Okay, let's break down the "why" behind separating grounds and neutrals, especially with pool circuits involved, in the context of your situation. You're right, understanding the underlying principles is crucial.

The Core Problem: Parallel Neutral Paths and Fault Current

The issue stems from the concept of parallel neutral paths and how they impact safety, especially in fault conditions. Neutral: The Normal Current Return Path. Under normal operation, the neutral conductor is designed to carry the unbalanced current back to the source (in your case, the barn's 200A disconnect). Ground: The Safety Net (Fault Current Path). The ground conductor, on the other hand, is not intended to carry normal current. Its primary purpose is to provide a low-impedance path for fault current in the event of a short circuit or ground fault. Think of it as a safety net that only comes into play when something goes wrong. The Parallel Path Problem

When you bond the neutral and ground at a subpanel, you create a parallel path for neutral current. Normal neutral current now has two routes back to the source: through the intended neutral conductor and through the ground conductor.

Why is this bad?1. Stray Voltage and Electromagnetic Interference (EMI)



Stray Voltage

Some of the neutral current will flow on the ground conductor. Because the ground conductor isn't designed to carry normal current, even a small amount of current flowing on it can create a voltage drop along its length. This creates stray voltage on grounded metal parts (enclosures, equipment frames, etc.). Even small amounts of stray voltage can cause nuisance tripping of ground fault circuit interrupters (GFCIs) and can sometimes be a shock hazard, especially in sensitive areas like swimming pools or areas where animals might be present.

EMI

Current flowing on the ground wire can radiate electromagnetic fields (EMI) that can interfere with sensitive electronic equipment.2.

Unpredictable Fault Current Path

During a ground fault (e.g., a hot wire touching a grounded metal enclosure), you want a clear, low-impedance path for fault current to flow back to the source. This high fault current is what triggers the circuit breaker or fuse to trip quickly, interrupting the fault and preventing electrocution or fire. With the neutral and ground bonded at the subpanel, the fault current will split between the neutral conductor and the ground conductor. This increases the overall impedance of the fault current path. The fault current might not be high enough to trip the breaker quickly, potentially leaving the fault energized for a longer period, increasing the risk of shock or fire.3.

Elevated Ground Potential

Bonding neutrals and grounds at multiple locations can result in differences in ground potential (voltage) between different parts of the electrical system. This is dangerous as someone touching two grounded objects in different locations could experience a shock due to the potential difference.4.

Risk of Electrocution

If the neutral conductor becomes loose or disconnected (a "floating neutral"), all of the neutral current will be forced to flow through the ground conductor. If the ground conductor is not sized to handle this current, it could overheat, and any metal parts connected to the grounding system could become energized, posing a serious electrocution hazard.

The Swimming Pool Factor

Swimming pools introduce several unique electrical safety considerations: Water Conductivity

Water is a good conductor of electricity, greatly increasing the risk of electrocution if there's a fault.

Proximity

People are often barefoot and wet around pools, making them more vulnerable to electric shock.

Bonding Requirements

Pool codes (NEC Article 680 in the US) require extensive bonding of all metallic parts of the pool structure, equipment, and surrounding areas to create an equipotential plane. This aims to minimize voltage gradients in the pool area, reducing the risk of shock.If you have a subpanel with bonded neutrals and grounds supplying pool equipment, you're introducing the potential for stray voltage and circulating ground currents into the pool's grounding/bonding grid. This can compromise the effectiveness of the equipotential plane and increase the risk of shock. GFCI protection becomes even more critical, but GFCI can nuisance trip when there are stray currents.

Why Separate Neutrals and Grounds? Dedicated Fault Path

Separating the grounds provides a dedicated, low-impedance path for fault current without the contamination of normal neutral current.

Minimizing Stray Voltage

It prevents normal neutral current from flowing on the ground conductors, eliminating stray voltage on grounded equipment.

Improved GFCI Performance

It reduces the risk of nuisance tripping of GFCIs due to stray currents on the grounding system.

Reliable Fault Clearing

Ensures that fault currents are high enough to quickly trip breakers, interrupting the fault and preventing hazards.

Single Point of Grounding (at the Service)

By bonding the neutral and ground only at the main service disconnect, you establish a single, well-defined reference point for the electrical system.

Your Specific Situation Your existing setup (grounds and neutrals bonded at the house panel and the subpanel) violates the principle of a single point of grounding. The neutral current has parallel paths and therefore the metal will have stray voltage on it. Because you have a ground rod at the house, you now have a very poor separately derived system where the neutral is not bonded to the ground rod, or if the neutral is bonded to the ground rod you now have a parallel neutral path. If you were to run circuits for a pool, this poses the safety risks outlined above, particularly the potential for stray voltage and compromised GFCI protection.

The Fix (Simplified)1. Separate Neutrals and Grounds

Disconnect the neutral-to-ground bond at the subpanel in the house.2.

Ensure a Four-Wire Feed

Your feed from the barn to the house should consist of two hots, one neutral, and one equipment grounding conductor.3.

Bond only at the Service

The only place where the neutral and ground should be bonded together is at the main service disconnect in the barn.

In summary: Separating grounds and neutrals creates a safer and more reliable electrical system by providing a dedicated, low-impedance path for fault current, minimizing stray voltage, improving GFCI performance, and preventing the risks associated with parallel neutral paths. The presence of a pool greatly amplifies the importance of these safety measures. Always consult with a qualified electrician to ensure that your electrical system meets code requirements and is safe. Flag for review