Why are we not allowed to measure voltage from the neutral wire to the ground wire in an electrical wiring system?

Measuring voltage between the neutral wire and the ground wire in an electrical wiring system is generally discouraged due to safety concerns. The neutral wire is intended to carry current back to the source (typically the transformer at the power distribution center), and it is usually near ground potential. The ground wire is connected to the ground, which is also considered to be at a reference potential.

Here are a few reasons why measuring voltage between the neutral and ground wires may be discouraged or problematic:

1. Safety Concerns: In a properly functioning electrical system, the voltage between the neutral and ground should be very close to zero. However, if there are faults or issues in the wiring, there's a potential for voltage imbalances or unexpected currents. Measuring voltage between neutral and ground may expose you to the risk of electric shock, especially if there's a fault in the system.

2. Fault Indication: If you measure a significant voltage between the neutral and ground, it could indicate a fault in the electrical system, such as a broken neutral connection. However, it's often safer and more effective to use other testing methods and equipment designed for fault detection rather than directly measuring voltage between neutral and ground.

3. Unreliable Readings: Voltage measurements can be influenced by various factors, and using the neutral and ground as reference points might not always provide accurate information about the health of the electrical system. It's better to use appropriate tools and techniques to diagnose electrical issues.

4. Code Compliance: In many electrical codes and standards, it's specified that the neutral and ground should be bonded at the main service panel but kept separate in subpanels. Measuring voltage between them may not be meaningful or compliant with these guidelines.

If you suspect a problem in your electrical system, it is recommended to consult with a qualified electrician who can use the appropriate tools and methods to diagnose and address any issues safely and in compliance with electrical codes and regulations.