This is a tricky situation, balancing NEC requirements with sensitive instrumentation needs. Let's break down the problem and potential solutions

Understanding the Issue
NEC 250.50 (Bonding of Electrode System)

This section mandates that all grounding electrodes present at a building or structure be bonded together to form a single ground electrode system. Your PE bus likely connects to the building's grounding electrode system, and NEC generally interprets "structure" broadly, potentially including your cabinet.

PE (Plant Earth)

This is your safety ground. It's intended to carry fault current back to the source in the event of an insulation failure, causing overcurrent protection (circuit breakers) to trip. It must be a low-impedance path for safety.

SH (Shield)

This is intended to provide a low-impedance path for high-frequency noise currents. The intention of isolating it with phenolic isolators is to prevent low-frequency ground loop currents from flowing through it. Ground loops are a major source of noise.

The Jumper

The blue bonding jumper, as currently configured, short-circuits the isolating effects of the phenolic isolators.

EMI/Noise

Ground loops, created by multiple paths to ground, can introduce significant noise, especially into sensitive analog signals (4-20mA, millivolt). This is what you're trying to prevent.

480V Equipment

The presence of 480V equipment increases the risk of high fault currents and the importance of a robust safety ground (PE).

Potential Solutions (and Considerations)

Here's a breakdown of how you can approach this, from least to most complex (and potentially effective):1. Verify Shielding Practices



Cable Shielding

Ensure all instrumentation cables (4-20mA, millivolt, digital) are properly shielded, and that the shields are connected to the SH bus only at one end. The best practice is to bond the instrumentation cable shields at the PLC end. Grounding shields at both ends can create ground loops.

Cable Routing

Keep instrumentation cables physically separated from power cables, especially those carrying 480V. Use dedicated cable trays or conduits. If crossing is unavoidable, cross at right angles.

Enclosure Shielding

Ensure the enclosure is properly grounded to the PE bus, so it can act as an effective shield.2.

High-Frequency Bonding (Capacitive Bonding)



The Concept

Instead of a direct, low-impedance connection, you can use a high-frequency capacitor to bond the PE and SH buses. This allows high-frequency noise currents to flow between the buses while blocking low-frequency ground loop currents.

Implementation

Replace the blue bonding jumper with a safety-rated capacitor (e.g., a Y-capacitor) with a voltage rating appropriate for the application (at least 480V). The capacitance value is crucial and depends on the frequency range of the noise you're trying to mitigate. Start with a small value (e.g., 0.01uF) and increase it if needed, monitoring the noise levels on your instrumentation circuits.

Safety

The capacitor must be safety-rated and designed for this type of application. A failure of the capacitor could compromise safety.

Important considerations

Calculate the impedance that the capacitor will provide at 60Hz/50Hz. This is the frequency of the ground loop currents that you are trying to prevent. You want this impedance to be significantly higher than the impedance of your safety ground. Calculate the fault current that would flow through the capacitor in the event of a short circuit. This fault current must be interrupted by the capacitor's fusing mechanism. Consult with a capacitor vendor to select the appropriate capacitor for your application.3.

Impedance Bonding with a Resistor



The Concept

Use a resistor instead of a jumper. This will allow current to flow between the two buses, but it will add impedance to the connection, which can help to reduce noise.

Implementation

Select a resistor with a low resistance value (e.g., 1 ohm, 1/2 watt). Connect the resistor between the PE and SH buses. Test the system to ensure that the noise is reduced to an acceptable level.

Safety Considerations

Make sure that the resistor is rated for the voltage and current that it will be exposed to. Consider the effect of the resistor on the fault current. You may need to increase the size of the grounding conductor to compensate for the increased impedance.4.

Consult with an Electrical Engineer and Grounding/Bonding Specialist



The Best Approach

Given the complexity and potential safety implications, the best course of action is to consult with a qualified electrical engineer specializing in grounding and bonding. They can: Perform a thorough site assessment. Analyze the existing grounding system. Measure noise levels. Design a grounding and bonding solution that meets both NEC requirements and your instrumentation needs. Provide documentation to support your changes.

Important Considerations and Warnings



Never Compromise Safety

Your primary responsibility is to ensure a safe electrical system. Do not make changes that could increase the risk of electrical shock or fire.

Document Everything

Keep detailed records of all changes you make, including the rationale behind them.

Testing

After any changes, thoroughly test the system to verify that it functions correctly and that noise levels are acceptable.

Local Codes and Regulations

Always comply with all applicable local electrical codes and regulations.

Vendor Consultation

Contact the cabinet vendor and explain your concerns. They may have specific recommendations for grounding their equipment.

PLC Manufacturer Consultation

Consult with Allen Bradley and/or a controls systems integrator who has a deep understanding of how to ground their I/O cards to avoid noise.

Why removing the jumper completely is generally not recommended

While tempting, simply removing the jumper can create a dangerous situation, especially with 480V equipment. If a fault occurs on the 480V side, the SH bus may become energized. Without a proper bond to the PE, there might not be enough fault current to trip the circuit breaker, creating a shock hazard. Also, it may not be NEC compliant.

In summary, the most responsible approach is to consult with a qualified professional, document your changes, and prioritize safety above all else. Starting with improved shielding practices is a good first step. Flag for review