Direct Answer: Type G-GC 3/C #2 AWG Specifications and Pilot Conductor Architecture 直接答案：Type G-GC 3/C #2 AWG 规格和导引导体架构

Engineering Specifications for Earth Fault Detection 接地故障检测的工程规格

The Type G-GC 3/C #2 AWG cable is a three-conductor power cable with an integrated pilot ground-check conductor designed specifically for underground mine power distribution and earth fault monitoring applications. The three main conductors each measure 33.3 mm² (2 AWG) cross-sectional area and carry three-phase power distribution. The pilot ground-check conductor measures 4 mm² (12 AWG equivalent) and serves as an independent monitoring channel for earth fault detection. The overall cable outer diameter (OD) is typically 18.5 mm to 20.0 mm (0.73 to 0.79 inches) depending on the specific insulation system. The overall weight is approximately 850 kg/km (570 lbs/1000ft). The cable outer sheath is typically XLPE (cross-linked polyethylene) rated for 600 volts continuous service with a safety protocol compliant with DIN VDE 0482-335-2 and IEEE 1202 standards for mine cables. The pilot conductor resistance is approximately 5.2 ohms per kilometer, which establishes the baseline sensitivity for ground fault detection circuits. The capacitance between main conductors and the pilot conductor is typically 120–140 pF/m, a critical parameter that determines the transient response characteristics of the earth fault detection relay.

The pilot ground-check conductor is electrically isolated from the three main power conductors throughout the cable length. It terminates at a dedicated safety relay that continuously monitors the potential difference between the pilot conductor and ground. When a ground fault occurs anywhere on the power circuit, earth current flows through the pilot conductor circuit, creating a measurable potential change that the relay detects and uses to trigger protective actions. The pilot conductor does not carry normal operating current—it functions exclusively as a measurement and diagnostic pathway.

1. The Earth Fault Crisis: Why Standard Cables Cannot Meet Modern Mine Safety Requirements 接地故障危机：为什么标准电缆无法满足现代矿井安全要求

Underground mining operations present electromagnetic and safety challenges fundamentally different from surface industrial installations. The mine environment combines high moisture, conductive dust, mechanical vibration, and extended cable runs in ways that create continuous risk of ground faults—dangerous conditions where electrical current leaks from the main power conductors to the mine structure. Traditional mine safety relied on overcurrent relays that detect fault current by measuring current magnitude. This approach is increasingly inadequate for modern mining operations because contemporary requirements demand not just fault detection, but rapid detection of small faults before they cause damage or create hazards. The solution requires fundamentally rethinking how we monitor ground faults—moving beyond simple current measurement to sophisticated diagnostic sensing using pilot conductors and multi-stage relay logic. Understanding this shift requires learning about the nature of earth faults in mining, why they are so dangerous, and how pilot conductors enable the detection capabilities that modern safety standards now require.

The earth fault problem in mining has become more acute rather than less despite improvements in cable insulation technology. Modern mining extracts ore from greater depths, operates more extensive electrical systems, and increasingly uses variable frequency drives and sophisticated automation that creates high-frequency disturbances. These factors increase the rate at which ground faults occur and make the failures more destructive when they do happen. A ground fault in a modern mine is not merely an electrical problem—it is a safety-critical event that can disable ventilation systems, compromise communications, create arc flash hazards, or trigger explosions if the fault occurs in a methane-prone environment. The industry response has been to require cables with dedicated pilot conductors and safety-rated relay systems designed to detect faults at early stages before they escalate to dangerous conditions.

2. Understanding Ground Fault Current Distribution and Earth Resistance Networks 理解接地故障电流分布和接地电阻网络

To understand how pilot conductors enable earth fault detection, you must first understand how ground fault current flows through a mine electrical system and how the earth resistance network distributes that current. This requires understanding electrical circuit topology in ways that differ significantly from surface industrial installations.

In a surface industrial facility, the earth/ground is typically a low-impedance reference—grounding rods, copper mesh, or connection to the utility company’s grounding system provide a well-defined return path for fault current. Current flowing from a faulted conductor to the earth has a clear path and a measurable magnitude. In an underground mine, the situation is fundamentally different. The earth is not a simple ground rod but rather the surrounding rock formation, which extends in all directions. When a ground fault occurs—when current leaks from an insulated cable to the mine structure—that current does not flow to a single ground point. Instead, it spreads throughout the rock formation in complex patterns determined by the rock’s electrical conductivity, the fault location, and the geometry of the mine structure.

This distributed current flow creates what we call an earth resistance network. The fault current flows out through the rock in all directions, with higher current density near the fault location and lower density farther away. At multiple points throughout the mine—at transformer terminals, at equipment frames, at intermediate earth connections—some portion of this fault current returns to complete the circuit. The magnitude of fault current measured at any single point depends on that point’s position relative to the fault location and its electrical distance through the earth resistance network. This distributed current flow means that overcurrent relays at the surface may measure very little current even when a significant ground fault exists deep in the mine, because the fault current is distributed across multiple return paths.

The pilot conductor solves this problem by providing a defined current path that does not depend on the distributed earth resistance network. When a ground fault occurs, the pilot conductor carries a signal that represents the fault condition regardless of where in the earth resistance network the fault current flows. The relay monitoring the pilot conductor measures a signal directly related to the fault location and magnitude, independent of the complex distributed current paths in the rock formation. This is why pilot conductors are indispensable for reliable fault detection in mining—they create a measurement pathway that is immune to the distributed earth resistance effects that make conventional current measurement unreliable.

3. The Pilot Conductor as an Independent Diagnostic Pathway 作为独立诊断路径的导引导体

The pilot ground-check conductor functions as an independent monitoring circuit that runs parallel to the three main power conductors but remains completely isolated from them electrically. To understand how this works, imagine the cable with three main conductors carrying power and a fourth smaller conductor—the pilot—running alongside them but never carrying main circuit power. At the power source (the surface) and at the load (the underground equipment), the pilot conductor connects to the safety relay that monitors it continuously.

The pilot conductor is grounded at both ends—at the surface through the relay’s measurement circuit and at the underground equipment frame. This creates a complete electrical circuit, but because the pilot conductor runs inside the cable sheath with the main conductors, any ground fault on the main circuit will also affect the pilot conductor. When a ground fault occurs on one of the three main conductors, a current path is created from that conductor to the mine structure. Because the pilot conductor runs close to this conductor through the cable, the fault creates an electrical coupling that appears as a voltage or current change on the pilot conductor. The safety relay detects this change and interprets it as evidence of a ground fault.

The critical advantage of this approach is isolation from the normal operating circuit. The pilot conductor is not affected by the normal operating current or normal load transients on the three main conductors. It responds only to ground fault conditions. This isolation allows the relay to establish very sensitive thresholds that would be impossible with a current transformer on the main circuit. A current transformer on the main circuit must filter out normal load variations, starting transients, and harmonic disturbances. A pilot conductor relay can be much more sensitive because it is looking for a specific signature—the coupling from a ground fault—rather than trying to distinguish fault current from load current. This is why pilot conductors enable significantly faster fault detection and much lower false alarm rates than conventional current-based protection.

4. Capacitive Coupling and the Problem of Transient False Alarms 容性耦合和瞬态误报的问题

One of the most subtle but critical problems in mine earth fault protection is distinguishing genuine ground faults from transient voltage spikes and capacitive coupling effects. This problem becomes acute with pilot conductors because their sensitivity to fault conditions also makes them sensitive to electrical transients if the relay is not properly designed.

Capacitive coupling occurs because the pilot conductor, running inside the cable with the three main power conductors, has inevitable capacitance between itself and the main conductors. This capacitance is small—perhaps 100–150 pF/meter—but it is sufficient to couple high-frequency transients from the main circuit to the pilot. When a load switches on, when a motor starts, or when any transient event occurs on the main circuit, high-frequency voltage components couple capacitively to the pilot conductor. If the safety relay simply responds to any voltage change on the pilot, it will trigger false alarms on every load transient in the mine.

The solution requires understanding the frequency content of genuine ground faults versus transients. A genuine ground fault is a sustained condition—once it occurs, the fault current flows continuously until the relay trips. The voltage and current signatures are relatively low frequency, typically DC or low-frequency (below 1 kHz) AC components. Transient events like load switching create very brief high-frequency disturbances that last for microseconds or milliseconds. A properly designed safety relay incorporates filtering that responds to sustained low-frequency changes (indicating a fault) while rejecting brief high-frequency spikes (indicating transients). This filtering cannot be performed in the cable—it must be performed in the relay electronics. Type G-GC cables are designed to work with relays that have this sophisticated discrimination capability built in.

5. How Safety Relays Interpret Pilot Conductor Signals: Multi-Stage Relay Logic 安全继电器如何解释导引导体信号：多阶段继电器逻辑

The safety relay that monitors the pilot conductor is not a simple electromechanical device. It is a sophisticated instrument that implements multi-stage logic to detect ground faults while maintaining reliability and preventing false alarms. Understanding how this logic works provides insight into why pilot conductors are so effective and how proper relay selection is essential.

The first stage of relay logic is signal conditioning. The raw pilot conductor signal enters an input circuit that filters out high-frequency noise through a combination of capacitive coupling and low-pass filtering. This removes transient spikes while passing the sustained voltage/current changes that characterize ground faults. The filtered signal then enters the detection logic.

The second stage implements a threshold detector. The relay measures the pilot conductor signal against a programmed threshold—typically set at a current equivalent of 20–50 milliamperes of earth fault current. If the signal exceeds this threshold and remains above it for a programmed duration (typically 50–100 milliseconds), the relay recognizes a potential fault condition. This duration discrimination is critical—transient spikes are rejected because they do not persist for the required duration, while true faults, which are sustained conditions, accumulate evidence over the discrimination period.

The third stage implements a confirmation logic that verifies the fault using a second monitoring channel. Many modern mine relays are designed with dual-channel architecture where the pilot conductor signal is measured through two independent measurement paths. Both channels must indicate a fault before the relay triggers. This redundancy prevents single points of failure where a relay malfunction could create false alarms or missed faults. Only when both channels agree that a sustained fault condition exists does the relay activate its output contacts and shut down the circuit.

This multi-stage logic is built directly into the relay electronics and cannot be implemented with conventional mechanical relays or simple solid-state devices. The relay communicates continuously with the cable system, accumulating evidence of fault conditions and making protection decisions based on sophisticated signal interpretation. This is why earth fault protection in modern mines requires safety-rated electronic relays designed specifically for pilot conductor monitoring rather than general-purpose relays.

6. Distinguishing Earth Leakage from True Ground Faults: Threshold Setting and Discrimination 区分漏电和真正接地故障：阈值设置和判别

An important distinction that safety relays must make is between earth leakage (slow insulation degradation where small currents leak to ground over time) and true ground faults (sudden insulation failure where substantial current suddenly flows to ground). Both conditions require attention, but they require different responses. Understanding how relays distinguish between these conditions reveals the sophistication of modern mine safety systems.

Earth leakage is a normal phenomenon in underground cables. Moisture, conductive dust, and vibration gradually degrade insulation, allowing small leakage currents to flow to ground. A cable with 500 volts applied across insulation rated for 600 volts might develop a few milliamperes of leakage current. This is a warning condition—it indicates the insulation is degrading and should be repaired—but it is not an immediate safety emergency. The cable can continue operating while repairs are scheduled.

A true ground fault is insulation failure, either sudden or progression from severe leakage. When leakage currents exceed a certain threshold—typically 50–100 milliamperes—the relay recognizes this as a fault condition that presents immediate danger. At this current level, the fault can generate heat that damages the cable, create arc flash hazards, or trigger equipment malfunctions. The relay responds by tripping the circuit immediately.

To accomplish this discrimination, the relay typically implements tiered thresholds. The first threshold (perhaps 20–30 mA) triggers a warning alarm that alerts the operator to degrading insulation but does not shut down power. The relay monitors how the pilot signal evolves—if the leakage remains below the warning threshold indefinitely, the system continues operating while the operator schedules maintenance. If the leakage increases and crosses a higher threshold (perhaps 50–100 mA), the relay immediately trips the circuit. This graduated response allows the mining operation to address insulation degradation during planned maintenance windows while maintaining the ability to shut down the system immediately if a dangerous fault develops.

7. Dual-Channel Verification: Why Redundant Monitoring Prevents Catastrophic Failure 双通道验证：为什么冗余监控防止灾难性故障

A recurring principle in safety-critical systems is that critical protection functions must not depend on a single measurement or a single device. If the only earth fault protection relies on a single sensor or a single relay, then a failure of that sensor or relay creates a situation where the mining operation is unprotected without the operator knowing it. This is unacceptable in safety-critical applications. The solution is redundant monitoring—using two independent channels to measure fault conditions, with the requirement that both channels must indicate a fault before protective action is taken.

In modern Type G-GC cable systems with safety-rated relays, dual-channel verification typically works as follows: the pilot conductor carries the primary monitoring signal, measuring earth fault conditions continuously. This is the first channel. A secondary measurement might be provided by a reference conductor or by a current transformer on the main circuit that provides a backup measurement of fault current. Both channels feed into the relay’s decision logic. When a ground fault occurs, it creates signatures on both channels. The relay compares these signatures and only triggers a protective shutdown when both channels report consistent evidence of a fault. If one channel fails or provides contradictory information, the relay does not act, but instead generates an alert that the monitoring system requires attention.

This redundancy does not slow down fault detection—detection still occurs within 100–200 milliseconds because the second channel operates in parallel with the first. But it provides assurance that false alarms from a single failed sensor cannot cause dangerous shutdowns, and it ensures that a single relay failure cannot prevent fault detection. This level of redundancy and cross-checking is what makes modern earth fault protection suitable for life-safety critical applications where mining operations depend absolutely on continuous safe operation.

8. Pilot Conductor Isolation Requirements and Grounding Architecture 导引导体隔离要求和接地架构

Like the shielding grounding requirements in VFD cables, pilot conductor effectiveness depends critically on proper grounding implementation. Many installations fail to provide adequate protection not because the cable is inadequate but because the grounding architecture is incorrect.

The pilot conductor must be grounded at two points: at the surface (through the safety relay’s measurement circuit) and at the underground equipment location (through the equipment frame or a dedicated ground point). These are not arbitrary grounding connections—they must be designed as part of an integrated earth fault protection system. At the surface end, the pilot conductor connects to a dedicated terminal in the safety relay that presents a defined input impedance (typically very high impedance for voltage-measurement relays or low impedance for current-measurement relays). This input circuit establishes the baseline potential reference for the pilot conductor monitoring. At the underground end, the pilot conductor connects to the equipment frame or to an independent ground bus that ensures the point of measurement is the actual earth fault location.

The critical error is grounding the pilot conductor through inadequate paths. If the underground end of the pilot conductor is grounded through a high-resistance connection—perhaps through corrosion products or loose bolts—the fault detection signal becomes attenuated and delayed. The relay may detect faults slowly or incompletely. Similarly, if the surface end of the pilot conductor connects to a noisy or inadequately filtered input on the relay, high-frequency noise can mask the fault signal. Proper pilot conductor grounding requires dedicated low-resistance connections at both ends, making use of the principle that this measurement circuit must have superior electrical quality compared to the main power circuit because it carries critical safety information.

The pilot conductor must also be isolated from the three main power conductors throughout the cable and at all terminations. If the pilot conductor accidentally becomes connected to one of the main conductors (through damaged insulation or improper termination), it ceases to function as an independent diagnostic pathway and becomes simply another phase conductor. This defeats its protective purpose. Cable installation specifications and acceptance tests must verify that the pilot conductor remains electrically isolated from all other conductors until intentional connection points.

9. Fault Location Accuracy: How Pilot Conductors Enable Precise Underground Diagnostics 故障位置精度：导引导体如何实现精确的地下诊断

Beyond fault detection, sophisticated mine safety systems use pilot conductors for fault location—determining where in the extended underground cable run a ground fault has occurred. This capability is valuable for maintenance and troubleshooting because it allows mining operations to focus repair efforts on the precise cable section requiring attention rather than searching throughout the entire cable run.

Fault location is possible because the pilot conductor is distributed throughout the cable length. At the moment a ground fault occurs on one of the main conductors, the fault couples electromagnetically to the pilot conductor at the fault location. The magnitude and character of the signal on the pilot depends on the distance from the measurement point (the relay at the surface) to the fault location. A fault near the surface end of the cable creates a different signal signature than a fault at the far end. By measuring the signal character and using knowledge of the cable’s electrical characteristics (its resistance, capacitance, and inductance per unit length), the relay can estimate the distance to the fault.

More sophisticated relays implement time-domain reflectometry—a technique borrowed from telecommunications where a pulse is sent down the pilot conductor and the relay measures how long the reflection from the fault point takes to return. From this time delay, the distance to the fault can be calculated. The accuracy of this method depends on knowing the cable’s propagation velocity precisely, but modern relays can typically locate faults to within 1–5% of their actual distance. In a 10-kilometer underground feeder cable, this means locating a fault to within 100–500 meters, enough accuracy to guide repair crews to the damaged section.

Fault location capability is not merely a convenience feature. It directly improves mine safety by reducing the time required to isolate and repair faults. When the fault location is known precisely, repair crews can dig to the exact location rather than excavating the entire cable route. This reduces safety risks to workers, reduces equipment downtime, and reduces operational costs. It also provides diagnostic information that helps identify why the fault occurred, whether it was due to external damage, insulation degradation, or other factors. This diagnostic capability contributes to preventing recurrence of similar failures.

10. Integration with Safety-Rated Relay Systems: Functional Safety Compliance 与安全级继电器系统的集成：功能安全合规

Type G-GC cables do not provide protection by themselves. They provide the sensing capability that allows safety-rated relay systems to implement earth fault protection. Understanding this relationship clarifies the division of responsibility between the cable and the relay and why both must work together as an integrated system.

A safety-rated relay is one that has been designed, tested, and certified according to functional safety standards such as IEC 61508 (for general industrial applications) or IEC 61511 (for process industry safety instrumented systems). In mining, the equivalent standard is typically a country-specific mining regulation or an international standard like ICMM guidelines. These standards define how the relay must be designed to provide reliable fault detection and to guarantee that it cannot fail in a way that masks dangerous conditions. The relay undergoes independent verification and testing to demonstrate that it meets these requirements.

The cable provides the sensory input that feeds into the relay’s protection logic. Type G-GC cables are designed to be compatible with safety-rated relays by providing a pilot conductor with specified electrical characteristics (capacitance, resistance, frequency response) that match what the relay’s input circuits expect. The relay can then analyze the pilot conductor signal with confidence that the signal accurately represents the earth fault condition.

The integration between cable and relay is specified in standards. DIN VDE 0482-335-2 (German mine cable standard) and corresponding international standards define the electrical and physical requirements for pilot conductors. The relay manufacturer specifies which cable types are compatible with their protection design and how the cable must be installed. This specification is not a guideline—it is a requirement. Using a non-specified cable with a safety-rated relay voids the safety certification. Similarly, using an inadequate relay with a properly specified Type G-GC cable fails to achieve the intended protection. Both components must be properly specified and properly installed for functional safety to be realized.

11. Installation Requirements for Type G-GC Cables in Hazardous Mine Environments 危险矿井环境中 Type G-GC 电缆的安装要求

Proper installation of Type G-GC cables in underground mine environments requires attention to details that differ significantly from surface industrial installations. These requirements exist because the mine environment presents unique challenges to cable integrity.

The first requirement is cable routing that protects the pilot conductor from mechanical damage. The pilot conductor, being smaller than the main phase conductors, is more susceptible to damage from mechanical stress, sharp edges, or crushing. Cable installation standards typically require protective conduit or cable armor in areas where mechanical damage is possible. The pilot conductor must not be routed differently from the main conductors—they must be installed as an integrated cable assembly to ensure that the geometric relationship between the pilot and the main conductors is maintained throughout the installation.

The second requirement is moisture management. Underground mine environments have high humidity and frequent exposure to water from groundwater seepage or mining operations. Moisture reaching the pilot conductor’s terminations can bridge the insulation and create false ground fault signals. Type G-GC cables must be terminated using moisture-sealed connectors that completely exclude water. The termination procedure must include steps to verify that moisture has not bridged any insulation. Some installations use desiccant packs or controlled atmosphere termination chambers to ensure dry conditions during termination work.

The third requirement is isolation from the main power system return. In some mine electrical systems, the mine structure itself is used as the return path for power (an “earth return” system). This common practice in mining creates a distributed earth potential that varies throughout the mine. The pilot conductor measurement must not be affected by this return current. This requires careful consideration of the grounding architecture and may require the use of isolated pilot conductor input circuits in the safety relay that do not share a common ground with the main power return.

The fourth requirement is avoidance of accidental parallel paths. If the pilot conductor accidentally connects to any other conductor at any point other than the intentional termination points, it ceases to function as an independent diagnostic pathway. Installation procedures must include careful documentation of all connection points and acceptance testing to verify that isolation is maintained. Cable tracing and continuity testing during commissioning are essential quality assurance activities.

12. Commissioning Procedures and Relay Response Time Verification 投运程序和继电器响应时间验证

Proper commissioning of an earth fault protection system requires systematic testing to verify that the pilot conductor, the relay, and the integrated system respond correctly to ground fault conditions. This testing cannot be performed with simple continuity checks—it requires sophisticated procedures that simulate fault conditions and measure relay response.

12.1 Pilot Conductor Isolation Verification 导引导体隔离验证

Before any earth fault testing, verify that the pilot conductor is properly isolated from all other conductors. Use an insulation tester (megohmmeter) set to 1000 volts to measure the insulation resistance between the pilot conductor and each main phase conductor. The resistance should exceed 100 megohms. If insulation resistance is lower, investigate for moisture, damaged insulation, or accidental connections that must be corrected before proceeding.

12.2 Pilot Conductor Resistance Measurement 导引导体电阻测量

Measure the pilot conductor’s DC resistance end-to-end. For a Type G-GC 3/C cable, the resistance should be approximately 5.2 ohms per kilometer. If the measured value is significantly different, investigate for broken strands, corrosion at terminations, or other damage. Record this baseline resistance for future reference and troubleshooting.

12.3 Relay Input Impedance Verification 继电器输入阻抗验证

Connect the relay to the pilot conductor and verify that the relay’s input circuit is functioning. The relay manufacturer provides specifications for the expected input impedance and input circuit behavior. Using a precision resistance box, apply known resistances to the pilot conductor input and verify that the relay responds with appropriate signal levels. This verifies that the relay can sense pilot conductor conditions correctly.

12.4 Simulated Fault Response Testing 模拟故障响应测试

The critical commissioning test is simulated fault response. Create a small induced voltage or current on the pilot conductor that simulates a ground fault and verify that the relay detects it and triggers its output within the specified response time (typically 100–200 milliseconds). This test confirms that the entire system—pilot conductor, relay, and protective logic—is functioning correctly. The test should simulate both small faults (at the sensitivity threshold) and larger faults (well above the threshold) to verify that the relay responds appropriately to faults of varying severity.

12.5 False Alarm Immunity Verification 误报免疫验证

Apply transient voltage spikes and high-frequency noise to the power circuit and verify that the relay does not trigger false alarms. This testing confirms that the relay’s filtering and discrimination logic are functioning correctly and that the system can tolerate the normal electrical disturbances expected in a mining operation without false shutdown events.

13. Maintenance, Testing, and Field Troubleshooting in Underground Operations 地下作业中的维护、测试和现场故障排除

Once commissioned, earth fault protection systems require ongoing maintenance and periodic testing to ensure continued reliability. The mining operation must establish procedures for routine verification that the protection system remains functional.

13.1 Periodic Earth Fault Testing 定期接地故障测试

At least quarterly, the mine operator should perform simulated fault testing to verify that the earth fault protection system responds correctly. This testing simulates the conditions that would occur during an actual ground fault and verifies that the relay detects and responds appropriately. The test should be documented and compared to previous test results to identify any degradation of system performance.

13.2 Pilot Conductor Continuity Trending 导引导体连续性趋势分析

Monthly resistance measurements of the pilot conductor can reveal gradual degradation. If pilot conductor resistance increases over time, it suggests moisture accumulation, corrosion at terminations, or other damage that requires attention. Trends in resistance measurements provide early warning of problems before they cause loss of protection.

13.3 Relay Self-Test Monitoring 继电器自检监控

Modern safety-rated relays include built-in self-test circuits that continuously verify the relay’s electronic components. The relay typically provides a status indicator showing whether self-tests are passing or if a failure has been detected. Mine operators should monitor this status continuously. Any indication that the relay’s self-test has detected a failure requires immediate investigation and likely replacement of the relay.

13.4 Troubleshooting False Alarms 假报警的故障排除

If the earth fault protection system triggers false alarms (trips without a genuine ground fault), systematic troubleshooting is required. First, verify that no actual ground fault exists by checking insulation resistance of the power circuit and examining cables for damage. If insulation is good, investigate sources of transient noise that might be coupling into the pilot conductor. Check for loose connections, corroded terminations, or moisture at termination points. Verify that the relay’s filtering and discrimination settings are appropriate for the mine’s electrical environment. Only after careful investigation should the relay be repaired or replaced.