You've hit on a good point and it's worth considering why this practice is acceptable in HRG systems with resistor health monitoring. Here's a breakdown of why it works and the considerations involved

Why it Works (And is Considered Acceptable)
High Impedance of the Monitoring Device

You're right to focus on the 20k ohm impedance. This is the key. While it is a parallel path, its extremely high impedance means it draws very little current compared to the NGR itself. The vast majority of fault current will still flow through the NGR, limiting the ground fault current to safe levels.

Purpose-Built Monitoring

The 20k ohm device isn't just a random resistor. It's specifically designed and manufactured for the sole purpose of monitoring the integrity of the NGR and neutral circuit. It is part of a larger system designed to improve safety and reliability.

Milliamps vs. Amps

A typical NGR might be sized to limit ground fault current to a few amps (e.g., 5A, 10A, etc.). A parallel path drawing only milliamps has a negligible impact on the overall system behavior.

Focus on Ground Fault Current Limitation

The primary goal of HRG is to limit ground fault current to a safe level that: Minimizes arc flash hazards. Reduces the risk of equipment damage. Allows time for a controlled shutdown instead of an instantaneous trip.The monitoring system, despite creating a parallel path, does not compromise this primary goal.

Enhanced Reliability and Safety

The small amount of current drawn through the parallel path allows the monitoring device to: Detect open circuits in the NGR. Detect degradation of the NGR. Alert personnel to potential NGR failures before a major ground fault event occurs.

Code/Standard Considerations (Why it's Usually NOT Specifically Addressed, But Implicitly Allowed) General Principles vs. Specific Implementations

Electrical codes and standards (like the NEC in the US) are generally written based on principles and broad requirements rather than prescribing specific designs for every possible application. They set the performance requirements, but generally don't dictate exactly how you achieve them.

Equivalent Safety

The installation of this parallel monitoring circuit can be argued to provide an equivalent or superior level of safety compared to an HRG system without monitoring. The potential for increased reliability and early detection of NGR problems offsets the slight deviation from a "pure" NGR configuration.

Listed Equipment

If the NGR and its monitoring system are listed by a reputable testing laboratory (e.g., UL, CSA), it means the entire system has been evaluated and found to be safe and compliant with applicable standards. The listing process typically considers the impact of the monitoring circuit. Installation instructions provided with the listed equipment must be followed.

Professional Engineering Judgement

Electrical installations often involve engineering judgment. A qualified electrical engineer can assess the specific circumstances and determine if the proposed design (including the monitoring system) meets the overall safety and performance requirements.

Addressing Your Concerns



Two Current Paths

You're correct that there are technically two paths. However, because of the impedance difference, the effect of the parallel path is minimal.

Code Violation? It's unlikely this would be considered a code violation, especially if the equipment is listed and installed according to the manufacturer's instructions. The key is that the monitoring system enhances safety, not compromises it. Mountain vs. Molehill

In most cases, this is a molehill. The impact of the monitoring circuit is usually insignificant. However, it's still important to understand the principles behind the design and to verify that the equipment is properly listed and installed.

In Summary:

The use of a high-impedance parallel path for NGR health monitoring is generally accepted because: It draws negligible current compared to the NGR. It enhances system reliability and safety by detecting NGR problems. It's often part of a listed system that has been evaluated for safety. It aligns with the overall goals of HRG, which are to limit ground fault current and improve safety.You're right to question this, but in this case, the benefits of the monitoring system outweigh the minor deviation from a theoretically "perfect" NGR configuration. If you have concerns about a specific installation, consult with a qualified electrical engineer. They can assess the specific circumstances and ensure the system is safe and compliant. Flag for review