Welcome to the forum! This is a fascinating situation, and you're right to be cautious about the client's proposed solution. Here's a breakdown of my thoughts and some things to consider

Understanding the Problem
Lightning's Unpredictability

Lightning is incredibly powerful and can induce voltage spikes in unexpected places. It doesn't always follow the path you expect.

Ground Loops

Your concern about creating ground loops is valid. Grounding the messenger cable at both ends can create a loop that can carry significant current during a lightning strike or even from differences in ground potential. This circulating current could then couple into other systems, potentially causing more damage.

Fiber's Isolation

While the fiber cable itself is dielectric (non-conductive), the network switch connected to it isn't. Induced currents from the lightning strike could have traveled through the switch's power supply, Ethernet cable, or even the fiber optic transceiver itself.

UPS's Role (or Lack Thereof)

The fact that the UPS wasn't affected doesn't necessarily rule out lightning as the cause. UPS units typically protect against voltage sags, surges on the power line, and power outages, but they may not be designed to handle the extreme, fast-rising surges induced by a nearby lightning strike. A good UPS will help, but may not be enough.

Why the Client's Solution is Potentially Problematic Creating a Lightning Rod

Grounding the messenger cable at both ends effectively turns it into a lightning rod. While this might attract the next strike away from the buildings, it also guarantees a massive current surge will flow through the messenger cable and into both ground points. This is dangerous.

Increased Ground Potential Difference

Lightning currents flowing through the earth create significant voltage gradients. Grounding at two points will result in significant voltage differences between the grounds during a strike, potentially causing damage to equipment connected to those grounds.

Introducing a Path to Ground

The building with the steel is likely already grounded as part of its electrical system. If lightning hits the messenger cable, it could travel through the messenger, to the building steel, and then to ground. Grounding the other end creates an additional path for this energy to travel, potentially exacerbating the problem.

Possible Causes (and Potential Solutions)1. Induced Voltage in the Network Switch



Likely Scenario

The most probable scenario is that a strong electromagnetic pulse (EMP) generated by the lightning strike induced a high-voltage surge in the network switch. This surge could have traveled through the Ethernet cable connected to the switch, damaging the switch's components.

Solution



Surge Protection

Install high-quality surge protectors on both the power and Ethernet ports of the network switches in both buildings. These should be designed for data lines and be properly rated for the expected surge current. Ensure they are properly grounded to a single, low-impedance ground point.

Fiber Optic Transceiver Protection

Some manufacturers offer surge protection devices for fiber optic transceivers. Look into these as an additional layer of defense.2.

Ground Loop Issues (Less Likely, But Possible)



Scenario

If there are existing ground loops in the building's electrical system, the lightning strike could have amplified those issues, causing voltage differences that damaged the switch.

Solution



Grounding Audit

Have a qualified electrician perform a thorough grounding audit of both buildings to identify and correct any existing ground loop problems. Ensure proper bonding of metallic components.3.

Direct Strike (Less Likely, But Possible)



Scenario

The video evidence the client has suggests a direct strike to the messenger cable.

Solution



Risk Assessment

Evaluate the risk of future strikes. If direct strikes are likely, consider a lightning protection system for the buildings themselves, which would involve air terminals (lightning rods) and a dedicated grounding system. This is a complex and expensive solution, but it's the most effective way to protect against direct strikes.

Recommendations to the Client1. Explain the Risks

Clearly explain the risks of grounding the messenger cable at both ends. Emphasize the potential for creating ground loops and attracting more lightning strikes.2.

Propose a Phased Approach



Phase 1: Surge Protection

Install high-quality surge protection devices on the power and Ethernet ports of the network switches in both buildings. This is the most cost-effective and often the most effective solution.

Phase 2: Grounding Audit

Have a qualified electrician perform a grounding audit of both buildings.

Phase 3: Lightning Protection System (If Necessary)

If problems persist or the risk of direct strikes is high, consider a lightning protection system for the buildings.3.

Emphasize Data Logging

Install a power quality meter at the network switches to record voltage surges, dips, and other anomalies. This data can help diagnose future problems and evaluate the effectiveness of the surge protection devices.4.

Upgrade UPS

Invest in more robust UPS systems designed to withstand voltage surges, especially those related to lightning.5.

Document Everything

Keep meticulous records of all work performed, including surge protector specifications, grounding audit results, and any other relevant information.

Important Considerations



Consult with a Lightning Protection Expert

For a comprehensive solution, consult with a qualified lightning protection expert. They can assess the risk of lightning strikes, recommend appropriate protection measures, and ensure that the system is properly installed and grounded.

Code Compliance

Ensure that all work complies with local electrical codes and regulations.*

Liability: Be clear about your liability. Document your recommendations and any decisions made by the client that deviate from your recommendations.In summary, grounding the messenger cable at both ends is likely a bad idea. A phased approach that focuses on surge protection, grounding audits, and a possible lightning protection system is a more effective and safer solution. Good luck! Flag for review