FLUKE
374 FC TRUE-RMS CLAMP METER
CLAMP — wrap around wire
SELECT A MODE
- - - -
click dial position to begin
COM
BLACK
BLACK
V/Ω
RED
RED
mA
RED
RED
CAT III 600V / CAT IV 300V
// MEASURING DC VOLTAGE — Use this to check 24VDC power (packer), loop supply. Crystallizer solenoids: use VAC mode (120VAC)
1
Set dial to VDC
Turn dial to V — position (the DC voltage setting). The — symbol means DC. The ~ symbol means AC. For 24VDC control circuits, always use V —.Meter powers on automatically when dial leaves OFF
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2
Insert leads — BLACK to COM, RED to V/Ω
Black lead plugs into COM jack (bottom left). Red lead plugs into V/Ω jack (center). This is for voltage — do NOT use the mA jack for voltage measurements.COM = common ground reference / V/Ω = voltage and resistance input
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3
Touch probes ACROSS the terminals — don't break the circuit
Place black probe on negative/COM terminal, red probe on positive/+24V terminal. Touch probes to both at the same time. You are measuring in parallel — circuit stays live and working during measurement.
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4
Read the display
LCD shows voltage. Packer solenoid (24VDC): expect 22–26V. Crystallizer solenoid: switch to VAC mode — expect 108–125VAC.
22–26V
HEALTHY 24VDC CIRCUIT
Power supply working, wiring intact, circuit live
0.0V
NO VOLTAGE
Check fuse, power supply, upstream breaker
18–21V
LOW VOLTAGE
Voltage drop — check connections, long cable run
1–5V
4-20mA LOOP (250Ω load)
1V=4mA=0%, 5V=20mA=100% (V=I×R)
⚡ Field Tips — DC Voltage
Check solenoid terminals when PLC output is ON — packer solenoids: expect 24VDC / crystallizer solenoids: expect 120VAC — use correct meter setting
Check voltage across a fuse — 0V across = good fuse (conducting). 24V across = blown fuse (open)
Measure panel power supply output before touching anything — confirm power is what you think it is
Negative reading means leads are reversed — swap probes. Not a problem, just orientation.
// MEASURING RESISTANCE — Use this on coils, RTDs, cables. POWER OFF FIRST.
⚠ CRITICAL — POWER OFF THE CIRCUIT FIRST
Measuring resistance on a live circuit will give wrong readings AND may damage the meter. Isolate the circuit, lock out if required, then measure. No exceptions.
1
Power off and isolate the circuit
De-energize the circuit. For solenoid coils: disconnect the connector or remove one wire at the terminal block. For RTDs: disconnect the transmitter terminals. Never measure resistance in a live circuit.
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2
Set dial to Ω
Turn dial to the Ω position. The meter auto-ranges. Black to COM, red to V/Ω jack — same jacks as voltage measurement.
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3
Zero the leads first
Touch the two probe tips together. Reading should be 0.1 to 0.5Ω. This is the lead resistance. Subtract this from your measurements on very low resistance checks.
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4
Touch probes across the component
Place probes on both terminals of the component being measured. Hold steady and read when display stabilizes. For RTDs hold for 3–5 seconds — reading may drift slightly.
85–120Ω
PACKER SOLENOID (24VDC)
Normal 24VDC coil resistance — packer area
1500–3000Ω
CRYSTALLIZER SOLENOID (120VAC)
Normal 120VAC coil resistance — much higher than 24VDC
100Ω
PT100 RTD @ 32°F / 0°C
138.5Ω @ 212°F / 100Ω @ 32°F
OL
OPEN CIRCUIT / BURNED COIL
Wire broken inside — coil is dead, replace it
<10Ω
SHORTED COIL
Wires touching inside — fire hazard, replace immediately
<1Ω
GOOD WIRE
Wire is intact, good continuity
0Ω
GOOD FUSE
Fuse conducting — OL means blown
⚡ Field Tips — Resistance
Check solenoid coil resistance before condemning it — Packer (24VDC): 85–120Ω good / Crystallizer (120VAC): 1500–3000Ω good — OL = burned, <10Ω = shorted
PT100 RTD check: 100Ω at ice bath (32°F) — carry a thermos of ice water on calibration days
Check wires for high resistance connections — corroded terminal = 5-10Ω instead of near 0Ω
// CONTINUITY — Is this wire actually connected end to end? Fast check. POWER OFF.
⚠ POWER OFF — Same rule as resistance
Continuity uses a small internal battery. Live circuits will override the reading and may damage the meter.
1
Set dial to continuity — the ))) symbol
Turn dial to the Ω position then press the button to cycle to the ))) mode (beeper symbol). Some meters have a dedicated continuity position. Black to COM, red to V/Ω.
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2
Touch probe tips together — confirm beep
Meter should beep immediately when probes touch. This confirms continuity mode is working. If no beep when touching tips, check lead connections or battery.
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3
Touch one probe to each end of the wire/path
Place one probe at one end of the wire (e.g. terminal block) and the other probe at the other end (e.g. device terminal). You're checking if current can flow end to end.
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4
Listen for the result
BEEP = wire is good (less than ~50Ω — connected). No beep + OL = open wire, broken, or not connected. No beep = problem.
🔔 BEEP
GOOD CONTINUITY
Wire intact, path conducting, connection good
SILENT + OL
OPEN CIRCUIT
Wire broken, loose terminal, or not connected
⚡ Field Uses — Continuity
Check Westlock switch — manually press cam, check continuity between COM and NO terminals
Trace ZSO/ZSC wires end to end — from Westlock terminal to PLC DI card terminal
Check fuse continuity — pull fuse, touch each end: beep = good, silent = blown
Check solenoid LED wiring — trace from PLC DO card terminal to solenoid T1
// MEASURING mA — 4-20mA loop current. You MUST break the loop. Use with caution.
⚠ MUST BREAK THE LOOP — Series measurement
Unlike voltage (parallel), current must be measured in SERIES. You break one wire, insert the meter, and current flows through the meter. The circuit stays powered — only one wire disconnected.
1
Set dial to mA DC
Turn dial to mA DC position. Red lead goes into mA jack (not V/Ω jack — wrong jack blows the fuse). Black stays in COM.
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2
Identify where to break the loop
Find a terminal block where the loop wire is accessible. The best place is usually at the transmitter's + terminal or at the PLC input card terminal. Loosen one screw terminal — don't cut the wire.
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3
Insert meter in series — one probe to each open end
Red probe touches the positive side (toward 24V supply). Black probe touches the wire going to the load/PLC. Current now flows through the meter. The loop is still live.
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4
Read the mA value and convert
Read the mA on the display. Convert to process value: % = (mA − 4) ÷ 16 × 100. Compare to what the HMI shows. If they match — transmitter and wiring are good. If they don't match — PLC input or signal conditioning issue.
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5
Remove meter and retighten terminal
Remove probes, retighten the terminal screw you loosened. Confirm process reading on HMI returns to normal. Don't leave a loose terminal.
4.0 mA
0% — LIVE ZERO
Transmitter alive at minimum range
12.0 mA
50% OF SPAN
Mid-range reading — normal operation
20.0 mA
100% — FULL SCALE
Transmitter at maximum range
0.0 mA
OPEN LOOP
No 24V power, broken wire, or failed transmitter
3.8 mA
SENSOR FAULT
Transmitter reporting sensor failure (below live zero)
>20 mA
OVER-RANGE / FAULT
Wiring fault, forced output, or sensor exceeded range
⚡ mA Formula Quick Reference
% span = (mA − 4) ÷ 16 × 100
Level: 0–10 ft, reading 8mA → (8−4)÷16×100 = 25% = 2.5 ft
Pressure: 0–100 PSI, reading 16mA → (16−4)÷16×100 = 75% = 75 PSI
Wrong jack = blown meter fuse. Always double-check red lead is in mA jack before measuring current.
// CLAMP CURRENT — Measure motor/load current WITHOUT breaking the circuit. Clamp around ONE wire.
1
Set dial to A (Clamp) position
Turn dial to the A position. No leads needed for clamp measurement — the jaws are the sensing element. The meter uses a Hall effect sensor in the jaw gap to detect the magnetic field around the wire.
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2
Zero the clamp — press ZERO button with jaws open
Open jaws away from any wire, then press the ZERO/REL button to null out any ambient field. Reading should show 0.0A. This prevents false offset readings.
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3
Clamp around ONE wire only
Open the jaw trigger and clamp around a single conductor. If you clamp around both supply and return wires, the fields cancel and you'll read 0A. Center the wire in the jaw for best accuracy.
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4
Read the current — compare to nameplate FLA
Read the amperage on the display. Compare to the motor nameplate Full Load Amps (FLA). Running above FLA = overloaded. Running significantly below = low load or issue. Use MIN/MAX to capture startup current.
⚡ Clamp Current Field Uses
Check conveyor motor current — compare to nameplate FLA, high current = mechanical binding
Check baghouse pulse-jet solenoids — clamp around supply wire, confirm they're cycling
Use MIN/MAX mode to capture motor inrush current on startup
Note: Fluke 374 clamp measures AC current best — for small DC mA signals, use the mA jack method instead