You're asking a very important question related to safety and compliance. Here's a breakdown of the bonding requirements for your industrial equipment, considering the electrical codes and best practices

Key Considerations:
Electrical Codes and Standards

The primary standard to consult is

NFPA 79 (Electrical Standard for Industrial Machinery). Also, you'll need to comply with the National Electrical Code (NEC) specifically Article 250, especially sections related to equipment grounding. UL 508A, particularly sections related to grounding and bonding, is also relevant if you are seeking UL Listing. Purpose of Bonding

Bonding creates a low-impedance path for fault current back to the source. This ensures that protective devices (like your 6A fuse) will quickly trip in the event of a ground fault, minimizing shock hazards and equipment damage.

Mechanical Fasteners as Bonding Means

This is the crux of your question. Generally,

mechanical fasteners alone are NOT considered a reliable or sufficient bonding means. There are exceptions, but they are specific and require rigorous adherence to certain standards.

Specific Requirements & Recommendations


Enclosure Bonding



Main Enclosure (Disconnect Switch Enclosure)

This enclosure must be bonded to the grounding electrode system (the ground bar where your main power is grounded). You are already doing this with the grounding wire from your valve ground block to the disconnect switch ground bar. This is critical.

Sub-Enclosures and Metal Panels

This is where it gets more nuanced.

Dedicated Bonding Jumpers Preferred

Best practice is to use dedicated bonding jumpers (wires or straps) between sub-enclosures and the main enclosure, and between metal panels and the enclosure they are mounted within. These jumpers should be sized appropriately for the fault current (consult NFPA 79 and NEC). Use ring terminals and star washers to improve the security of the bond connections.

Fasteners (With Caveats)

You might be able to rely on properly installed fasteners in some limited cases if you meet the following stringent conditions:

Listed Enclosures

The enclosures themselves must be listed for grounding. This means the manufacturer has tested and certified that the mechanical connections provide a reliable ground path. Look for markings on the enclosure.

Clean Surfaces

Mating surfaces MUST be clean, free of paint, oxidation, and any non-conductive coatings at the point of contact. Paint should be removed. Use a corrosion inhibitor to prevent future oxidation.

Proper Fastener Type

Use fasteners specifically designed for grounding. These often include serrated washers or other features that ensure good electrical contact.

Torque Requirements

Fasteners must be tightened to the manufacturer's specified torque. Use a calibrated torque wrench to verify.

Reliability

Consider the long-term reliability of the connection. Vibration, temperature changes, and corrosion can degrade the connection over time.

NEC 250.96 and 250.8

These sections of the NEC address bonding of metal parts and enclosures. Consult them carefully. NEC 250.96(A) allows for metal raceways, cable trays, cable armor, cable sheath, enclosures, frames, fittings, and other metal non-current-carrying parts to be connected to an equipment grounding conductor or to each other, if approved. The "approved" part is key; it means the Authority Having Jurisdiction (AHJ) or a qualified testing lab like UL has to sign off on the method.2.

Individual Components (Valves, Sensors, etc.)

You mentioned each valve has a green ground wire going to a central ground block. This is good. The ground block itself must be securely bonded to the main enclosure's ground.3.

Grounding Wire Sizing

Your grounding wires must be appropriately sized based on the overcurrent protection (your 6A fuse). NFPA 79 and the NEC provide tables for this.4.

Testing

After installation, perform continuity testing to verify the integrity of your grounding and bonding system. Your <1 ohm reading is a good start, but you need to measure between every point you consider bonded and the main ground point to confirm consistency. Test after any modifications or maintenance.

Why Dedicated Bonding Jumpers are Highly Recommended, Even if Fasteners Might Be Acceptable



Safety Margin

Dedicated jumpers provide a much higher safety margin. They are less susceptible to corrosion, loosening, and other issues that can compromise a fastener-based connection.

Code Compliance

It makes demonstrating code compliance much easier. It's a clear and unambiguous approach.

Long-Term Reliability

Jumpers provide a more reliable long-term connection, reducing the risk of future ground faults.

Ease of Maintenance

Jumpers make it easier to inspect and maintain the grounding system.

Best Practice

It's considered a best practice in industrial electrical installations.

Specific Advice for Your Situation


Err on the Side of Safety

Given the potential for human contact with your equipment, err on the side of safety. Adding dedicated bonding jumpers, even if it seems like "overkill," is a worthwhile investment.2.

Consult the AHJ

Talk to the Authority Having Jurisdiction (the electrical inspector in your area) early in the design process. Get their input on your grounding and bonding plan. This can save you time and money in the long run.3.

Consider UL Listing

If you plan to sell your equipment commercially, consider getting it UL listed. UL certification will require you to meet strict grounding and bonding requirements.

In summary: While mechanical fasteners might be acceptable in very specific and controlled circumstances, dedicated bonding jumpers provide a more reliable, safer, and easier-to-verify grounding solution. It is highly recommended* to use jumpers for all sub-enclosures and metal panels within your equipment. Flag for review