NGR Continuity Monitoring: Respond to an Open or Bypassed Resistor

1. Monitor the grounding path itself

A neutral current relay can detect an earth fault only within the conditions of its designed grounding system. Little current does not prove that the neutral grounding resistor is healthy: the resistor or its connection may be open. A bypass can instead raise fault current above the intended limit. Continuity monitoring answers a different question from earth-fault current detection. The Bender monitor manual describes resistance, voltage and current measurements. This guide addresses loss of the grounding path and its response; the grounding-transformer and NGR sizing guide covers the original grounding design.

2. Identify the monitored boundary

Draw the star point, neutral conductor, resistor assembly, earth conductor, sensing connections, CT and isolation devices. Decide which connections are included in the monitor's measurement; an intact resistor element does not prove an intact earth connection outside that boundary. Obtain system voltage, nominal resistance, resistor thermal duty, charging-current study, operating modes and permissible parallel sources. The NGRM500 product page limits that model to high-resistance-grounded systems. Select hardware for the actual grounding method rather than using one product name for both high- and low-resistance grounding. Confirm the manufacturer's sensing components and installation limits.

3. Illustrative consequence of a bypass

Assume a 6.6 kV line-to-line system designed for 10 A resistor current at full neutral displacement. Using 1.732 for the square root of three gives approximately 3811 V phase-to-neutral and a nominal resistor of 381.1 Ω.

If an unintended parallel path lowers the effective resistance to 190.55 Ω, the simplified total current becomes 20 A and total grounding-path dissipation becomes 76.22 kW. This is the combined path, not the original resistor alone. With two parallel 381.1 Ω branches at 3811 V, each branch carries 10 A and dissipates 38.11 kW; the original element's heating has not doubled at that fixed voltage.

This is a rounded resistive illustration, not a network fault study. Source impedance, capacitive current and the actual bypass arrangement change the result. An open resistor is not merely the opposite numeric case; it changes grounding behavior and can defeat the assumptions of current-only protection.

4. Separate fault signatures

The Bender manual describes active and passive methods. Passive voltage/current inference cannot establish resistance reliably when useful excitation is absent. Active measurement can provide continuity evidence without an ordinary earth fault, within the supported installation. Multiple monitors must be coordinated so their injected measurements do not interfere.

5. Define a grounding-failure action

Create separate cause-and-effect rows for earth fault, resistor failure, sensing failure and monitor supply loss. Decide whether each state alarms, blocks energization, initiates controlled shutdown or trips, using the site protection policy and applicable jurisdiction. Do not assume that a policy allowing operation through one earth fault also permits operation with a lost resistor. Assign acknowledgement, escalation and restoration authority. The intended response should remain visible when SCADA is unavailable. For tied generators or transformers, the Bender multiple-source guidance demonstrates that active/passive operation requires topology-aware coordination; verify the actual manufacturer's arrangement rather than improvising injection switching.

6. Commission and hand over the boundary

Use the manufacturer's approved test method under an isolated, authorized plan. Demonstrate the resistance-fault output, earth-fault output, sensing invalidity and supply-loss response separately. Verify downstream isolation or shutdown logic and the prevention of unintended re-energization. Never create an actual live open-neutral condition simply to prove an alarm. Record the baseline resistance and temperature, sensing circuit, configuration, topology states and expected contacts. Include evidence that a correct resistor reading covers the required connections. Reassess any REF protection affected by grounding changes. Procurement should require maintainable sensing components, replacement instructions and an unambiguous fault indication.

7. Questions for operations

Does zero neutral current prove continuity?

No. It can mean no fault, an open path or a failed measurement. Resistance supervision supplies separate evidence.

Should every NGR fault immediately trip?

The action depends on the system and operating policy. Specify it deliberately; no universal delay is justified here.

Can parallel sources make a good resistor appear low?

Yes, a changed effective grounding path can change the measured condition. Validate topology before changing an alarm threshold.

8. Primary references

• Bender NGRM500/NGRM550
• Bender LINETRAXX NGRM500
• Bender — Generator Monitoring