started snap­ping off my an­chor light ca­ble I had to re­vise that be­lief. I also had to climb the mast to re­place them so, yep, I re­placed the ca­ble too... just in case.

Cop­per braid of this coax­ial ca­ble dis­in­te­grated in a salt wa­ter en­vi­ron­ment Ð though the core is still in good con­di­tion

The best of ca­bles: 19 strands of sil­ver plated cop­per in­su­lated with PTFE

David and Ann Berry keep their Moody Eclipse Aderyn Glas in the Io­nian

This ca­ble had run through the an­chor locker and had ar­eas that were clearly swollen. When I cut into it this is what I found: the cop­per strands were slowly dis­in­te­grat­ing into a mass of cop­per salts

Wires that ter­mi­nate at switches or tags should be sol­dered and pro­tected with heat­shrink sleeves

Chocolate blocks are quick and easy to use but have built-in long term prob­lems

Con­duit is cheaper and eas­ier to re­place than ca­bling

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