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newSchoolWorks.com › VoTechWorks › Building & Construction › Elevator Mechanic (pre-apprentice) › Elevator Controls: Circuits, Controllers, and Signal Systems › Lesson 5
BLD-ELEV10-04 · Lesson 5 · 12 clock hours · practice

Test Instruments and Measurement Procedures on Control Circuits

Objective. Students will correctly select meter function and range, then measure AC/DC voltage, current (using a clamp meter), resistance, and continuity at designated test points on a powered trainer circuit without causing a fault or shorting any terminal.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Test Instruments Matter in Elevator Control Work

Elevator controllers contain a mix of AC and DC circuits, relay coils, solid-state boards, and signal wiring—all operating in close proximity. A wrong measurement technique can blow a fuse, latch up a safety circuit, destroy a sensitive component, or, most seriously, expose you or a coworker to an electrical shock hazard. Selecting the right instrument, the right function, and the right range before you touch a probe to any terminal is not a formality—it is the core skill that separates a safe, effective technician from one who creates new problems while diagnosing old ones.

Lockout/Tagout Before You Touch Anything for Resistance or Continuity

The single most important rule: never measure resistance or continuity on an energized circuit. Current from the circuit will damage your meter and can injure you. Before switching your meter to the resistance (Ω) or continuity (diode/beep) function, verify the circuit is de-energized and apply lockout/tagout (LOTO) per your site energy-control procedure. Place your own lock on the disconnect, verify absence of voltage with your meter in AC voltage mode, then proceed. For voltage measurements the circuit must be energized—keep one hand in your pocket or away from grounded metal to reduce the path for current through your body.

Personal Protective Equipment

Wear insulated gloves rated for the voltage level present (check the voltage rating marked on the gloves), safety glasses, and arc-flash PPE appropriate to the panel's incident-energy level. Use test leads with insulated, finger-guarded probes (CAT III or CAT IV rated for the environment). Never use leads with cracked insulation or exposed metal at the probe shank.

Your Digital Multimeter (DMM): Functions and Ranges

AC Voltage (VAC): Select the VAC function. If your meter has manual ranging, start at the highest range and work down to get a precise reading. Most modern meters are auto-ranging—still confirm the displayed range makes sense. Elevator control circuits commonly include 120 VAC for coil supplies and 208/230 VAC for motor feeds; read the wiring diagram so you know what to expect before probing.

DC Voltage (VDC): Select VDC. Controller logic circuits often operate at 24 VDC or lower. Polarity matters—if the meter displays a negative value, you have the leads reversed relative to circuit polarity. This is a reading, not a fault; simply note the polarity.

Resistance (Ω): De-energize the circuit. Touch probes across the component. A relay coil in good condition will show a resistance in the range printed on its data sheet (commonly tens to hundreds of ohms). An open coil reads OL (overload/open). A shorted coil reads near zero. A corroded or burned contact will show higher resistance than the near-zero value a good closed contact produces.

Continuity: De-energize the circuit. Select the continuity or diode function (symbol is usually a speaker or diode icon). The meter beeps when resistance is below a threshold (typically under about 30 Ω on most meters), confirming a complete path. Use this to quickly verify wire runs, fuse integrity, and contact closure without reading a number.

The Clamp Meter: Measuring Current Without Breaking the Circuit

A clamp meter measures AC current (and on better models, DC current) by sensing the magnetic field around a single conductor. The jaw must encircle only one conductor. If you clamp around a two-conductor cable (both the supply and return conductors together), the fields cancel and you read zero. Open the jaw fully, position it around one wire, close it fully, and read the display. Select the appropriate AC or DC current function and a range above the expected current before clamping. Clamp meters are non-contact for current—you do not break the circuit, so LOTO is not required for the measurement itself, but follow all energized-work PPE requirements.

Reading the Wiring Diagram First

Before every measurement, consult the wiring diagram for the trainer. Identify the test point, the expected signal type (AC or DC), the nominal voltage or resistance value, and whether the circuit should be energized or de-energized for that measurement. Write the expected value on your station card before you probe. This gives you a target to compare against and alerts you when something is genuinely wrong—versus when you simply selected the wrong function.

Recording and Interpreting Results

Record three things for every measurement: (1) the expected value from the diagram, (2) the measured value with units, and (3) pass or fail against the station tolerance. A reading outside tolerance is a diagnostic clue, not a test failure on your part—unless you made the measurement incorrectly. Common abnormal findings and their meanings:

Technique Summary

1. Read the diagram; identify expected value and signal type.

2. Select the correct meter function and range.

3. Apply LOTO if the measurement requires de-energization.

4. Don appropriate PPE for energized or de-energized work.

5. Place leads correctly—line to line for voltage, across the component for resistance, jaw around one conductor for current.

6. Read, record, and compare to expected value.

7. Remove leads before changing function or range on an energized circuit.

Key terms9

Auto-rangingA DMM feature that automatically selects the best measurement range; the technician still verifies the displayed range is appropriate for the circuit.
CAT ratingA safety category (CAT I through CAT IV) marked on test leads and meters that indicates the overvoltage and transient environments they are designed to withstand safely.
Clamp meterA test instrument that measures current by clamping its jaw around a single conductor and sensing the magnetic field, without interrupting the circuit.
Continuity testA de-energized measurement that produces an audible beep when a complete, low-resistance electrical path exists between two test points.
Lockout/Tagout (LOTO)An energy-control procedure that isolates equipment from its energy source and secures the isolation device with a personal lock before de-energized work begins.
OL (Overload/Open)A DMM display indicating the measured value exceeds the selected range, or that resistance is too high to measure—indicating an open circuit in a resistance measurement.
Test pointA labeled terminal, wire junction, or component terminal identified on a wiring diagram as a designated location for taking a measurement.
Voltage dropA reduction in voltage across a component or conductor caused by resistance; excessive voltage drop across a contact indicates high contact resistance.
Relay coil resistanceThe DC or AC resistance of a relay's electromagnetic coil, measurable with a DMM on a de-energized circuit; a value outside the manufacturer's specification indicates a faulty coil.

Worked examples4

1. A station card states the expected voltage at test points TP3 and TP4 is 120 VAC ±5%. Your DMM reads 114.5 VAC. Does this pass?
  1. Calculate the lower tolerance limit: 120 VAC × (1 − 0.05) = 120 × 0.95 = 114.0 VAC.
  2. Calculate the upper tolerance limit: 120 VAC × (1 + 0.05) = 120 × 1.05 = 126.0 VAC.
  3. Compare the measured value: 114.5 VAC is between 114.0 VAC and 126.0 VAC.
Answer: Pass. 114.5 VAC falls within the ±5% tolerance band of 114.0–126.0 VAC.
2. The wiring diagram shows a relay coil rated at 180 Ω ±10%. After applying LOTO, your DMM resistance reading is 205 Ω. Does the coil pass, and what could a reading of OL indicate?
  1. Calculate the lower limit: 180 Ω × 0.90 = 162 Ω.
  2. Calculate the upper limit: 180 Ω × 1.10 = 198 Ω.
  3. Compare: 205 Ω exceeds the upper limit of 198 Ω, so the coil fails the resistance check.
  4. OL on a coil resistance measurement means the coil winding is open (broken wire internally); no current can flow and the relay will not energize.
Answer: 205 Ω fails (acceptable range is 162–198 Ω). An OL reading on the same coil would indicate an open winding—the coil must be replaced.
3. You use a clamp meter to measure current in a 24 VDC control circuit wire. The meter is set to DC Amps, jaw closed around both conductors of the twisted pair running to a door-lock relay. The display reads 0.00 A. Is this a real reading or a technique error?
  1. Recall the operating principle: the clamp meter senses the net magnetic field around the conductors inside the jaw.
  2. When equal and opposite currents flow in two conductors (supply and return), their magnetic fields cancel, producing a net field of zero.
  3. The jaw must encircle only one conductor to obtain a valid current measurement.
  4. Re-clamp around the supply conductor only, keeping the return conductor outside the jaw.
Answer: Technique error. Clamping both conductors cancels the magnetic fields. Re-measure with only one conductor inside the jaw to get the true current value.
4. A closed safety-chain contact reads 4.7 Ω on your DMM (de-energized). The station card states a good closed contact should read ≤ 0.5 Ω. What does this reading suggest, and is it within the ±5% station tolerance?
  1. Note that the tolerance of ±5% applies to the nominal value: 0.5 Ω × 1.05 = 0.525 Ω upper limit.
  2. 4.7 Ω far exceeds 0.525 Ω, so the reading does not pass.
  3. A closed contact with resistance nearly 10 times the acceptable maximum indicates heavy oxidation, pitting, arcing damage, or a poor mechanical connection.
  4. Diagnose as a high-resistance contact requiring cleaning, resurfacing, or replacement before return to service.
Answer: Fail. 4.7 Ω far exceeds the ≤ 0.5 Ω specification. The contact surface is likely corroded or arc-damaged, causing excessive resistance that will produce voltage drop and possible intermittent safety-chain faults.

The math in this lesson1

Learners check meter readings against ±5% and ±10% tolerance bands on control circuit test points.
7.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstration of proper multimeter and clamp-meter technique at the controller trainer: selecting correct function, range, and lead placement for each measurement type; students observe and record the procedure on a technique checklist. · Structured measurement rotation: students rotate in pairs through five labeled test stations on the trainer, each requiring a different measurement type; at each station they record expected value (from the wiring diagram), measured value, and pass/fail against a tolerance stated on the station card. · Interpretation exercise: students receive a table of six abnormal readings (e.g., unexpected voltage at a coil terminal, higher-than-normal resistance across a contact) and must write a one-sentence diagnosis for each based on the wiring diagram and measurement theory covered.
Materials: Digital multimeters (one per pair), clamp meters, controller trainer, measurement station cards with tolerance specs, technique checklists, wiring diagrams matching the trainer
Check: Instructor observes each student performing at least two unsupervised measurements (one voltage, one resistance); checks for correct lead placement, range selection, and reading recording; pass = all four measurement types performed correctly with readings within ±5% of instructor-verified values and no unsafe probing technique.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and safe work practices
1. Before measuring the resistance of a relay coil in an elevator controller, the FIRST action a technician must take is:
Troubleshooting and diagnostics
2. A DMM set to resistance function displays 'OL' when probed across a relay coil. This MOST likely indicates:
Electrical theory and measurements
3. When using a clamp meter to measure current in a control circuit, the jaw must encircle:
Electrical theory and measurements
4. The station card for test point TP7 states the expected DC voltage is 24 V ±5%. Which measured value FAILS this specification?
Troubleshooting and diagnostics
5. A technician measures 4.2 Ω across a normally closed safety contact that should read near zero ohms. The BEST one-sentence diagnosis is:
Safety and safe work practices
6. Which PPE item is specifically required when taking voltage measurements on an energized elevator controller panel?
Electrical theory and measurements
7. A student selects the VDC function on a DMM and probes a 24 VDC logic output. The display shows −23.8 V. The CORRECT interpretation is:
Troubleshooting and diagnostics
8. An elevator controller wiring diagram shows that a particular coil terminal should be de-energized when the car is parked at a floor. During troubleshooting, the technician measures 120 VAC at that terminal. Which diagnosis is MOST consistent with this finding?
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