Electricity: Back to basics

April 1, 2004

Avionics Technology

Technician with amp clamp and scope meter. Sometimes you have to check multiple values at the same time.

Back to Basics

by Jim Sparks

Like it or not, aircraft of today are becoming more and more electric. At one time calling an electrician was an easy out for resolution of problems with components that involve wires. Many technicians I have worked with over the years have a sound understanding of hydraulic systems but when it comes to devices operating on electrons rather than fluid they feel out of place. Troubleshooting electrical issues is really not that much different than hydraulic problems. Many of the rules are the same. Although electrons move a bit quicker than fluid, and electron leaks are detected by smoke rather than drips.

A pump is a device that converts mechanical energy into fluid movement so a pump installed in a hydraulic system creates the flow and it is the restriction or resistance to fluid movement that causes the pressure increase. In order to take advantage of hydraulic power a specific force or pressure needs to exist in order to move the fluid allowing restrictions to be overcome. This is however, only part of the requirement. Flow and pressure are the two variables and with both of these elements present work can be accomplished. Electrical power is also the result of combining electrical flow and pressure.

Inventors and definitions
Count Alessandro Volta was an Italian physicist and among his credits was the forerunner of the modern battery. This device was built in 1800 and as a result the basic unit of electrical measurement was named in his honor. In fact it is this very unit that is used as a reference for electrical pressure. The formal definition of a volt is joule per coulomb. Simply put this is the potential difference between two points. A pressure gauge employs the same principle. As the pressure being monitored is always referenced as either atmospheric pressure or a second point to illustrate pressure differential.

The ampere is the standard unit of measure for electrical flow and where a hydraulic system may use gallons per minute an ampere is the number of electrons flowing through a conductor in a specific time.

Georg Simon Ohm experimented with early batteries in the early 1800s and based on his observations some very interesting and pertinent conclusions were reached. Ohm noted that the amount of current flow in a wire was directly proportional to the diameter of the conductor and inversely proportional to the wire length. This was the foundation of Ohm’s law, which states the voltage consumed by a device is a result of the amount of current multiplied by the internal resistance of the component.
In other words E = I x R, where E is voltage, I is current, and R is resistance. The relationship of these simple three letters when understood provides the key to solving many of the discrepancies that plague aircraft electrical systems.

One coulomb is the amount of charge accumulated in one second by a current of one ampere. Electricity is actually a flow of charged particles, such as electrons, protons, or ions. The charge on one of these particles is a whole-number multiple of the charge on a single electron, and one coulomb represents a charge of approximately 6.241 506 x 1018 electrons. The coulomb is named for a French physicist, Charles-Augustin de Coulomb, who was the first to measure accurately the forces exerted between electric charges.

Power supply and breakout boxes enable testing a circuit under realistic conditions. Generator shunt attachment to aircraft structure. Dissimilar metals may cause corrosion resulting in resistance.

Construction and troubleshooting
Circuit construction plays an important role in the voltage amperage relationship. Resistances wired in series (that is end to end) will have a constant electrical current through all components in the circuit and this current is limited by the greatest resistance value. The voltage will decrease as the current passes through each element of the circuit. This means that if a 28-volt power supply is connected to four 7-ohm resisters all wired in series the total circuit resistance would be 28 ohms and there would be a 1-amp flow when the system is in operation. Each resistor would consume one-fourth of the voltage.

Components wired in parallel (side by side) will cause a circuit to have a variable current flow but a constant electrical pressure or voltage present at each device.

In the event a cabin reading light will not illuminate, the initial reaction would be to plug in a new lamp and try it again. When that component change does not resolve the problem, troubleshooting begins. A reading light circuit contains two wires, a lamp, switch and connections to a power source, and a ground. Removing the lamp and installing a digital voltmeter across the contacts of the bulb socket reveals the presence of bus voltage. On a hunch another new bulb is obtained and installed but still the light does not illuminate. Removing the replacement bulb and once again installing the voltmeter across the socket contacts displays once again a normal voltage present. Now what?

Going back to hydraulics, if a pump is operating and supplying fluid through a restrictor (the diameter of a number 90 drill bit) to a supply manifold and if there is no demand for fluid flow from the manifold, the internal pressure of the manifold will be the same as the pump output. When a valve on the manifold is opened the pressure within drops and the only flow is that through the restrictor. Now back to the reading light. If the control switch has a high resistance across the ON contacts there may be inadequate current flow to illuminate the bulb but when the light is removed electrical pressure or voltage will be observed.
Installing the bulb and reading the voltage across the lamp socket would reveal a significant drop.

A circuit often includes a connection to a power source which may be some form of a protective device such as a circuit breaker or a fuse and like its hydraulic counterpart an electrical circuit protector will interrupt current flow in the event too much flow is detected. One of the most important and yet frequently overlooked rules in troubleshooting is that whatever flows into a circuit should also flow out. When input current exceeds output there must be an electron leak. Many airframe manufacturers use aircraft metal structure to supply a return path for electrons to flow back to their source of power. This may aggravate conditions where wiring is routed in close proximity to components where chafing of the insulation occurs. As most metals are capable of carrying an electric current this deterioration of insulation will often cause an electron leak. Locating short circuits is often a challenging experience and may involve separating parts of the circuit at bulkhead connectors, terminal strips, or splices. In some cases it may become necessary to cut a wire to decide if the short is in the front or back end of the circuit. High resistance shorts between different conductors can also be mind boggling and may involve isolating each and possibly every electrical circuit in the aircraft.

Connections and resistance
Connections in circuits are often a source of unwanted resistance. Copper is a common conductor used in aircraft wiring and when a terminal end made of some other material is installed on the wire, and a steel bolt is used to connect that terminal to an aluminum structure, a small amount of moisture will no doubt invite galvanic action. This causes the beginning of corrosion resulting in a relatively small amount of resistance, which of course increases with time. This newfound resistance is now in series with all the other components in the system and will account for its own share of power consumed. When a high resistance connection is consuming voltage it means there is now less potential for all the other elements of the circuit. In some cases this type of resistance may be tough to spot using an ohmmeter.

Thrust reverser switches. Switch location is exposed to high vibration and temperatures. Technician testing thrust reverser switches using a test box. This will put a high load on the switch contacts which will show high internal

When a voltmeter is used to monitor the voltage drop across an electric connection a real-time observation can be made regarding restriction to flow based on how much voltage is displayed. A perfect electrical connection is unobtainable; however, very low resistance connections are possible and are highly desirable. In most cases a millivolt drop across the link is tolerable. Manufacturers that use airframe grounds often issue recommendations for installation of electrical terminations to ground. In some cases sacrificial metal is installed to bear the brunt of the dissimilar metal corrosion and as a result this material is replaced periodically. Sometimes sealing products are applied in an attempt to reduce exposure of the junction to moisture or corrosive substances. Specific manufacturer’s recommendations should be closely adhered to.

The measurement of amperage can often be a good way to evaluate an electrical malfunction. If there is no current flow in a specific system or device proper operation can not be expected. Also many circuits can not work normally with too little amperage and some malfunction will probably be observed. Excessive current flow can be detrimental to components. Current flow results in heat and high temperatures can cause component failure. Many multimeters have the ability to display amperage readings and in most cases the maximum current is limited to 10 amps or less. It is always important to be familiar with the limitations of any test equipment prior to use. When in doubt never jeopardize the aircraft or tester and keep in mind Ohm’s law may provide a suitable alternative. When a resistor of a known value is installed in series with an electrical load, the current through the resistor is the same as the circuit and the voltage the resistor consumes will be proportional to the amperage. If a resistance of 1/2 ohm is installed in series with a circuit that draws 5 amperes the resulting voltage drop across the resistor will be 2 1/2 volts.

Use of the ohmmeter is very common when looking for high resistance in an electrical circuit. In systems where current flows are low this method will often produce the desired results. Unfortunately they are less effective on systems where higher amperage is required. Most of us have experienced a clogged drain in a kitchen sink. If you fill a small cup with water and slowly pour it into the sink, the drain appears to work. It is only when a high volume of water is present that the restriction makes itself known. Oftentimes the small battery within the ohmmeter does not produce adequate current to observe the true nature of an electrical restriction to current flow. Using a voltmeter across a suspected high resistance will reveal a voltage drop. This is often a much more exacting method of fault isolation than attempting to read resistance. A test light or continuity tester may provide a quick and accurate means of locating an obstruction to flow. The important thing during testing is to try to simulate as closely as possible the conditions that exist within the circuit.

There are many specialty testers available which can provide a means to view each and every reaction that takes place within an electric circuit. The most advanced and sophisticated tester will still be ineffective if the user is not knowledgeable of its operation. Using test equipment that you understand combined with an understanding of the basic principles of electrical laws will provide a good foundation for solving electrical problems. Relating electrical systems to their hydraulic counterparts often provides the clues needed to get the clog out of the drain and flow back to normal.