7.2.4 Student Book © 2005 Propane Education & Research CouncilPage Identifying Procedures for Measuring Current Between Two Given Points in an Electrical Circuit Making current measurements is useful for checking proper operation of electrical motors, heating elements, and setting the heat anticipator on some electro-mechanical thermostats. In this module, you will learn to: 1.Identify electrical current and its unit of measurement 2.Identify electrical current classifications 3.Connect the meter to directly measure electrical current 4.Apply meter input protection 5.Use principles of indirect measurement of current values 6.Use circuit current measurement to set a thermostat heat anticipator

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 1 Identifying electrical current and its unit of measurement Electrical current flow is the flow of electrons from a negative to a positive charge. Electric current flowing through an electrical circuit may be compared to the flow of gas through a piping system. There is one primary difference— In a gas system the gas flows from the source to the load (burner), and in an electrical system the current flows from the voltage source through the load and then back to the voltage source. Figure 1. Comparing Gas Flow with DC Electrical Current Flow The unit of measurement for electrical current is the ampere. The ampere is the rate at which electrons move past a given point. The mathematical symbol for amperes is the letter "I." (The letter "I" stands for intensity of electron flow).

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 2 The name ampere is often shortened to amps and is abbreviated "I" or "A." The milliampere (MA) is one thousandth (.001) of an ampere. The microampere () is one millionth ( ) of an ampere. In other words, there are 1,000 milliamperes or 1,000,000 microamperes in an ampere.

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 2 Classification of electrical current Electrical current is commonly classified as either direct current DC or alternating current AC. The measurement of current in this instruction sheet is discussed primarily in terms of digital meters. Current measurements are different from other measurements made with a digital multimeter. Direct current measurements are taken by placing the meter directly in series with the circuit being measured, thus allowing all circuit current to flow through the meter circuitry. This method requires special precautions and procedures. Indirect current measurement does not require the circuit to be opened nor does it require the meter to be placed into the circuit. Indirect current measurement does require the use of a current probe or a clamp meter.

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 2 Connecting the meter to directly measure electrical current To protect yourself and the ammeter the following steps should be followed: Step 1Turn off the power supply or remove the power source from the circuit to be tested. One method of removing the power source is to disconnect one leg of the circuit from the power source terminal. Step 2Break the circuit (current path)at the point where the current is to be measured. Do this by cutting, disconnecting or unsoldering the connecting wiring in the circuit. The ammeter is connected in series with the leg of the circuit being tested. Step 1: Disconnect the Power Source Step 2: Break the Circuit

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 3 Step 3Select Amps AC (A~) or Amps DC (A ) as required. Step 4Plug the black test probe into the COM input jack. Plug the red test probe into the highest expected range jack—this might be the 10 amp (10A) or 300 milliamp (300mA) input jack. (Clamp meters will typically not have input jacks for direct current measurement.)

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 3 Step 5Connect the ammeter into the circuit. (WARNING: If an analog meter is used—check to insure the meter is properly connected by observing the polarity. The ammeter has two terminals; one is labeled negative, ( ‑ ) and the other terminal is labeled positive (+). Current must flow through the ammeter from the negative terminal to the positive terminal. Step 5: Connect the Ammeter Into the Circuit

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 3 Step 6Connect the circuit or turn the circuit power supply on, and read the current value on the ammeter scale. NOTE: On a digital meter, if the test leads are reversed for a DC measurement, a “-“ will show in the display. Step 6: Connect the Circuit Wiring

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 3 Applying meter input protection A common and costly mistake made with multimeters is to leave the test leads plugged into the current input jacks and then to attempt a voltage measurement. This causes a direct short across the source voltage through a low-value resistor inside the meter called a current shunt. As a result, a high current flows through the meter, and if the meter is not adequately protected, can cause extreme damage to the meter and to the circuit, and injury. Extremely high fault currents can occur if industrial high voltage circuits are involved. Meters without fuse protection should not be used on high- energy electrical circuits (  240V ac). Make it a consistent practice to remove test probes from input jacks after completing measurements.

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 4 Always make sure the power is off before cutting or unsoldering the circuit and inserting the meter for current measurements. Even small amounts of current can be dangerous. Never attempt a voltage measurement with the test probes plugged into the current measurement jacks. Meter damage or personal injury may result!

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 4 Principles of indirect measurement of current values In higher current applications (typically over 2A), where high accuracy is not needed, a current probe or clamp meter is very useful. With a clamping meter or current probe, it is not necessary to break the circuit. Instead a current flowing through a single connecting wire is indirectly measured by the magnetic field that is produced by an induction coil that comprises the clamp or current probe. Figure 2. Measuring Current Draw on a Branch Circuit (Display reads 0.7A)

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 4 Figure 3. Measuring Current Draw on one Lead of a Hot Surface Igniter

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 5 Using current measurement to set a thermostat heat anticipator Technicians must measure the current amp draw of the gas valve and blower fan to properly set the thermostat heat anticipator. The amp draw of these components is measured in the R-W circuit using a clamp multimeter. Accurate amp draw readings can be obtained as a direct current measurement at the thermostat sub-base terminals R and W. Figure 4. Measuring Amp Draw for Heat Anticipator Setting

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 5 Most clamp meters and current probes measure current in values higher than those of an ammeter direct measurement. However, a clamp meter can be configured to more accurately measure current flow by wrapping a wire carrying the current around one jaw of the clamp, thus increasing the strength of the magnetic field induced in the clamp coil. Figure 4 illustrates an easy method of obtaining actual amp draw. Take the amp reading after blower motor has started.

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 5 The actual value to use in setting the heat anticipator is obtained by dividing the reading on the meter display by the number of wire turns around the clamp jaw.

7.2.4 Student Book © 2005 Propane Education & Research CouncilPage 6 Figure 5. Thermostat Anticipator Adjustment

7.2.4 Student Book © 2005 Propane Education & Research CouncilPages 7 & 8 Time to See If You Got the Key Points of This Module… Complete the Review on page 7. See if you are ready for the Certification Exam by checking off the performance criteria on page 8.