11 Analyze Circuits
The trick to effective troubleshooting electrical equipment and circuits is to zero in as quickly as possible on the problem. Using an electric meter will allow you to effectively test the components that are most likely to be the cause of the problem before you unnecessarily dismantle the equipment and replace parts.
Safety
Even though you may normally deal with small voltages and currents, the values are never far away from lethal levels. You can receive a shock or burn from any common electrical circuit. The severity of the electrical shock depends on a number of factors:
- the amount of current that passes through the body
- the path that the current takes through the body
- type of voltage—AC or DC
- voltage strength
- the length of time that the current flows within the body
- condition of the skin and the body’s chemical makeup
- area of contact
Normal household current (plugs and light circuits) is generally limited by a circuit breaker to a value of 15 amperes. This device has been designed to trip and open a circuit if the 15 ampere value is exceeded, and it is designed to protect against property damage. It is possible to cause a fatal injury with a current flow of only 50 milliamperes (mA) or five one-hundredths of an ampere. The body is sensitive to relatively small values of current. In comparison, a 100 watt lightbulb draws approximately 0.85 ampere (850 mA) of current when connected to a 120 volt source. Remember, there are 15 amperes available in each standard house circuit.
Electrical shocks, electric burns, and other related injuries occur far too often and in most cases go unrecorded. If an accident happens:
- Don’t touch the person and don’t use a conductive tool to free the person that may be electrically energized.
- Shut off the power or pull the plug if it is safe to do so. If you are not able to, get help.
- Remove the person from the contact point using a non-conductive object such as a dry piece of wood or a leather belt.
- Call 911 for help if the person is obviously injured (loss of consciousness, significant trauma, etc.)
- Seek medical attention (first aid) in any case of injury such as an altered mental state (confusion, slow/slurred speech) or other obvious injury (laceration, burn, etc.).
When performing maintenance or doing repair work, or when a machine is in an unsafe state, it is vital to eliminate the possibility of the machine being energized unexpectedly. In order to create a safe work environment, workers need to guard against contact with electrical voltages and control electrical currents.
Make the environment safer by doing the following:
- Protect portable electrical equipment with an approved ground-fault circuit interrupter (GFI) when using the equipment outdoors.
- Ensure all the cords are in good condition, with the caps and plugs well secured on the cables. Ensure the proper U-ground plug is in good working condition.
- Use cords of sufficient gauge for the amount of current used by the tools they are powering. Each tool is labelled with the power that it draws.
- Treat all conductors and bare wires—even apparently de-energized ones—as if they are energized until they are locked out and tagged.
- Do not make any electrical measurements without specific instructions from a qualified person.
- When servicing equipment be sure it is “locked out,” meaning the electrical service is shut off at a disconnect panel whenever possible, the panel is locked, and the only key is kept by the person working on the equipment.
- When replacing components on mobile equipment, disconnect the battery.
Troubleshooting principles
IMPORTANT: The first step in all testing or troubleshooting is to use a “non-contact voltage tester” to determine if the circuit is energized or de-energized. All workers who have contact with any electrical components should carry one of these devices. It will not tell you the level of voltage, but it will tell you if a voltage is present.
There are really only two rules for troubleshooting using a voltmeter. They are simple and always true:
- If you measure a voltage across a switch, the switch is open.
- If you measure a correct voltage across a load and the load doesn’t work, the load has failed.
With digital meters, voltage readings that are considered as zero will often indicate very small voltage readings. For example, when reading across a closed switch, a very small reading could indicate a very slight resistance across the switch contacts or even a meter inaccuracy.
Notice that the first rule does not say that if you read zero volts across a switch, the switch is closed. There are many situations in which you might read zero volts across an open switch.
The second rule indicates that the load has failed. This only means that the problem is with the load and you don’t have to look anywhere else for the problem. The actual remedy still has to be determined. This may require a replacement of the load, but there may be other possibilities. For example, there may be an overload that needs resetting.
Always look for the easy fix first. Check components that are easily accessible first that might explain the symptom that you have observed. For example, one of the first checks is to verify the power supply.
Voltage tests
You can troubleshoot a problem using either volt or ohms tests. It is most practical to choose voltage testing. With a resistance test, you have to first disconnect the component being tested from the circuit, and while you are removing the wiring you could jostle things and possibly change the circuit, which may temporarily remedy the problem. In other words, you may not really find the problem.
When you use your voltmeter to troubleshoot, you will find either a switch that is open or a load that has failed. You can do this without moving any wires and without changing the circuit in any way. You may then remove the device and double check it with your ohmmeter.
Voltage drops in series circuits
In series circuits, the total voltage is the sum of the individual voltage drops in the circuit, and the equation E = IR is used to calculate the voltage drop across each resistor. Since the current is the same through each resistor, the voltage drop across each resistor is directly proportional to the value of resistance. In other words, the greater the value of a resistor in a series circuit, the higher the voltage drop. Consider the simple series circuit in Figure 1.
Polarity in a parallel circuit
Just as in series circuits, electrical current flows “from negative to positive” through each of the load components in a parallel circuit. As illustrated in Figure 6, electrons leave the negative terminal of the source and flow from negative to positive through each of the load resistors. Note that the polarity of each of the resistors is the same as the polarity of the source.
6. Polarity test
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- Tie one side of the power sources together.
- Before connecting the paralleling jumper between the remaining two terminals, insert a voltmeter between these two points. See Figure 7.
- If the polarity is incorrect (Figure 7b), the voltmeter indicates two times the source voltage, because the equal EMFs aid each other. Do NOT connect across these terminals.Polarity in a parallel circuit
Polarity is always expressed from one point of a circuit relative to another point with a different electrical potential. Note that in Figure 6 the top side of each resistor, which is marked negative, is in effect the same point. No difference in potential exists between any of these like terminals.
Also notice that the individual currents through each resistor (I1, I2, I3) together constitute the total current (IT) drawn from the source. When the total current required to operate each of these parallel loads exceeds the current output rating of the one source, you will need to increase the source output.
Polarity test for parallel voltage sources
Voltage sources are connected in parallel whenever it is necessary to deliver a current output greater than the current output a single source of supply can provide, without increasing voltage across a load.
- Power sources are connected in series to increase the voltage output.
- Conversely, power sources are connected in parallel to increase the current capacity.
An advantage of parallel-connected power sources is that one source can be removed for maintenance or repairs while reduced power to the load is maintained. If a 6 V battery has a maximum current output of 1 A, and if it is necessary to supply a load requiring 2 A, then you can connect a second 6 V battery in parallel with the first.
If there is any doubt about the polarity of the two batteries, then you can do a simple voltmeter test for correct polarity.