Overloaded or poorly maintained wiring can lead to fire
By Paul Abelson
A dozen or so years ago, at a meeting of the Society of Automotive Engineers (SAE), I helped organize a panel discussion about sleeper cabs. I brought three owner-operators for a dialogue with truck builders’ cab engineers. After hearing how sleepers had been made larger and more comfortable, Dennis Mitchell of Blissfield, Mich., stopped the engineers in their tracks. He asked if any of them knew how long it took for a sleeper cab to burn down to the frame rails.
Dennis reported seeing it happen in less than five minutes.
Since then, improvements have increased safety and reduced escape time significantly. Federal Motor Vehicle Safety Standard (FMVSS) 302 requires combustible materials used in trucks to burn no faster than four inches per minute. Flame resisting thermosetting plastics are used in many structural areas, replacing thermoplastics that melt. Of course, liquids like anti-freeze, washer fluid and motor oil, if stored in the cab, can help feed a fire, but for the most part, trucks as delivered are significantly safer against fires. Most of us tend to think of truck fires being the result of crashes, but in reality, the greatest cause of truck fires is overloaded or poorly maintained wiring.
Energy and heat
Look at an ordinary incandescent light bulb. A wire filament has an electric current passing through it. The wire is very thin, so its resistance to current flow is high. The current heats the wire white hot. The more current there is, the brighter the wire will be. Voltage is constant, so higher wattage comes from more current, resulting in brighter light. A 60 watt bulb burns roughly 50 percent brighter than a 40 watt bulb. Filaments radiate enough heat to burn your fingers on the glass. If the glass didn’t keep the filament in a vacuum, the fine wire would burn through in a fraction of a second.
Whenever electricity passes through a wire, it converts some of its energy to heat. How much depends on the diameter of the wire, its length, and the amount of current flowing through it. Wires in trucks are sized and insulated for the current they are expected to carry. Insulation protects wire from wear and holds in the little heat a properly sized wire will generate. In order to allow insulation to flex with the wire, it must be made of soft, flexible thermoplastic, often vinyl.
Insulation can wear or melt, exposing wire. Exposed wire can contact metal, causing a short circuit. Current, following the path of least resistance, flows to the contact point, overloading the wire and its insulation. That’s why it’s important to protect wiring. Always use grommets when routing wires and cables through a bulkhead where chafing can occur.
When possible, use mounting plates with strain relief. If wires are exposed to weather, put a drip loop so salt and chemical spray will be directed away from connectors. If possible, bundle wires together using wire ties and cover bundles with conduit tubing. Don’t let wires droop. Hold them as securely as possible, allowing only minimal movement for flexing and vibration. Whenever possible, cut wires and cables to proper length and use sealed terminals. Don’t loop excess cable. It may look neat, but the coils create electrical interference, reducing efficiency.
If protecting wire from external damage is important, keeping current within limits is critical. How many of us have piggy-backed multiple 12-volt plugs to run more and more accessories without any regard to current load. I know I have. Have you ever tried connecting a CB radio, a cell phone charger, GPS navigation, a back massager, heated seat and portable refrigerator, all leading to the same power plug? I’m sure I could think of more devices, but these are what it took for me to start blowing fuses.
I was faced with two choices: eliminate some devices from the circuit or replace the blown 15-amp fuse with a 20- or 30-amp. That could have caused a fire. Never increase the size of a fuse. Always locate fuses as close to the batteries as possible. Use fused plugs. They are there to protect the wiring and the plug assembly. Having five separate cords, each with its own 15- or 20-amp fuse will not protect a circuit designed for 15 to 20 amps total current. A panelist for a TMC seminar on cab safety told of a driver came into a shop with eight devices leading from a single accessory plug.
The longer the cord, the greater the resistance. Instead of using a 12-foot extension cord to reach the bunk for an electric blanket, consider a separate fused circuit near the bunk.
Inverters are becoming more and more important in today’s trucks. Devices that run on household current are less expensive and more reliable than their 12- volt counterparts, and amp draw for a 120-volt item is only a tenth of what 12-volt appliances draw. But inverters use a great deal of amperage from the battery, and require heavy cables. Bruce Purkey of Purkey’s Fleet Electric is known at TMC as Dr. Electricity. He uses “00” cables for all inverter installations. Bruce places a fuse as close to the battery as possible. He determines the size fuse required by dividing the inverter’s rated wattage by 10.
Make sure there is enough air flow to cool the inverter. Have a good ground from the inverter to the chassis. To power a CPAP machine, medical gear, or computers, you’ll need a pure sine wave inverter. A modified sine wave can damage computers or chips in an electronic gear.
The temptation is great to pile on electric cab conveniences, but you have to be sure you don’t overload any circuits. Fire is a risk, but you can manage that risk with proper wiring methods.
Look for corrosion at terminals. Trailer nose boxes, 7-pin plugs and sockets and every lamp connection are all vulnerable to corrosion. Phillips Industries has J-560 plugs and trailer nose boxes with replaceable parts. Truck-Lite’s “Smart Box” nose box eliminates rewiring when replacing receptacles. Either one makes repairs easy and isolates any corrosion.
Check for damaged wire and splices that can be conduits for internal corrosion. If wires go through holes in chassis or body parts, make sure the frame grommets are used and in good condition.
Examine chassis ground wires. Replace corroded or damaged parts. If the chassis is corroded where the ground connects, clean it with a wire brush and emery cloth. Then, re-prime, repaint, and re-attach with new connectors. It’s good practice to paint over the connection with liquid vinyl, sold in parts and hardware stores as liquid electrical tape to prevent new corrosion.
To avoid the need to splice wire, buy replacement lamps with molded plugs or sockets, rather than those with wire pigtails. Protect plugs and sockets from corrosion with dielectric grease.
Never penetrate the insulation or use connecters that bite through insulation. Continuity testers with pointed probes are designed for use with metal sockets and plugs. When pushed through insulation, they leave a pinhole that conducts chemicals and can lead to corrosion.
Protect wires from abrasion by bundling them together inside plastic slit-tube conduit available at hardware stores. Use grommets whenever wires are routed through openings in frames, cross members, firewalls, brackets, or panels.
Senior Technical Editor Paul Abelson is a member of the Technology & Maintenance Council, industry professionals who develop best practices for trucking equipment and maintenance.