Troubleshooting
45 Using Meters for Troubleshooting
Voltmeter
A voltmeter is designed to be used on a live circuit. When working on live circuits, all safety precautions and PPE (personal protective equipment) requirements should be adhered to.
A voltmeter is essentially a very high value of resistance. When the voltmeter leads are connected to two points in a circuit, that high value of resistance is connected in parallel with that point and, due to Kirchhoff’s Voltage Law, experiences the same value of voltage. By measuring how much current flows across the meter’s internal resistance, it can calculate the voltage value.
Voltmeters typically read zero volts when measuring between points of equal potential. Closed switches and connected wires are examples of components that, when energized, are of equal potential.
Voltmeters typically read line voltage when measuring between points of different potential. In a control circuit this is line voltage. In the power circuit, this is a phase-to-phase voltage, which is different from their phase-to-ground voltage.
Open switches and contacts are examples of components that a voltmeter would measure line voltage across.
When current flows through a circuit, it drops off voltage proportional to the resistance of the device it flows through. Since closed switches and contacts offer nearly zero resistance, there will be no voltage drop measured across them. In every branch of a control schematic, there must be only one load to limit the current, and this device will have line voltage dropped across it when the circuit is operating. The coils of motor starters, control relays, and timer relays are examples of loads that when energized would have full line voltage dropped across them. Pilot lights are another example of resistive loads that would experience full line voltage.
Ohmmeter
Disconnect circuit from power supply first! Before using an ohmmeter in a circuit, use a voltmeter to confirm that power is off and that there is zero potential difference between the two points you wish to measure.
An ohmmeter works by using an internal voltage source to push a small DC current through its leads. By measuring the value of current, it can display a calculated value of ohmic resistance. Because it has an internal voltage source, ohmmeters cannot ever be connected in live circuits as they could cause damage to equipment or injury to the operator.
When using an ohmmeter in a control circuit, there are three typical readings you can get:
- Close-to-zero ohms: This means that the leads of the ohmmeter are connected across two points that are electrically common. The two terminals of a closed switch or contact would give an ohmmeter reading close to zero ohms.
- Very-high-to-infinite ohms: An open in the circuit would allow zero current to flow and so would read as infinite ohms. The terminals of open switches and contacts would give very high ohmic readings.
- Some ohms: An ohmmeter measuring across a load such as a pilot light would read a very high (approximately megaohms), but not infinite, value of ohms. This is one way of confirming that a pilot light is in good working condition. The terminals of a coil should have continuity and a low (approximately tens to hundreds of ohms), but not zero value of resistance. If a coil is shorted and reads zero ohms, it needs to be replaced.
When using an ohmmeter to test fuses, first confirm they are removed from the circuit. If a fuse is in good condition, it should give a close-to-zero reading of resistance. If the fuse has blown due to a fault, then it should behave as an open and give an infinite resistance reading.
A device testing and measuring the potential difference (voltage) between two points. Leads are connected in parallel with the circuit, and the meters very high internal resistance will draw a small current which can be used to determine the level of voltage.
Can be digital or analogue and measure either AC or DC.
The opposition to the flow of current in an electric circuit, measured in ohms (Ω).
In electrical terms, refers to a connection where current has more than one path to flow.
Loads connected in parallel will experience the same potential difference (voltage), but may draw different values of current depending upon their individual resistance.
"The sum of the voltage rises must equal the sum of the voltage drops in a circuit."
In a series circuit, the total voltage at the source is equal to each of the individual voltage drops in any loads.
E total = E1 + E2 +E3...
Loads connected in a parallel circuit will experience the same potential difference.
E total = E1 = E2 = E3...
The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).
A device for making or breaking the connection in an electric circuit.
In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.
In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself. Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor.
The conducting part of a switch that makes or breaks a circuit.
A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.
Used when additional auxiliary contacts are needed in a control circuit, a control relay is a magnetic contactor which is not designed for the energization of motors, and does not have built in overload protection.
A small lamp connected in the control circuit to indicate the status of a motor or other situation.
A device used to measure the resistance of a circuit. Ohmmeters must not be used on live circuits. Ohmmeters connect a small internal voltage source to the circuit that is being measured or tested, and determine the value of resistance or continuity by measuring what value of current flows through the meter.
Can be either digital or analogue.
Referring to two or more points in a circuit which have no loads or switches between them and have no potential difference between them.
The unit used to measure electrical resistance (Ω). It takes one volt to push one amp through one ohm of resistance.
An insulated tube containing a strip of conductive metal that has a lower melting point than either copper or aluminum. It protects a circuit from damage because it will melt in overload or overcurrent situations and break the connection with the rest of the circuit.
