Meters and electrical measurement

An elevator mechanic's meter is the most important diagnostic tool in the van. It is also the tool that most often puts a mechanic's hands inside an energized cabinet. Priya spends a full session on meters, because a mechanic who does not understand how a meter measures, what it can and cannot be connected to, and how it can mislead will either miss faults or hurt herself finding them. Tamsin bought her own meter at the end of her first year on Ray's advice: a true-RMS digital multimeter with a low-impedance voltage mode, rated for the circuits she will meet, with a clamp for current. This lesson explains why each of those features matters.

Voltage, current and resistance measurements

A voltmeter measures the potential difference between two points, so it is connected across them, in parallel with whatever is between them. It has a very high internal resistance, typically ten megohms on a digital meter, so it draws almost no current and does not disturb the circuit. Measuring voltage is the safest and most useful measurement in elevator work, because the leads only touch the circuit at two points and the circuit is not changed.

An ammeter measures the current through a conductor, so the current must pass through it: the circuit is opened and the meter connected in series. Its internal resistance is very low so that it does not drop much voltage. Connecting an ammeter across a supply, in parallel, is a short circuit through the meter. A good meter protects itself with a high-energy fuse, but a meter with the wrong fuse, or one used with its leads in the current jacks while measuring voltage, can explode in the hand. For that reason Northcrest mechanics rarely use the series current function on power circuits. They use a clamp meter.

A clamp meter measures current by sensing the magnetic field around a conductor. The jaws are closed around one conductor, and the meter reads the current without any connection to the circuit. It works on AC and, with a Hall-effect sensor, on DC too. The jaws must enclose only one conductor of the circuit: if they enclose both the outgoing and return conductors, the fields cancel and the meter reads zero. A clamp around all three phase conductors of a motor feed reads near zero for the same reason, unless there is leakage current to ground, which is how leakage clamps find ground faults.

An ohmmeter measures resistance by passing a small current from its own battery through the component and measuring the voltage. It must only be used on a circuit that is locked out and verified dead. Any voltage from the circuit will give a false reading and may damage the meter. Resistance readings are also affected by parallel paths: a coil measured while still connected may read low because another component is in parallel with it. When the reading matters, one end of the component is disconnected.

Meter categories and ratings

Meters, test leads and accessories are rated under the international standard for electrical test equipment by measurement category and voltage. The category reflects how much energy a transient or fault can deliver at that point in the installation. Category IV is the origin of the installation, at the service entrance and utility meter. Category III is the distribution level inside the building: panelboards, feeders, motor starters and fixed equipment such as an elevator controller and its mainline disconnect. Category II covers equipment plugged into receptacles. A meter rated CAT II is not suitable for a controller even if its voltage rating is high enough.

For elevator work, the meter and leads should be rated at least CAT III for the system voltage, and for 480 V equipment Northcrest specifies CAT III 1000 V or CAT IV 600 V. The rating of a measurement set is the lowest rating of any part of it, so a cheap set of leads or an alligator clip with a lower rating downgrades the whole set. Test leads should have shrouded connectors, finger guards and short exposed tips, often four millimeters or less, which reduce the chance of an accidental short between two terminals.

Ghost voltages and low-impedance mode

A digital voltmeter's high impedance is usually an advantage, but it has a side effect. Conductors run side by side in a conduit, a raceway or a traveling cable are coupled to each other by capacitance. A conductor that has been disconnected at both ends, or whose switch is open, can pick up a voltage from its live neighbors through that coupling. The coupling can supply only a tiny current, but a ten-megohm meter draws almost nothing, so the meter reads a substantial voltage that is not really there. These ghost voltages are common in elevator wiring, which is full of long parallel runs.

The danger is in both directions. A ghost reading can make a dead circuit look live and send a mechanic looking for a fault that does not exist. More rarely, a mechanic who is used to ghost readings may dismiss a real voltage as a ghost. The cure is a meter with a low-impedance voltage mode, often marked LoZ, which puts a load of a few kilohms across the leads. A real voltage source holds up under that load; a capacitively coupled ghost collapses to near zero. The code below models the coupling as a small capacitor between the live conductor and the open one.

# Ghost voltage on an open conductor coupled to a live neighbor (illustrative)
import math
f = 60.0                # Hz
v_live = 120.0           # V on the neighboring conductor
c_coupling = 2.0e-9     # F, roughly 2 nF over a long parallel run
xc = 1 / (2 * math.pi * f * c_coupling)    # coupling reactance, ohms
for name, z_meter in (("high impedance, 10 Mohm", 10e6), ("low impedance mode, 3 kohm", 3e3)):
    # the meter and the coupling form a divider (treat both as magnitudes in quadrature)
    v = v_live * z_meter / math.sqrt(z_meter ** 2 + xc ** 2)
    print(f"{name:<28} reads {v:6.1f} V")
print(f"coupling reactance {xc / 1e6:.2f} Mohm")

high impedance, 10 Mohm      reads  119.0 V
low impedance mode, 3 kohm   reads    0.3 V
coupling reactance 1.33 Mohm

The high-impedance meter shows most of the neighboring voltage on a conductor that is not connected to anything. The low-impedance mode reads less than a volt. When Tamsin finds an unexpected voltage on a conductor that should be dead, Ray has her switch to LoZ and measure again before drawing any conclusion. Low-impedance mode should not be used on sensitive electronic circuits, because its load can disturb them, and it should not be left connected to a live circuit for long, because it draws real current.

RMS and drive outputs

Alternating voltages and currents are stated as RMS values, the root-mean-square, which is the value that would produce the same heating as a DC value of the same size. Lesson 5 explains where the numbers come from. Inexpensive meters do not measure RMS directly. They measure the average of the rectified waveform and multiply by a factor that is correct only for a pure sine wave. On a clean sine wave they are accurate. On the distorted current drawn by a rectifier, or the pulse-width-modulated output of a variable-frequency drive, they can be wrong by a large margin.

A true-RMS meter calculates the RMS value from the waveform itself and stays accurate on distorted waveforms. Even then, the output of a drive is a series of high-frequency pulses, and many meters read it erratically. Better meters have a low-pass filter setting, sometimes marked VFD, that removes the switching frequency so that the meter shows the fundamental voltage the motor actually responds to. Priya's rule is that motor voltages on the output side of a drive are read with the filter on, or not trusted.

Insulation resistance testing

An insulation resistance tester, often called a megohmmeter, applies a high DC voltage, typically 500 or 1,000 V, between a conductor and ground or between two conductors, and measures the very small current that leaks through the insulation. Good insulation reads in the hundreds or thousands of megohms. Damp, contaminated or damaged insulation reads lower. The test finds faults that a normal ohmmeter, which tests at a few volts, cannot see, such as a hoist motor winding that has absorbed moisture or a traveling cable conductor with a nicked jacket.

The test voltage is high enough to destroy electronic equipment. Before testing a motor or a cable, the conductors are disconnected from the drive, the controller boards and any surge suppressors, and the circuit is locked out and verified dead. After the test, the conductor is discharged to ground, because the cable and winding capacitance stores a charge from the tester. Readings are recorded with the temperature, because insulation resistance falls steeply as temperature rises. A common rule of thumb is that it halves for every 18 degrees Fahrenheit of temperature rise, so readings are corrected to a reference temperature before they are compared with earlier ones.

# Insulation resistance of the Tillicum car 1 hoist motor over three years
# readings at 1,000 V, one minute, corrected to 104 F with a halving per 18 F rule
readings = [  # (year, measured Mohm, winding temperature F)
    (2024, 850.0, 71.6),
    (2025, 610.0, 82.4),
    (2026, 140.0, 87.8),
]
for year, mohm, t in readings:
    corrected = mohm * 0.5 ** ((104 - t) / 18)
    print(f"{year}: measured {mohm:6.0f} Mohm at {t:5.1f} F -> {corrected:6.0f} Mohm at 104 F")
first = readings[0][1] * 0.5 ** ((104 - readings[0][2]) / 18)
last = readings[-1][1] * 0.5 ** ((104 - readings[-1][2]) / 18)
print(f"change since first test: {100 * (last / first - 1):.0f} %")

2024: measured    850 Mohm at  71.6 F ->    244 Mohm at 104 F
2025: measured    610 Mohm at  82.4 F ->    266 Mohm at 104 F
2026: measured    140 Mohm at  87.8 F ->     75 Mohm at 104 F
change since first test: -69 %

The corrected figures make the trend clear. The winding is still well above any minimum, but its resistance has fallen sharply in a year, and the trend matters more than the absolute value. Ray puts a note on the Tillicum file to inspect the motor for moisture and contamination, and to retest after cleaning and drying.

Continuity, diode and other tests

Most meters have a continuity function that beeps when the resistance is below a few tens of ohms. It is quick for checking a fuse, a wire or a closed contact on a dead circuit. The diode test applies a small current in one direction and reports the forward voltage of a diode, about 0.5 to 0.7 V for a silicon diode, and an open reading in the reverse direction. It is used on the rectifiers that feed brake coils and on the suppression diodes across DC relay coils. Some technicians also carry a small oscilloscope or a meter with a recording function, which can capture an intermittent dropout that is too quick to see on a display.