Tech Page: Electrical Test Instruments

February 13, 2002
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If you want to know, you have to test. The professional hvacr service technician/installer uses electrical test instruments (ETIs) to diagnose, test, and measure equipment performance, controls, circuitry, and wiring. From troubleshooting direct digital control (ddc) systems, to setting a thermostat heat anticipator, to verifying proper voltage at a transformer, servicers/installers are knee-deep in situations that demand the use of ETIs every day.

The use of an ETI involves the ability to recognize what is being tested or measured, and to interpret that information and its relationship to overall system operation.

On this Tech Page, we will discuss components that make up electrical circuitry, what is being measured, and the instrumentation used to take these measurements. One reminder before we proceed: While many fine brands and different types of ETIs are available, human appendages are not one of them. Always use extreme caution when working with electricity. Read the instrument’s instructions thoroughly and follow directions diligently.


An electrical circuit is where most of the action takes place when using an ETI. The following are major components of the circuit:

  • Conductor — Carries electricity or current from the power source to the load. Copper wiring is one example of a conductor.
  • Load — Equipment that consumes the current, such as a motor.
  • Switch — Device that either starts or stops the current flow. A thermostat is a type of switch.
  • Continuity — Refers to a circuit that is continuous (no opens or breaks).
  • Power source — Electrical power originates at power generation plants. For our discussion, power originates at the building’s electrical panel.


    The following is terminology used to describe the rate of current, what pushes it through a circuit, and what forces work to hold it back.

    The ETI measures this data. It is up to you, the servicer/installer, to interpret it and apply it to the situation at hand.

  • Ampere (A) — The rate of flow of the current;
  • Volt (V) — A unit of electrical pressure;
  • Ohm — The amount of resistance in an electrical circuit;
  • Watt (W) — A unit of electrical power; and
  • Farad — The unit of capacity of a capacitor.

    Sometimes we need to measure a fraction of an ampere or volt. The following two prefixes are then used:

  • Milli — Means one thousandth; examples are millivolt and milliamp.
  • Micro — Means one millionth; examples are microvolt and microamp.

    The following are the two types of current encountered in the hvacr field:

  • Direct current (dc) — Electrons continually flow in one direction. Voltage generated by a thermocouple when sensing the pilot flame is dc. Automobile batteries are also dc.
  • Alternating current (ac) — The flow of current alternates direction. Nearly all residential furnaces and condensers run on ac.


    Digital multimeter (DMM) — This ETI is a combination of several different meters. Its versatility makes it one of the most commonly used meters in the hvacr field today. The number of its functions are in direct correlation with the cost of the meter.

    A basic DMM will read ac and dc volts, ohms, and continuity. Additional features may include ac and dc current, frequency, and capacitance.

    Typical uses of the DMM are:

  • Measuring for proper input voltage;
  • Checking for open circuits;
  • Measuring resistance of motor windings; and
  • Measuring microamps of a flame current.

    Some DMMs will have an “autoranging” feature. The meter automatically sets the range of current or voltage to be measured.

    Clamp-on ammeter — This ETI is also very commonly used. A clamp-on ammeter can measure voltage, resistance, and continuity; however, its most distinguishing characteristic is its ability to clamp around a conductor and measure current. This is useful when measuring items like:

  • Current draw of a compressor;
  • Current draw of a furnace blower motor; and
  • Amperage at an electrical panel.

    Capacitor tester — This ETI measures the capacitance value of a capacitor in microfarads. It also can determine if a capacitor is good or bad.

    Compressor analyzer — This ETI tests compressors for continuity, resistance, and amperes. Compressor analyzers contain a start capacitor and a reverse switch to assist in starting single-phase compressors that have become stuck.

    Megohmmeter — This ETI measures electrical insulation resistance in ohms or megohms. It can detect motor winding breakdown and buildups of moisture and acid.

    Watt meter — This ETI measures watts consumed by an electrical device.

    Could you imagine your family doctor not using a stethoscope? Electrical test instruments are like stethoscopes and blood pressure/heart rate monitors. They let you know what is happening within the circuit or system.

    Rothacker is a director for AREA51HVAC.COM. For questions or comments on the Tech Page, contact Rothacker at (e-mail).

    Publication date: 02/18/2002

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