about electrical systems…
APPARATUS/EQUIPMENT
Although electrical systems tend to be one of the least understood of all fire apparatus/systems, routine maintenance of them is a must to prevent possible disaster.
Unlike mechanical systems which often give warning of a forthcoming failure, electrical systems generally are either operable or inoperable; and many times the result of inoperable systems can be disastrous.
Maintenance of electrical systems on fire apparatus can be performed routinely, as with all systems. However, electrical systems are probably the least understood of all systems, since electricity is an abstract quantity. Nevertheless, electrical systems can be understood by most to the degree that relatively simple, routine maintenance of their components will prevent a large portion of failures.
Fundamentals of electricity
A direct current (d.c.) is used for applications requiring an easily portable power supply which produces a constant, steady electrical potential. The d.c. cannot be used efficiently over great distances because of losses caused by its transmission. Since the d.c. system is the primary cause of nearly all electrical maintenance problems, and since fire trucks use batteries to power their electrical systems, this article will deal only with battery supplied d.c., which provides the flow of electrons as a result of chemical reaction. Batteries are a compact system, require no motive power for use, are simple to maintain and are relatively inexpensive.
There are three basic d.c. circuit elements: current (I), voltage (E) and resistance (R). These elements always have a direct relation to each other: voltage (in volts = current (in amperes) X resistance (in ohms), or E = IR. As long as there is an electrical potential, this relationship is true in any point of the electrical system, whether it is operable or inoperable.
Voltage is the electrical potential difference between two points. This means that it is the energy needed to transfer one amp from one point to another through one ohm of resistance. It is analogous to the required pressure to produce a given flow in a fire hose.
Resistance, a natural characteristic of any material, opposes the flow of electrons. It is measured in ohms and is analogous to the friction in a fire hose.
The ampere is a unit of electricity moved through the resistance of one ohm when acted upon by one volt. It is analogous to gallons per minute of water through a fire hose.
Photo by Ron Jeffers
Electricity is a potential energy source, “there” waiting to be used. The wires, terminals and battery which comprise an electrical system are analogous to a conventional pressurized plumbing system.
On fire apparatus, the carriers of electricity are the chassis circuits and the circuits installed by the apparatus manufacturer. Since the chassis manufacturer’s circuits are specialized in themselves this article will center on only the apparatus manufacturer’s circuits.
The circuits are comprised of various sizes and types of conductors (wires), which are generally graphite, copper, aluminum or steel, and are sheathed by an insulator (a nonconductor) used to confine and define the flow of electricity from one point to another. The wires used are manufactured to certain Society of Automotive Engineers (SAE) codes and national electrical codes, which require that the sheathing material withstand oil, fumes, heat, cold, moisture, etc., as well as insulate the electrical potential to be used.
In basic d.c. circuits (voltage = amperes X resistance), the voltage from the battery is constant and the amperes is the required flow to make an accessory operable. Therefore, any change in resistance will upset the planned circuit. This leads to the conclusion that most times the key villain in electrical service/maintenance work is resistance. Since everything naturally is a resistor, anything can cause diversion — low resistance (current follows this path as in a short circuit) or high resistance (current is restricted from flow entirely as in corroded battery terminals).
The sheathing is designed so that its resistance is greater than the potential of the electricity to be carried (wire to ground as measured in volts) and, therefore, will conduct the electricity through the confines of the wire. Should the sheathing deteriorate or be fractured so that its resistance is lower than the electrical potential carried, a path through this area of least resistance to ground is created. This area is called a short circuit or short, lust as water will leak through a hole in a fire hose, electricity also will follow the path of least resistance.
Data has shown that a large percentage of the problems that occur in electrical systems on fire trucks are caused by moisture or moisture-induced reactions. In a fire truck’s natural habitat, moisture conditions, which promote deterioration and/or failure, are just short of being perpetual. Therefore, to properly maintain and service electrical systems, moisture should be considered a catalyst whose end result is failure. Just as in a battery, moisture between two dissimilar nontouching metals sets up a galvanic cell which results in corrosion. This is the reason wiring terminals, splices and plugs must be kept tightly together.
Potential problem areas
A recent survey revealed that electrical problems can account for up to 8 percent of the total problems reported. In a typical apparatus wiring system, electrical problems generally will occur at one of four areas: (1) at any place a wire has an “end,” (2) at any place a wire goes through a panel, (3) at any place a wire spans an area, or (4) within electrical components and/or accessories.
The wire’s end is either a plug, a terminal or a splice. All of these must be securely attached for physical strength as well as for making a conductive connection. This is the point where many failures occur due to high resistance. Some common terminals and connectors and their possible problems include:
Checklist of Maintenance Conditions
Wires and wire assemblies:
- Loosely supported wires will allow snow and mud to accumulate on them, thereby exceeding their mechanical strength.
- Carelessly hanging wires can touch or burn on mufflers and exhaust systems.
- Wires can chafe across sharp edges, causing shorts.
- Pinched and/or mechanically broken wires can result from being entwined in moving components such as drive lines.
- Continual flexing of lines used on items such as telescoping light poles, spotlights, hose bed lights, etc., can cause mechanical breakage of wire and/or sheathing as well as chafing.
- Battery terminals should be periodically cleaned and treated to deter corrosion.
- Terminal ends on all electrical accessory items such as hose reel motors and solenoids carrying high amperage should be checked for tightness. If loose, remove the terminal from the post, brush clean and replace.
- Exposed terminals such as those used in backup light switches mounted on transmissions should be kept clean.
- Toggle switches, even of the waterproof type, should have their terminals checked, the outside kept clean and wiped dry when finished with use.
- Terminal ends of all wiring should be checked and tightened or replaced if necessary. Above all, if corrosion occurs, remove the terminal from the post, brush clean and replace.
- Wires which are clamped in place, such as automatic transmission lock-up wiring, should be checked to ensure that they have not loosened and are not in danger of being melted or shorted out.
Corrosion of primary wire terminal ends:
Switches, electrical accessories and devices:
Crimp-on – Failure usually is caused by corrosion due to exposure, breaking due to flexing, or the wire pulling out because of insufficient crimp.
Soldered – Failure usually is due to exposure of the wire next to the connection.
Screw clamp – Failure is due to clamping devices coming loose. This causes high resistance (nonconductance) and can allow corrosion.
Bayonet piercing — Failure is due to flexing and pulling of the wire. This reduces the contact area, causing the bayonet to pull out of the wire, resulting in insufficient contact. It is not advisable to use this type of terminal.
Molded – Generally there are no problems with this terminal unless the pins are pulled slightly out of their socket, thereby exposing them. This then can allow moisture to enter the socket, resulting in corrosion.
“Spring” clamp type-Failure occurs because of insufficient clamping force due to flexing and pulling of wires. This connector is not recommended.
For terminals attached by soldering, crimping or clamping, routine maintenance should include checking the fastening process. Connecting methods used should retain the manufacturer’s recommendations; for example, crimp-on terminals should have a minimum pull-off force of approximately 35 pounds. Clamp types should be checked to see that the method used for clamp has not worked loose. If a molded plug is observed to be pulled slightly apart, remove, dean terminals and refasten. Solder types are excellent, usually done on a bench operation, and should retain integrity and not show corrosion within the ends or around the insulation/ terminal interface. Improper attachment of any kind can cause high resistance resulting in improper readings on gages, improper/premature failure of accessories, or inoperable equipment.
The second area of potential problems is where wires pass through panels. Again, this generally is caused by movement of the wire relative to the panel through which it passes. This movement, called chafing, can wear through grommets, looms and insulation causing shorts of the main conductor to the body or chassis frame. It is advisable to visually examine grommets, harnesses and looms at all points where they go through a panel to see that they are intaa and do not exhibit visible wear. Also, look for “pinched wires,” which usually occur in unsuspecting areas and, again, can cut through insulators, harnesses and looms causing a short of the conductor to the chassis frame. A common example of this is the hose deck grille lying on top of the water tank sender wire.
Third, examine the wires themselves for mechanical strength. Mud and snow accumulations can cause wires to sag, resulting in either shorting out by chafing or actual breaking. If any wires are noticed to be exposed in such a manner that they could be damaged, clamp them up immediately. If the maintenance or repair requires alternate routing or replacement of wires, select a path which provides a structural support to which the wires can be attached. Loom the wire assemblies if necessary and securely clamp for support. Do not wrap wires around hydraulic lines, chassis cross members or any other flexing, moving part. Do not drape wires across unprotected spans or areas. Proper maintenance is as important as the original design and selection of components. Routine maintenance and a familiarity with the wiring diagrams and corresponding coded circuits for the apparatus will result in less downtime and easier, quicker repair when necessary.
Last, examine the electrical components purchased as complete units and installed. There are key areas in and around the electrical components where moisturerelated failures can occur. Components can include within themselves switches, solenoids, electrical switching devices, relays, etc. The failure of these usually can be traced to one of two things. First, either they were not originally intended to be used in the environment or surrounding elements; or second, the original protection afforded through seals and/or coatings has since deteriorated, allowing moisture to enter, corrosion to occur and failure to set in. Maintenance should include examination of these components to ensure that protective covers and boots originally furnished are intact and properly installed.
Many commmercially available aids can be purchased locally to help in preventive maintenance. These include types of waterproofing (spray or paint on); strain relief devices (grommets, bushings, pliable sealants that harden); wiring clamps (tie wraps, clip-on hangers, pressure-sensitive hangers); and corrosion preventives for battery terminals, wire terminals and plugs (spray-on, salve type). Use them! Electrical maintenance now could prevent a disastrous failure later.
