Electrical Sensors Selection Guide for Industrial Automation

Current transformer (CT): Measures AC current. Output: 1A or 5A (standard).

Voltage transformer (VT) / Potential transformer (PT): Measures AC voltage. Output: 100V or 110V (standard).

Hall effect sensor: Measures DC and AC current. Output: voltage or current proportional to measured current.

Temperature sensor: RTD (PT100), thermocouple, NTC/PTC thermistor. For monitoring temperature of equipment.

Ratio: Primary current / secondary current. Example: 100/5A (ratio 20:1).

Accuracy class: 0.2, 0.5, 1.0, 3.0. 0.2 for metering, 1.0 for protection.

Burden: VA rating of secondary circuit. Must be ≤ CT rated burden.

Type: Wound type (for low current), toroidal (split-core or solid-core), bar type (for high current).

RTD (PT100): Accuracy ±0.1°C to ±0.5°C. Range: -200°C to +600°C. 3-wire or 4-wire configuration.

Thermocouple: Type K (-200°C to +1200°C), Type J (-40°C to +750°C). Lower accuracy than RTD, but higher temperature range.

NTC thermistor: Negative temperature coefficient. High sensitivity, but non-linear. For temperature compensation.

Application: PT100 for switchgear (monitor busbar temperature), thermocouple for transformer (monitor winding temperature).

CT installation: Never open secondary circuit when CT is energized (dangerous high voltage!). Always short secondary before disconnecting.

VT installation: Never short primary or secondary (CT is short-circuit device, VT is open-circuit device).

Temperature sensor: Ensure good thermal contact (use thermal paste, mount close to heat source).

Wiring: Use shielded cable for low-voltage signals. Keep away from power cables.

1. Wrong CT ratio (meter reading incorrect).

2. Undersized CT burden (accuracy degraded).

3. Wrong sensor type (measuring AC with DC sensor).

4. Poor installation (inaccurate readings).

5. Not calibrating (drift over time).