CST Load Cell Wiring & Troubleshooting 3 — Questions and Answers
Question 1: Which symptom most strongly suggests moisture ingress into a load cell or junction box?
- Erratic, drifting zero with no applied load (Correct answer)
- Consistently high output at zero load
- No output under any load condition
- Output that doubles when load is applied
Correct answer: Erratic, drifting zero with no applied load
Moisture causes leakage currents across the bridge circuits, producing an unstable, drifting zero reading.
Question 2: What is the purpose of a shielded cable in load cell wiring?
- To reduce electromagnetic interference (EMI) on the signal lines (Correct answer)
- To increase the mechanical strength of the cable
- To carry the excitation current separately
- To provide a redundant signal path
Correct answer: To reduce electromagnetic interference (EMI) on the signal lines
Shielding surrounds the signal conductors and is grounded at one end to divert induced EMI noise away from the load cell output.
Question 3: During troubleshooting, a technician finds that shorting the signal wires together causes the indicator to read exactly zero. What does this confirm?
- The indicator and its wiring back to the junction point are functioning correctly (Correct answer)
- The load cell itself is working normally
- The excitation supply is within specification
- The cable has no leakage resistance to ground
Correct answer: The indicator and its wiring back to the junction point are functioning correctly
Shorting the signal inputs confirms the indicator circuit reads zero when given a true zero-differential input, isolating the fault to the load cell side.
Question 4: What value should the input resistance (signal+ to signal−) of a healthy 350 Ω load cell measure?
- Approximately 350 Ω (Correct answer)
- Approximately 700 Ω
- 0 Ω (dead short)
- Infinite (open circuit)
Correct answer: Approximately 350 Ω
For a 350 Ω load cell, the signal bridge output resistance (input resistance) is nominally 350 Ω when measured across the signal terminals with excitation disconnected.
Question 5: Which load cell specification directly describes the maximum combined error from non-linearity, hysteresis, and repeatability?
- Combined error or accuracy class (Correct answer)
- Rated output (mV/V)
- Creep specification
- Temperature effect on zero
Correct answer: Combined error or accuracy class
Combined error (accuracy class) is the single specification that encompasses non-linearity, hysteresis, and repeatability as a percentage of rated output.
Question 6: A technician suspects a load cell cable has a wire-to-shield short. Which test will confirm this?
- Measure resistance from each conductor to the shield with the cell disconnected (Correct answer)
- Apply full excitation and observe mV output
- Compare rated output to measured output under half load
- Measure AC voltage between signal wires
Correct answer: Measure resistance from each conductor to the shield with the cell disconnected
Measuring resistance from each conductor to the shield with the load cell disconnected directly reveals any wire-to-shield fault.
Question 7: What is the effect of connecting the cable shield at both ends (indicator and load cell) in a noisy industrial environment?
- Creates a ground loop that can introduce noise and hum (Correct answer)
- Doubles the shielding effectiveness
- Provides redundant EMI protection
- Reduces cable capacitance
Correct answer: Creates a ground loop that can introduce noise and hum
Grounding a shield at both ends allows circulating ground loop currents induced by potential differences to flow through the shield and couple noise onto signal wires.
CST Load Cell Wiring & Troubleshooting 3
Which symptom most strongly suggests moisture ingress into a load cell or junction box?