You've touched upon a very important and often misunderstood aspect of grounding and bonding. You're right to question the seemingly counterintuitive idea of intentionally lowering resistance when dealing with high-voltage events like lightning strikes. Let's break down why a low-impedance grounding system is crucial, even if it seems to increase current in some scenarios.Here's the key: The goal of a low-impedance grounding system isn't to prevent current flow during a fault (like a lightning strike), but to control and route that current in a predictable and safe manner.

Let's dissect your concerns and the principles at play

1. The Purpose of Bonding and Grounding:
Bonding

Connecting all metallic parts (enclosures, conduits, equipment frames, etc.) together electrically. This creates an equipotential plane. The goal is to ensure that all these parts are at virtually the same voltage at any given instant.

Grounding

Connecting the bonded system to Earth (via ground rods, ground grid, etc.). This provides a low-impedance path for fault currents to return to the source (usually the utility transformer) and to dissipate into the earth.

2. Why Low Impedance is Crucial



Minimizing Voltage Differences (Step and Touch Potential)

This is the most important reason. During a fault (like a lightning strike or a ground fault in equipment), a substantial amount of current flows to ground. If different metallic parts aren't bonded together, significant voltage differences can develop between them. Imagine touching two different metal enclosures that are at different voltages - that's a shock hazard. A low-impedance bonded system ensures that all touched metal parts are at nearly the same potential, minimizing the risk of dangerous step and touch potentials. This is why we use a low-impedance ring around the plant. By minimizing voltage difference, step and touch potential are minimized.

Facilitating Fast Overcurrent Protection

A low-impedance fault path allows a large current to flow quickly. This large current triggers overcurrent protection devices (circuit breakers, fuses) to trip faster. A faster trip time reduces the duration of the fault, minimizing damage to equipment, reducing the risk of fire, and improving safety. If the fault path has high impedance, the fault current might be too low to trigger the overcurrent protection, leading to a prolonged and potentially catastrophic fault.

Controlled Current Path

A low-impedance network provides the lowest resistance route for fault current. Therefore, the majority of the current will flow in that route. This prevents the random and potentially harmful flow of current through other paths such as through human bodies.

3. Addressing Your Lightning Strike Scenario



Yes, Current Will Be High

You are absolutely correct. With E = IR, and 'R' being very low, a lightning strike (high 'E') will result in a very high current ('I'). This is unavoidable, and it's better to have a controlled path for that current. The low resistance allows the current to quickly dissipate to ground instead of traveling throughout the plant seeking alternate, less-controlled paths.

The Goal Isn't To Prevent Current, But to Manage It

The low-impedance grounding system is not intended to reduce the total amount of current from a lightning strike. That's impossible! The goal is to:

Provide a defined path

Ensure the current flows along the intended grounding conductors, rather than through less predictable paths like equipment frames, process piping, or (worst case) human bodies.

Distribute the current

A well-designed grounding system will distribute the current, which will reduce the voltage rise at any one location. This minimizes potential differences and associated hazards.

Encourage rapid dissipation

A low-impedance path to earth allows the current to dissipate into the ground more quickly.

Why Higher Impedance Is Worse in This Case

Imagine if the grounding system had a high resistance. The current would still flow somewhere. It would be less predictable where it would flow, increasing the chance of it traveling through equipment or people. The voltage at different points within the plant would be much higher, increasing the risk of dangerous touch potentials. The ground fault protection might not react as quickly, extending the fault duration and increasing damage.

4. Analogies and Visualizations



Think of Water Flow

Imagine a river. A low-impedance grounding system is like a wide, deep channel. A flood (lightning strike) will still cause a lot of water to flow, but it will be contained within the channel. A high-impedance grounding system is like a small, shallow, and poorly defined channel. The flood will overflow, causing damage and potentially flowing through unintended areas.

Think of a Pressure Relief Valve

In a high-pressure system, a relief valve provides a controlled path for excess pressure to escape. The valve doesn't eliminate the pressure, but it prevents a catastrophic explosion by releasing it in a safe direction. A low-impedance grounding system is similar to a pressure relief valve for electrical faults.

In Summary:

While it might seem counterintuitive, a low-impedance grounding and bonding system is crucial for safety in industrial plants, especially when dealing with the potential for high-voltage events like lightning strikes. It doesn't eliminate current flow, but it controls and manages that current to minimize voltage differences, facilitate rapid overcurrent protection, and prevent hazardous conditions. The high current flow in a low-impedance system is a feature*, not a bug, as it ensures the fault is cleared quickly and safely. Flag for review