The Sensor That Randomly Triggered
We used a proximity sensor (article 108) on a machine. The sensor worked intermittently — it triggered randomly (no part in front). The sensor’s cable was shielded (article 166). The problem: the shield wasn’t grounded. The shield (a metal foil around the signal wire) blocks noise. If the shield isn’t grounded, it picks up noise (like an antenna). The random triggers were from the servo drive’s noise (article 168, which emits electrical noise). We grounded the shield (one end, to the panel’s ground bar). The random triggers stopped. The mistake was leaving the shield ungrounded (it was a noise antenna).
Grounding and bonding directs the noise to ground. This article covers the practice.
Grounding (PE)
Every machine has a ground (PE, protective earth) connection. The ground wire (green-yellow) connects the machine’s metal frame to the plant’s ground bar. The ground is for safety (if a wire touches the frame, the ground trips the breaker). It’s also for noise (the ground provides a reference).
Bonding (Equipotential)
Bonding connects the machine’s parts (the panel, the frame, the conveyor) to the same ground (equipotential). All the metal parts are at the same potential (no voltage difference). If two parts are at different potentials, the noise flows between them (through the cable shields, or the mounting). Bonding (a braided strap, a copper wire) connects them.
Step 1: Ground the Shield (One End)
A shielded sensor cable (article 166) has a shield. Ground the shield at one end (the panel end, to the ground bar). Don’t ground both ends (that creates a ground loop — a current flows in the shield, which adds noise). One end (the panel) grounds the shield. The sensor end (in the field) is not grounded (the shield floats).
The grounding rule: Ground the shield at one end (the panel). The sensor that randomly triggered had an ungrounded shield (a noise antenna). One-end grounding blocks the noise. Don’t ground both ends (ground loop).
Step 2: The Ground Bar
The panel has a ground bar (a copper bus). All the grounds (PE, shields, bonds) connect to the bar. The bar connects to the plant’s ground. Don’t daisy-chain (one ground to another). Star-ground (each ground goes to the bar directly).
Step 3: Isolate the Power (Article 188)
The 24V power supply (article 188) has a ground. The logic (PLC) and the power (sensors) share the ground. Keep the power ground (the high-current return) separate from the signal ground (the low-current return). If they share a wire, the power current (large) creates a voltage drop (which the signal sees as noise).
Step 4: The Servo Drive’s Noise
A servo drive (article 132) emits electrical noise (the switching current). The drive’s power cable (motor cable) is shielded (grounded at both ends — the drive and the motor). The encoder cable (signal) is shielded (grounded at the drive end). Separate the power cable (motor) from the signal cable (encoder) — don’t bundle them (article 166).
| Cable | Shield Ground |
|---|---|
| Sensor signal cable | One end (panel ground bar) |
| Servo motor power cable | Both ends (drive + motor) |
| Servo encoder cable | One end (drive) |
| 24V sensor power | Not shielded (twisted pair) |
A Grounding Checklist
- Is the machine grounded? (PE?)
- Are the parts bonded? (Equipotential?)
- Is the shield grounded? (One end?)
- Is it both ends? (Ground loop? Fix?)
- Is there a ground bar? (Star grounding?)
- Is the servo cable shielded? (Both ends?)
- Is power separated from signal? (Article 166?)
- Is the random trigger gone? (Test?)
- Is the ground resistance low? (Measured?)
- Is the panel grounded? (Article 168?)
The Bottom Line
Grounding and bonding directs the noise. The sensor that randomly triggered had an ungrounded shield. Ground the shield at one end (the panel). Bond the parts (equipotential). Use a ground bar (star). The sensor that stopped randomly triggering wasn’t the best cable — it was grounded.
