Question: I recently had a marine surveyor inspect my boat to determine a condition and value for my insurance company. One of the suggestions he made has me scratching my head, because as a long-time boater I’ve never heard of it.
He says on his report that I should have all of the terminals in my battery cable runs tightened with a torque wrench to ensure that they are adequately tightened. He says that the ABYC recommends doing this in its electrical standards. Is this really true? The photo I sent in shows one of my batteries and the connections to a fuse in line on the way to the back of my battery switch for this battery. The surveyor says all of these connections should be torqued properly.
Answer: Your surveyor’s statement regarding the proper tightening of battery cable terminations has some merit, but he is a bit confused about the ABYC recommending this procedure in its standards. I believe he may be thinking about an old legacy question found on the ABYC’s Marine Electrical certification exam that suggests that a torque wrench is the tool of choice for tightening cable lugs on battery switches. Most marine electricians will tell you that this is not something they do on a regular basis -- or ever, for that matter. This includes me, and I’ve been involved in this sort of work for over 40 years.
It’s important to understand the origin of this whole idea of using a torque wrench for this purpose. The requirement has to do with ISO-9000 certification for boatbuilders. What this means in a nutshell is that, according to ISO-9000 standards, the builder must be able to document each and every step in their manufacturing process to ensure consistency and show that each boat is assembled to the same set of exact standards. Using a torque wrench to tighten battery cable lugs will guarantee that each and every one of those cable terminations is exactly the same tightness.
The problem with all of this in field applications is that it is rather difficult to find torque specifications that are appropriate for these terminals. This is significant because if you were to use the more readily available torque specifications for steel hardware on some of these electrical terminals, which are often made of brass or tin-plated copper, you would consistently be breaking off the studs. Copper and brass are much softer and require less torque to be considered tight. The reason this is important is because loose connections at these terminals can generate significant heat and maybe even arcing in extreme cases, which could cause a fire or even explosion under the right conditions.
Experienced technicians develop a feel for when bolts and nuts are adequately tightened, but the inexperienced typically either over-tighten and break things or under-tighten because they’re afraid of breaking something. This methodology is all a bit too subjective to comply with ISO-9000 standards requirements. To give you a point of comparison, Blue Sea Systems, one of the only manufacturers I know of that provides torque specifications for the studs on their battery switches, recommends either 120 or 140 inch pounds (13.6 newton meters) of tightening torque for their switches using 3/8” studs, and 220 inch pounds (24.8 newton meters) of torque for their switches using ½” studs. Grade-5 steel (the most commonly used mild steel grade for fasteners) bolts in 3/8” size would require 33 foot pounds, and ½” size would require 78 foot pounds of torque by comparison. A good resource to find this information online for the various metals we deal with in marine applications is the Engineer’s Handbook.