What meter must never be connected to a circuit when the circuit power is turned on?

A 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.

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A voltmeter is essentially a very high value of . When the voltmeter leads are connected to two points in a circuit, that high value of resistance is connected in with that point and, due to , experiences the same value of . 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 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 this is line voltage. In the , this is a phase-to-phase voltage, which is different from their phase-to-ground voltage.

Open switches and 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 , , and timer relays are examples of loads that when energized would have full line voltage dropped across them. are another example of resistive loads that would experience full line voltage.

Ohmmeter

Disconnect circuit from power supply first! Before using an 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 . The two terminals of a closed switch or contact would give an ohmmeter reading close to zero .
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 , 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.

There is a range of meters used for measuring various electrical quantities. These meters are often expensive and delicate pieces of equipment, so they should always be handled with care and used according to the manufacturer's instructions.

Meters are available with two different forms of readout:

analogue – readout on a graduated scale with a pointer operated by a magnetic field
digital – readout on a digital screen.

Analogue display

An analogue meter must be positioned correctly. If placed on its side or an angle, the pointer could be affected by gravity and give an incorrect reading.

When viewing the scale, position yourself directly in front of the scale. If viewed from an angle, you will get an incorrect reading – the greater the angle the more inaccurate it will be. This is because the pointer is separated from the graduation scale by a short distance. This is called the parallax effect.

An analogue meter must be checked for zero before being used. There is usually a zero set screw adjustment to ensure that the pointer starts on the zero calibration before measurement is taken. Any time you change scales when measuring resistance, you must always set the zero again.

Always connect meters with the correct polarity – black to negative and red to positive. Failure to do this could damage the meter, particularly an analogue meter.

Digital meters are difficult to read in direct sunlight. Most have a battery to supply power. Make sure that the battery is not flat and that you have a spare available.

The four most common meters are:

voltmeter
ammeter
ohmmeter
multimeter.

Voltmeter

A voltmeter is used to determine the voltage (electrical pressure) present at any point in a circuit.

Connecting the voltmeter across the battery measures the voltage present in the battery.

Measuring battery voltage

Connecting one lead of the voltmeter to the input side of a globe and the other lead to earth results in a reading of the voltage present at the globe. Voltmeters are mostly used to measure voltages across components. They are connected in parallel with the component.

Measuring voltage at the globe

Ammeter

An ammeter measures the amount of current flowing through a circuit.

An ammeter must be connected in series with the circuit. An ammedter has a very low internal resistance and must never be connected directly to a power supply without being in series with a load. The ammeter used must be capable of carrying the current that you expect to read.

To connect an ammeter, the circuit needs to be disconnected (broken) and the ammeter placed in series with the circuit.

Measuring current flow

Ohmmeter

An ohmmeter is used to measure the amount of resistance in a component or section of circuit.

An ohmmeter has its own power supply. The circuit being tested must always be de-energised (turned off) when using the ohmmeter.

Measuring resistance of a globe

When using an ohmmeter:

isolate the component or circuit section that requires measurement to ensure a correct reading occurs
avoid touching the metal probes of the ohmmeter as this may lead to a false reading.

Warning

Failure to disconnect or isolate the component being measured from the power supply will cause damage to the ohmmeter.

Multimeter

A multimeter, as its name suggests, has multiple uses. Some of the quantities it can measure include voltage, current and resistance. Multimeters are available with analogue or digital readouts. Digital multimeters are commonly used as they give accurate results and are easy to read, except in direct sunlight.

Multimeters

The two different multimeters shown can measure a range of electrical quantities including voltage (AC or DC), ohms, continuity, capacitance, frequency and current (AC or DC).

The rotary switch is used to select the quantity (and range) you wish to measure. The left side meter is auto ranging and the user needs only to select the type of quantity to be measured. For the right hand side meter, the user needs to select the electrical quantity as well as an appropriate range based on the value expected. It is best to start with a high range and then progressively rotate the switch to a lower range.

The test leads of the multimeter need to be connected into the meter in the appropriate positions. The black lead connects into the COM (common) position and the red lead into either V, A or mA position, depending on whether you are measuring voltage (V), resistance (Ω) or current (A or mA).

When testing a circuit with a multimeter, you need to connect the leads to the circuit appropriately depending on whether you are measuring voltage (V), resistance (Ω) or, current (A or mA). The connections are the same as for the voltmeter, ammeter and ohmmeter described above.