Impedance Monitoring

Your Grounding System Depends
on an Intact Resistor Path

An open or bypassed neutral grounding resistor changes how the system responds to a ground fault. Continuous resistor path monitoring helps identify that loss of protection so the installation can alarm or trip as required.

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Would You Know If Your Protection Failed?

A Failed Resistor Path Can
Compromise Your Grounding System

In both high resistance grounded (HRG) and low resistance grounded (LRG) systems, an open circuit in the neutral grounding resistor (NGR) or its connections can leave the system ungrounded. A short circuit that bypasses the resistor can create a solidly grounded path, removing the intended limit on ground-fault current. Without resistor path monitoring, either condition can remain undetected until a ground fault occurs.

Protective functions should never be assumed to work. They should be verified.

Resistor Path Failure Modes

Failure mode
Effect on the system

Open resistor path

The system can become ungrounded, increasing susceptibility to overvoltages to ground and making ground faults harder to locate.

Short circuit bypassing the resistor

The system loses the resistor’s intended current limitation, allowing substantially higher ground-fault current.

Continuous monitoring identifies these changes so personnel and protection circuits can respond when the grounding path is compromised.

High resistance grounding is I‑Gard’s engineering focus. Explore the benefits of HRG →

Required in Canada.
Essential to Reliable Resistance Grounding.

CEC 2024, Section 10 and Table 17 establish monitoring and protective-response requirements for impedance-grounded installations. In a resistance-grounded system, the monitoring arrangement must detect failures of the resistor path and provide the alarm and de-energization response applicable to the installation.

These requirements address a fundamental concern: the grounding protection must remain effective throughout operation. An installed resistor can develop an open circuit, be bypassed by a short circuit, or lose a connection. Continuous monitoring makes those conditions detectable.

Compliance depends on the installed arrangement, including the monitoring relay, sensing components, settings, and connections to the alarm and trip circuits.

As a Canadian manufacturer, I‑Gard advocates continuous resistor path monitoring wherever resistance grounding is used. The need to verify the grounding path is universal.

SIGMA3-C

Ground-fault protection and resistor path monitoring for HRG and LRG systems.

SIGMA3-C continuously monitors the neutral-to-ground path, including the resistor and its connections, to detect conditions that compromise resistance grounding. It provides the monitoring function and protective outputs needed to support compliance with the installation requirements of CEC 2024, Section 10 and Table 17.

  • The relay monitors resistor path integrity during normal operation and ground-fault conditions.
  • An adjustable resistor path fault trip delay of up to 48 hours allows the response to be coordinated with the applicable installation requirements.
  • SIGMA3-C supports high resistance grounded (HRG) and low resistance grounded (LRG) systems, with pulsing control for fault location in suitably equipped HRG systems while operation continues.
  • I‑Gard manufactures the relay and the NGR it monitors, with sensing components selected to match the application.

SIGMA3-C Instruction Manual SIGMA3-C Product Brochure SIGMA3-C Calibration Guide SIGMA3-C Troubleshooting Guide
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SIGMA3-C in Action

Reliable resistance grounding depends on detecting ground faults and verifying the resistor path. SIGMA3-C monitors both and, in suitably equipped HRG systems, also helps personnel locate the fault.

1. Ground-Fault Protection

Detects ground faults with an adjustable pickup level and a trip delay of 60–9,300 ms.

2. Resistor Path Monitoring

Continuously monitors the neutral-to-ground path, including the resistor and its connections, for abnormal resistance and open- or short-circuit conditions.

3. Pulsing for Fault Location in HRG Systems

With a pulsing NGR and contactor, SIGMA3-C controls a repeating change in ground-fault current. A technician traces the pulses with a clamp-on ammeter to locate the fault while operation continues.

SIGMA3-C provides separate trip, ground-fault, resistor path fault, and watchdog outputs. These connect to the installation’s alarm and trip circuits to implement the required protective response.

Two Complementary Modes for Continuous Monitoring

Mode
Active When
How

Measurement

Current through the NGR exceeds 5% of its rated let-through current.

Calculates resistance from current and voltage measured during operation.

Injection

Current through the NGR is below 5% of its rated let-through current.

Uses high-frequency AC injection and a DC-based measurement to assess resistor path integrity, including open-path detection where parallel capacitance affects the AC response.

Together, these modes monitor the resistor path during normal operation and ground-fault conditions.

System Capacitance Should Not Mask Resistor Path Condition

System capacitance can provide an additional path to ground for the high-frequency monitoring signal. This changes the conductance measured by the relay, even when the resistor path itself remains unchanged. Reliable monitoring must distinguish the influence of capacitance from changes in the resistor path.

SIGMA3-C supplements high-frequency AC monitoring with DC injection during Injection Mode. Because capacitance does not carry steady-state DC current, this provides an additional measurement for assessing the resistor path where capacitance affects the AC response. The monitoring settings are adjusted to suit the application.

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Engineered to Work Together

Reliable monitoring depends on correctly matching the relay and sensing components to the NGR. System voltage, resistor characteristics, and current sensor selection determine the appropriate configuration.

I‑Gard manufactures these components and coordinates their selection for the application. Our engineering team can also review monitoring requirements where an NGR has already been specified or purchased.

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Sensing and Display Components

SIGMA3-C uses an NGRXS sensing resistor and a ZSCS current sensor to monitor the neutral-to-ground path. The SIGMA3-C-TDM touchscreen brings system readings, alarms, and relay settings into view, helping personnel assess conditions and respond.

Component
Role

NGRXS Sensing Resistor

Provides the neutral-to-ground voltage input and couples the monitoring signal into the resistor path. It is selected to suit the system voltage.

ZSCS Current Sensor

Measures current through the NGR, providing the current input for ground-fault detection and resistor path monitoring.

SIGMA3-C-TDM Touchscreen Display

Displays system readings, alarms, and event history, and provides access to relay settings.

I‑Gard selects the sensing components to match the application’s voltage and current requirements.

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Compliance & Standards

CSA logo
IEEE logo
NEC / CEC logo
NFPA 70E / CSA Z462 logo

Resistor monitoring (Canada)

CSA C22.1, §10 & Table 17

Resistance-grounded systems

IEEE 242-2001 §8.2.4 · IEEE 142-2007 §1.4.3

Wiring & grounding

NEC (US) · Canadian Electrical Code (Canada)

Arc flash mitigation recognition

NFPA 70E (US) · CSA Z462 (Canada)

FAQ

Frequently Asked Questions

Answers to our most commonly asked questions. Can’t find yours? Ask here

CEC 2024, Section 10 and Table 17 establish resistor monitoring requirements for resistance-grounded installations. SIGMA3-C provides the monitoring function needed to support compliance when installed and configured with the appropriate sensing components, alarm circuits, and trip circuits.

An open circuit in the NGR or its connections can leave the system ungrounded. A short circuit that bypasses the resistor removes the intended limit on ground-fault current.

SIGMA3-C monitors the neutral-to-ground path to detect these conditions and provides outputs for the installation’s alarm and protection circuits.

Yes. When current through the NGR is too low to calculate resistance from current and voltage measurements, SIGMA3-C uses Injection Mode. It applies a high-frequency signal through the NGRXS sensing resistor and evaluates the response to monitor resistor path integrity.

When current exceeds 5% of the NGR’s rated let-through current, Measurement Mode calculates resistance from measured current and voltage.

SIGMA3-C uses an NGRXS sensing resistor and a ZSCS current sensor. The NGRXS provides the neutral-to-ground voltage input and couples the monitoring signal into the resistor path. The ZSCS measures current through the NGR.

The sensing components must be selected to suit the installation’s voltage and current requirements.

The TDM provides a 7-inch touchscreen for system readings, relay settings, alarms, and trip history. It stores up to 1,000 trip-history records, replacing the oldest records as new ones are added.

The supplied IX cable carries both power and communication between the relay and display. The TDM also supports Modbus TCP/IP over Ethernet for connection to external monitoring and control systems.

The adjustable delay allows the response to a resistor path fault to be coordinated with the installation requirements of CEC 2024, Table 17. The maximum setting is 48 hours; the selected delay must suit the application and required protective response.

This is separate from the ground-fault trip delay, which is adjustable from 60 to 9,300 ms.

RPF monitoring is enabled at the factory for Canadian orders and disabled for US orders. The setting is controlled by DIP switch 9 on SW1.

During commissioning, verify that monitoring is enabled where required and that the relay settings and output connections meet the installation’s protection requirements.

Compatibility depends on the NGR’s electrical characteristics and the sensing components used with the relay. Contact I‑Gard with the system voltage, NGR resistance, rated current, and duty rating so the application and required configuration can be reviewed.

In an HRG installation equipped with suitable pulsing hardware, SIGMA3-C can control a pulsing signal that a technician traces with a clamp-on ammeter.

This function supports fault location while a suitable HRG system continues operating. LRG applications require fault clearing within the applicable protection and resistor duty limits, making pulsing-based fault location generally unsuitable.

Injection Mode monitors resistor path integrity in HRG and LRG systems. It applies a high-frequency signal through the NGRXS sensing resistor when current is too low for Measurement Mode.

Pulsing helps locate ground faults in HRG systems. With a pulsing NGR and contactor, SIGMA3-C varies ground-fault current to create pulses that a technician traces with a clamp-on ammeter.

No. Resistor path monitoring checks the integrity of the neutral-to-ground path. Insulation resistance and dielectric withstand testing evaluate different aspects of connected equipment.

Where such testing is performed, follow the SIGMA3-C manual’s preparation and isolation instructions to protect the relay and sensing components.

No. A normal indication reports the status of the monitored functions. Establishing an electrically safe work condition requires the applicable isolation, lockout, and absence-of-voltage verification procedures.

Working to a Project Specification?

Send I‑Gard your resistor monitoring requirements. Our engineering team can help confirm the SIGMA3-C configuration, sensing components, and display options for your equipment.

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