Okay, let's break down the best practice for connecting the neutral drain wire in your medium voltage transformer setup, given the information you've provided.

Understanding the Situation Medium Voltage Transformer

You're working with voltages significantly higher than standard residential or light commercial levels. This introduces safety considerations.

High Resistance Grounding (HRG)

This is a grounding system where the neutral is connected to ground through a resistor. HRG is used to limit ground fault current, reducing the risk of arc flash and equipment damage. The resistor value is carefully selected.

Bonding Strap Removed

This is crucial. The original bonding strap at the transformer XO connects the transformer's neutral point to the transformer's case (ground). Removing this strap isolates the neutral from the transformer case. This is a necessary step with HRG.

Neutral (Drain) Wire in Phase Conductor

This is common in some types of medium voltage cable. The drain wire provides a continuous grounding path for the cable shield, even if the shield itself isn't perfect.

The Correct Action: Remove the Neutral Drain Wire Connection at XO.Reasoning1. Intent of HRG

The entire point of HRG is to precisely control the current flow in the event of a ground fault. The resistor is the defined path. Connecting the drain wire at XO would create a parallel path to ground that bypasses the resistor. This defeats the purpose of HRG and could lead to unintended consequences.2.

Maintaining Defined Ground Path

The HRG system requires that all grounding current flows through the grounding resistor. Any other connection between the neutral and ground will bypass the resistor and compromise the HRG system's protective function.3.

Stray Currents and Noise

Connecting the drain wire to the XO could introduce stray currents onto the neutral system, potentially creating noise issues or other problems with sensitive equipment.4.

Proper Shield Grounding

While the neutral drain wire is integral to the cable assembly, its primary grounding function is for the cable's shield and is typically handled at the cable termination points within the switchgear or other equipment enclosures. Connecting the shields at the enclosures provides shielding from EMI and RFI and safely carries any fault current to ground.

Where Should the Drain Wire Be Connected? Termination Points

The neutral drain wire should be connected to ground at the equipment termination points (switchgear, panels, etc.) where the cable enters.

Cable Grounding Kits

Proper cable grounding kits are designed to connect the cable shield (via the drain wire) to the enclosure's ground bus. These kits are essential for maintaining the integrity of the shield and providing a safe path for fault current.

Important Considerations and Recommendations Consult with a Qualified Engineer

Medium voltage systems are complex and dangerous. Always consult with a qualified electrical engineer experienced in medium voltage HRG systems before making any modifications or connections. They can review your specific system design and ensure the proper grounding configuration.

Review the System Design Drawings

The system design drawings should clearly indicate how the neutral is to be grounded and where the drain wires are to be connected.

Follow Manufacturer's Instructions

Always follow the cable manufacturer's and the grounding resistor manufacturer's instructions for grounding and termination.

Verify Grounding Impedance

After installation, it's critical to verify the impedance of the grounding resistor to ensure it's within the specified range.

In summary: Remove the neutral drain wire from XO. Connect the drain wires only at the equipment termination points using appropriate cable grounding kits, ensuring they are connected to the equipment's grounding bus. Flag for review