A conveyor can look grounded and still be isolated. Paint on a frame, a nylon roller, a plastic carrier or a loose braid can break the path. The only way to know is to measure it.
Key takeaways
- Ground the frame at multiple points, not at one end.
- The carrier must be dissipative and must contact a grounded part of the conveyor.
- Measure resistance from the product surface to earth, not just from the frame to earth.
- Moving joints and painted surfaces are the usual failure points.
The ground path
The path runs from the product to the carrier, from the carrier to the conveyor frame, from the frame to the earth bus, and from the bus to the building earth. Each step has a resistance, and the total must be below the limit set by your ESD control plan, typically 1×10⁹ Ω for the whole path and much lower for the frame bond.
The carrier is often the weak link. A plastic carrier is an insulator unless it is made from a dissipative material. A carrier with a metal insert can be grounded only if the insert contacts the product and the frame. Check the contact points for wear and contamination.
Frame bonding
Bond each frame section to the earth bus with a braided conductor, not a single wire. A braid has lower impedance at high frequency and is more resistant to vibration. Remove paint under the terminal and use a star washer. Bond both ends of a long conveyor, and bond across any flexible joint.
Do not rely on the conveyor's structural bolts for electrical continuity. Bolted joints can be insulated by paint, corrosion or a gasket. Use a dedicated bonding conductor.
Measurement
Use an ESD resistance meter with a 100 V test voltage for surface resistance and a low-resistance meter for bonding. Measure at several points along the line, including the load station, the middle, the unload station and any transfer. Record the readings with the line ID and date.
Measure with the carrier in place and, if possible, with a representative product on the carrier. A reading taken on an empty frame does not tell you what the product sees.
| Measurement | Typical target | Frequency |
|---|---|---|
| Frame to earth | <1 Ω | At installation, annually |
| Carrier surface | 1×10⁶–1×10⁹ Ω | Monthly |
| Carrier to frame | <1×10⁹ Ω | Monthly |
| Product to earth | <1×10⁹ Ω | Monthly and after cleaning |
| Wrist strap | 0.8–1.2 MΩ | Daily |
Common failures
The most common failures are painted contact surfaces, worn carrier contacts, insulated rollers, broken braids and cleaning agents that leave a film. Less common but more serious is a ground loop or a difference in potential between two earth points, which can cause current to flow through the product.
If two conveyors are joined, bond them together and to the same earth bus. If they are on different earth systems, consult an electrician; a potential difference can damage sensitive components.
Ionisation and insulators
Some insulators cannot be eliminated, such as display films, plastic housings and tape. Ionisers neutralise static on these surfaces, but they must be placed correctly and maintained. An ioniser that is dirty or out of balance can make the problem worse.
Use ionisers at stations where insulators are handled, and verify their performance with a charged plate monitor. Do not use ionisers as a substitute for grounding; they complement it.
Frequently asked questions
Can a stainless conveyor be grounded through its frame?
Yes, if the frame sections are bonded and the contact surfaces are clean. Stainless is conductive, but paint, passivation and bolted joints can still interrupt the path.
Do plastic rollers need to be conductive?
If the product contacts the rollers, yes, or the carrier must provide the ground path. A plastic roller is an insulator and will not drain charge from the product.
How often should ESD grounding be checked?
At installation, monthly for carriers and product paths, and after any cleaning, repair or modification. Keep a log with the readings.
Send your ESD control plan and conveyor layout to our engineers through the contact page; we will specify the grounding kit and the measurement points with the line.
