Troubleshooting Basic DC Electrical Problems

The last few times you used your forward cabin lights you noticed they flickered and were not as bright as they should be. Today when you switched them on the circuit breaker popped. You reset it and it popped again! What now?

Here is a word of caution before we begin. AC wires on boats use the following color convention, black = hot or current
carrying, white = neutral and green = earth ground. DC wiring on older boats use red = hot or current carrying, black = ground. So be careful and ensure the black wire you are handling is indeed part of a DC circuit and not the hot part of an AC circuit. In newer model boats they have adopted yellow as the DC ground wire color code which means that if you are handling a black wire on these boats it is likely to be an AC hot wire.

Troubleshooting DC electrical problems can be a simple affair if you understand the basic concepts and are familiar with the terminology. Before discussing troubleshooting methods lets define a few terms and look at some basic circuit examples. A common misconception is that the current flows from the positive to the negative in a DC circuit, however the reverse is true. But for our purposes we will stick with the commonly accepted direction of current flow, positive to negative. In a DC electrical circuit the current will flow if there is a closed loop or path to ground. What this means is that the electrons leave the positive terminal on the battery, travel through the circuit breaker, then down the positive wire through closed switches to the load then travel through the negative wire back to the negative ground terminal on the battery.

Positive wire = red wire = hot wire
Negative wire = black wire = ground wire

Load
Load refers to any item using the DC current such as a light bulb, electronic device, heater or pump motor etc. When DC current passes through a load, there will be a voltage drop from one side of the load to the other.

Ampere (amp)
A measure of the amount or rate of electrical current (electrons) flowing through a medium. The medium is normally the boat's wiring and in some cases the bilge water and water surrounding the boat. In measuring the rate of current flow, the term amp is used and the conventional symbol is I.


Volts
For the electrons to flow we need a force to cause them to move. This force is caused by the difference in voltage between one side of a load versus the other side and is known as the electrical potential difference. It is also commonly referred to as potential or voltage drop. It is this difference in voltage that is the driving force, not the voltage itself, which pushes the current through the DC circuit. The unit of measurement used to describe this voltage drop is the Volt with the conventional symbol of E or V. We will use V to denote volts since it is more intuitive.

Voltmeters use simple math to compute voltage. The VOM measures the voltage on the black probe and subtracts that from the voltage measured on the red probe, the result is displayed as volts. In the diagram below you can see the Voltmeter measuring the difference between the two different sides of the load, 12.4 V - 0 V = 12.4 V.

Attachment 981


Resistance
Resistance is the opposition to the flow of electrons. In a conductor (wire), it is commonly caused by the wire's composition, cross sectional area (size) and length. Resistance increases as wire sizes get smaller and lengths (runs) get longer. The standard unit for resistance is the ohm with the symbol omega (Ω) or R.


This leads us to the Ohm's Law formula:



The equation will be discussed in more detail later but if any two values are known, the equation can be solved to find the third.


To effectively troubleshoot DC electrical systems you need to learn to use a Volt/Ohmmeter (VOM). The VOM is a highly versatile combination meter that will perform many different functions based upon the switch settings. For now we are only interested in the Voltmeter and Ohmmeter functions. We aren't going to concentrate on the specifics of the VOM meter itself; instead we will discuss the concepts and what you can expect to see. For your specific meter, read the instructions very carefully or you can damage the meter.

Voltmeter
We will begin by looking at a healthy circuit containing a power source (battery), a switch (toggle) and the load (a light bulb). For our examples we will use a 12 volt circuit but the test procedures essentially remain the same for 24 volts.

On a DC circuit you can essentially think of it as two sections, the positive 12 volt section (red) and the negative, ground potential 0 volt section (black). A VOM in the volts setting measures potential or difference, so with the leads of the meter placed as shown by meter A you can see 12.4 (positive) - 0 (negative) = 12.4 volts. Apply the same logic to meter B and you will note 12.4 - 12.4 = 0 volts. Note the other meter positions and readings.

Attachment 982

Now note the change in indications on meters C and D with the toggle switch selected off. With the switch off, the voltage downstream of the switch drops to ground potential or 0 volts.

Attachment 983

Ohmmeter
The ohmmeter setting on your VOM is also an essential tool for troubleshooting. In the ohm's position the meter will measure the resistance in the circuit allowing you to check the continuity of the electrical components. The ohmmeter has an internal battery that sends a small electrical current through the circuit. The meter measures how much of this current is lost due to resistance and displays accordingly. Warning: power must be removed from the circuit before using the ohmeter or damage will result.

Attachment 984

In the diagram above meter C shows 0L or over-limit. This is the way some ohmmeters display infinity or unlimited resistance and means the wire is broken (no continuity) and the resistance value is above the meters limits. Meter D shows a typical resistance reading found when checking various pieces of equipment such as the filament in a light bulb.


Closed Circuit
A circuit providing a complete "uninterrupted" path between the positive and negative terminals of the power source, in this case a battery.

Attachment 985

Open Circuit
A circuit that does not provide an "uninterrupted" path between the positive and negative terminals of the power source. An open circuit may be intentional such as with the toggle switch selected OFF in the illustration below.

Attachment 986

Or the open circuit may be due to an electrical problem such as the broken wire shown in the next illustration.

Attachment 987

Short Circuit
A short circuit is a circuit with an inadvertent direct connection between the positive and negative where the current bypasses the load, in this case the light. These can be commonly caused by a faulty switch. On older boats you may find wires with chafed insulation that are touching or a screw may have penetrated them as components have been added over the years.

Attachment 988

Short circuits normally trip circuit breakers and the load item does not work, for instance in the example above the light is not working.

Ground Fault (Leak)
Circuits with a ground fault allow a lower resistance path to ground than the actual ground wire. With a ground fault the current flowing through the positive does not equal the current flowing through the negative. Some of this current is escaping (leaking). This can be caused by a number of things such as chafed or poorly insulated wires. On boats, one of the most common causes of a ground fault is the poorly insulated bilge pump wires sitting immersed in water allowing the current to escape.

Attachment 989

It can be confusing at first to understand the difference between a short circuit versus a ground fault. With a short circuit the load does not work at all while an item with a ground fault may continue to work but not as well as it should. Ground faults are not likely to trip circuit breakers unless they are the GFCI type. Short circuits are a severe fire hazard since they may put an excessive load on a portion of the circuit not designed to handle it (smaller wire). For this reason if you find melted wiring thoroughly test all branches of the circuit and check any adjacent wiring.



General Troubleshooting
If a piece of equipment fails to work and is not providing hints to help us find out why, then a more general troubleshooting procedure can be used. A piece of equipment needs two things for it to work, electrical current and a path for the current to follow both to and from the equipment.
Attachment 924
Attachment 925
Attachment 927


Troubleshooting An Open Circuit
When troubleshooting with an Ohmmeter do not forget to remove power from the circuit or you risk damaging your meter!
Attachment 926


Troubleshooting A Short Circuit
When troubleshooting with an Ohmmeter do not forget to remove power from the circuit or you risk damaging your meter!

In example 1 the meter shows 0L or over-limit which means infinite resistance which it should with the load removed (open circuit). This indicates the circuit is good since no current is finding its way back to the negative side and the short circuit is in the load itself.

Attachment 990

In example 2 the meter shows a very small ohm reading. This means that even though the load has been removed (light bulb) current is still finding its way back to the negative side of the circuit. In this case through a screw that has penetrated the wires. A reading of less than infinity or 0L (over-limit) generally means there is a problem with the wiring.

Attachment 991

Now that we have determined that we have a short circuit in the wiring, how do we find it?

Isolating a Short Circuit
When troubleshooting with an Ohmmeter do not forget to remove power from the circuit or you risk damaging your meter!

Attachment 976

If a short circuit is not present the meter should show an OL or over limit since the circuit should not be able to complete the path. In the example above the meter indicates a short is present (yellow path) with an indication of 0.62 ohms. A lower meter reading normally indicates a more severe short.

To continue to isolate the location of the short:

If after breaking the circuit, the meter indication does not change, then the short is between the meter and the switch/breaker as shown below (see yellow path).

Attachment 978

If after breaking the circuit the meter indication changes to OL over limit then the short is on the other side of the switch as shown below. In this instance the switch has broken the circuit.

Attachment 979If the short is on the other side of the switch as shown above, turn the switch back ON and break the circuit further along, in this case turn OFF the breaker. If the meter indication does not change then the short is between the breaker and the switch. If the meter indication jumps to OL overlimit the short is somewhere prior to the breaker.

Troubleshooting a Ground Fault
When troubleshooting with an Ohmmeter do not forget to remove power from the circuit or you risk damaging your meter!

If the VOM shows any voltage you have a leak, see the illustration below!

If the VOM still displays a voltage then the leak is on the battery side of the switch. Normally the only thing wired prior to the battery switch is the bilge pumps. In the illustration below you can see the battery switch has been turned off and the VOM still indicates a voltage reading, a faulty connector leaking current into the bilge water is the guilty culprit. When the positive battery cable is reconnected to the positive battery terminal the current flows down the red wire to the bilge water, then into the bilge water where it finds its path to ground via the bad connector on the black ground wire then back to the battery negative terminal. The current could also find its path to ground through anything that is in the bilge water, prop shaft, thru-hull etc.

Attachment 993

If the VOM no longer displays a voltage indication with the battery switch selected OFF, then the leak is in the boat's wiring as shown in the following illustration. At some point on the boat past the battery switch the wiring is allowing positive current to find a path to ground. Once again this path to ground may or may not be through the negative wire.

Attachment 994

Note: If you are troubleshooting circuits with light fixtures you can get a false positive since the filament in some types of bulbs will allow the path to be completed to ground. The way to overcome this is to turn the light switch off but then you will not be testing the wiring downstream of the switch to the bulbs. The other method is to leave the light switch on but remove the bulbs.

Attachment 995

If the leak is in the boat's wiring accomplish the following steps to isolate the bad circuit.

6. Turn the battery switch ON.

7. On the DC panel pull one circuit breaker at a time. The VOM should continue to indicate the leaky voltage until you pull the circuit breaker of the bad circuit, then the VOM should indicate 0 volts.

8. Once you find the leaky circuit you can use the troubleshooting procedures for Isolating a Short Circuit listed above to help find the leak. Since a leak is essentially a minor short circuit the ohmmeter readings will be higher.

If you pull all of the circuit breakers and the VOM still indicates the leaky voltage, then either you still have something powered on the boat or the leak is in the wire from the battery switch to the DC panel.

These troubleshooting procedures are just an example of several methods that can be used to troubleshoot DC electrical problems. Hopefully this article will help you become more proficient at tracking down problems.

Download a pdf version of the article here!