Your concerns are valid and touch upon a complex issue in distribution system design, balancing safety and protection. Let's break down the considerations

Your Core Concern:

You're worried that grounding metallic crossarms in high-resistivity soil can create a dangerous ground potential rise (GPR) near the pole during a fault. Instead of quickly tripping the upstream breaker, a fault to the crossarm could electrify the surrounding earth, creating a shock hazard.

Why Metallic Crossarms Are Sometimes Grounded



Fault Clearing

The primary reason for grounding is to provide a low-impedance path back to the source substation in case of a fault. This low impedance is crucial for the overcurrent protection devices (fuses, reclosers, circuit breakers) to detect the fault and quickly trip, isolating the faulted section.

Lightning Protection

While you're right about lightning arresters needing a good ground, grounding the crossarm can also help provide a path for lightning strikes to ground, even if the strike isn't directly on the arrester.

Safety (Theory)

The idea is that by grounding the crossarm, a fault to the crossarm will immediately create a near-zero voltage potential between the crossarm and the earth immediately adjacent to the pole. Theoretically, this should reduce the touch potential (the voltage a person would experience if they touched the crossarm and were standing on the ground) and step potential (the voltage between a person's feet if they are standing near the grounded structure).

Shielding

The grounded structure can provide some shielding to conductors from EMI (Electromagnetic Interference).

Why Grounding in High-Resistivity Soil is Problematic



Poor Grounding Impedance

In high-resistivity soil, achieving a truly low-impedance ground connection is difficult and expensive. You might drive ground rods deep and still not get the resistance below the desired level.

Elevated GPR

With poor grounding, a fault current flowing through the limited ground connection creates a significant voltage rise around the pole (GPR). This voltage is distributed through the soil, creating dangerous touch and step potentials.

Delayed Fault Clearing

If the ground impedance is too high, the fault current might not be high enough to quickly trip the upstream protection. This extends the duration of the fault, increasing the hazard.

Your Proposal: Isolating Crossarms (with Exceptions for Lightning Arresters)

Your proposal to isolate the crossarms (except for lightning arresters) is based on valid safety principles and is worth considering: Reduced GPR

By isolating the crossarm, you prevent a fault to the crossarm from directly energizing the earth around the pole.

Reduced Touch and Step Potential

If the crossarm is isolated and far enough from accessible areas, the risk of someone touching it during a fault is lower, and the step potential near the pole is also significantly reduced.

Alternative Fault Path

The concern becomes: how will the fault be cleared? The assumption is that if a conductor falls and touches the crossarm, the conductor itself is still coupled to the earth - either directly or through trees, etc. Fault current will flow and a fuse or breaker will trip.

Key Considerations and Mitigation Strategies

1.

Fault Clearing Time

The most crucial factor is ensuring that a fault to an isolated crossarm will still reliably trigger the upstream protection. You need to analyze the potential fault currents and ensure they are high enough to operate the protection devices. This might require adjustments to the protection settings at the substation.2.

Clearance Distances

If you isolate the crossarms, you MUST ensure that they are located at a safe height (2.5m according to 250.110) and distance from any accessible areas to prevent accidental contact. Use insulating material and extra clearance for phases.
3.

Material Choice

Non-metallic crossarms are an excellent alternative. They provide insulation by design and eliminate the grounding issue altogether. Fiberglass reinforced polymer (FRP) is a common choice.4.

Grading Rings

To mitigate the high electric field around the insulators at the poles, consider using grading rings or corona rings, particularly at higher voltages. These help to distribute the voltage more evenly and reduce the risk of flashover.5.

Insulated Conductors

Use covered conductors with high withstanding voltage between conductors and earth.6.

Lightning Protection

Ensure that lightning arresters have dedicated, low-impedance ground connections.7.

Ground Potential Rise (GPR) Studies

Perform thorough GPR studies for your system. This will help you understand the potential voltage rise around grounded structures during fault conditions.8.

Touch and Step Potential Analysis

Analyze touch and step potentials around the poles to determine the safety risks.9.

Local Regulations

Check local regulations and utility practices regarding grounding of distribution systems.10.

Risk Assessment

Conduct a formal risk assessment to weigh the pros and cons of grounding versus isolating the crossarms.11.

Bonding of Metallic Hardware

Any other metallic hardware on the pole (e.g., down guys, metal conduit) must be carefully considered. You might choose to bond them to the same ground as the lightning arrester or isolate them as well, depending on the specific situation.

Recommendations



Start with a Thorough Analysis

Don't make a blanket decision to isolate all crossarms. Analyze your specific system, soil conditions, protection settings, and clearance distances.

Consider Non-Metallic Crossarms

This is often the simplest and safest solution, especially in high-resistivity soil.

If Grounding, Improve Grounding

If you decide to ground the crossarms, invest in improving the ground impedance. This might involve multiple ground rods, chemical treatment of the soil, or even ground grid installations.

Optimize Protection Settings

Carefully review and adjust the protection settings at the substation to ensure that faults to isolated crossarms will be detected and cleared quickly.

Consult with Experts

This is a complex topic. Consult with qualified electrical engineers experienced in distribution system design and grounding practices.

In summary: Your concerns are valid. Isolating crossarms is a viable option in high-resistivity soil if* you carefully address the implications for fault clearing and safety. Thorough analysis, risk assessment, and expert consultation are essential. Non-metallic crossarms often provide the best balance of safety and reliability. Flag for review