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How to Terminate a Sea Cable

July 2, 2010

In retrospect, noise had been creeping into our CTD measurements and bottles were mysteriously misfiring so it wasn’t a complete surprise when ultimately the lab software lost communication with the instruments. We had sent the CTD down to 65m when the pump suddenly shut off and the live-plotting data went haywire.

“This is a problem that restarting probably won’t fix.”

We brought the CTD back on board and checked all of the wet-mateable connections. The pump power connection seemed like it may have had a little moisture in it so we dried it out and decided to try putting it in the water again. It didn’t even make it past the surface before we observed the exact same behavior.

Upon closer examination, we found that the insulation had been melted on part of the sea cable where it sometimes rubbed against the termination. It appeared that a hole had developed and then a spark had exacerbated the problem. We quickly patched the hole with scotchkote and tape to do one quick CTD at that station.

Immediately after we got the CTD back on board (approx. midnight), we began the preparations to re-terminate the cable. First, we cut the cable above the damaged segment where we would start our new connection. This cable has three wires inside that are covered in a very tough, rubbery insulation. Outside of that are two layers of steel wire wrapped in opposite directions. This cable provides communication to the instruments while they’re being deployed in addition to serving as the load-bearing cable running from the winch.

Next, we nicked the steel wire using an air-powered grinder in order to carefully bend off about 3ft of the steel wire without cutting the insulated cable inside. Then we bent back the ends of the steel wire to create a “bird’s nest” around the insulated cable. This bird’s nest had to fit inside of the cable termination – this big stainless steel block that is bolted onto the CTD cage.

In order to hold the cable in the termination, we filled it with a metal alloy that melts at 160 degrees F. We heated the metal to about 230 degrees F and then pre-heated the termination using a propane blowtorch.

Once all of that was done and the metal had cooled, we soldered the insulated cable to a “pigtail” and connected it to the instruments. The three wires inside the insulated cable were twisted together and soldered to the live side. The other side was soldered to a piece of sacrificial wire which was screwed to the termination to act as ground. In case water gets into the ground wire, the sacrificial wire protects the rest of the pigtail from damage. The easiest way to make sure you hook it up right is to check the pigtail that you just removed and do it the same way.

After everything was soldered, we covered all of the connections with a couple layers of scotchkote and self-vulcanizing rubber tape. After we topped it off with a layer of electrical tape we were able to hook everything back up and *tada* it all worked again. However, this time we zip-tied some padding to the termination in hopes of preventing any re-occurrence of this problem.

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