CST Load Cell Wiring & Troubleshooting 2 — Questions and Answers
Question 1: What is the typical color code for the excitation positive (+EXC) wire in a standard 4-wire load cell cable?
- Red (Correct answer)
- Green
- White
- Black
Correct answer: Red
In most standard load cell color conventions, red is used for the excitation positive (+EXC) lead.
Question 2: A technician measures 0 mV output from a load cell under full rated load. Which fault is most likely?
- Open excitation circuit (Correct answer)
- Shorted signal wires together
- Excessive cable length
- Incorrect strain gauge orientation
Correct answer: Open excitation circuit
An open excitation circuit removes the bridge supply voltage, resulting in zero output regardless of applied load.
Question 3: When wiring multiple load cells in a parallel junction box, what must be matched between cells to ensure accurate summing?
- Input impedance of each load cell (Correct answer)
- Cable color coding only
- Physical mounting orientation
- Indicator display resolution
Correct answer: Input impedance of each load cell
Matching input impedance ensures equal current distribution across parallel-connected load cells for accurate summing.
Question 4: What does a negative mV/V output reading indicate when a load cell is under a compressive load that should produce a positive reading?
- Signal wires are reversed (Correct answer)
- Excitation voltage is too high
- Load cell is overloaded
- Calibration factor is incorrect
Correct answer: Signal wires are reversed
A negative output when positive is expected means the signal+ and signal− leads have been swapped.
Question 5: Which instrument is most appropriate for measuring the zero balance output (offset) of a load cell in mV?
- Precision digital multimeter (Correct answer)
- Clamp-on ammeter
- Megohmmeter
- Oscilloscope set to AC mode
Correct answer: Precision digital multimeter
A precision digital multimeter with mV resolution is the correct tool for measuring the small DC offset voltage of a load cell at zero balance.
Question 6: What wiring configuration is used in a 6-wire load cell to compensate for cable resistance errors?
- Separate sense lines feed back excitation voltage to the indicator (Correct answer)
- A sixth wire provides an independent ground reference
- Two extra wires carry the temperature compensation signal
- Three pairs of twisted wires reduce EMI inductively
Correct answer: Separate sense lines feed back excitation voltage to the indicator
6-wire load cells add sense lines that allow the indicator to monitor and regulate the exact voltage at the load cell terminals, eliminating cable resistance drop errors.
Question 7: A junction box trim potentiometer is adjusted to correct for what specific condition in a multi-load-cell system?
- Corner or span errors between individual load cells (Correct answer)
- Indicator display backlighting brightness
- Excitation voltage magnitude
- Cable insulation resistance
Correct answer: Corner or span errors between individual load cells
Trim potentiometers in a summing junction box allow individual cell output levels to be equalized, correcting corner and span errors.
CST Load Cell Wiring & Troubleshooting 2
What is the typical color code for the excitation positive (+EXC) wire in a standard 4-wire load cell cable?