The 10 Pitfalls That Cause Nuisance Trips & Downtime in Mining Motor Protection (and How to Fix Them)

In mining, even a single nuisance trip can halt production and rack up staggering costs. Industry studies put the average cost of unplanned downtime around $180,000 per incident (R3 million), and a single conveyor failure at a major copper mine recently slashed output by 36%. Protective relays are meant to save motors, but if they’re poorly coordinated or configured, they can become part of the problem. Below we highlight 10 common pitfalls that lead to false trips, overheated motors, or missed faults, and how to fix each issue to keep your mining operations running smoothly.

Safety note: The guidance below is general. Always follow your site’s standards, OEM specs, and commissioning procedures.

Pitfall 1: Treating VFD-Driven Motors Like DOL Circuits for Earth-Leakage Protection

Symptom: 

Unexplained earth-leakage trips at startup or during low-load operation, especially on Variable Frequency Drive (VFD) feeds.

Cause:

VFDs inherently produce high-frequency leakage currents through cable capacitance and filters. If you treat a VFD-fed motor like a direct-on-line (DOL) circuit, a standard earth-leakage relay will sense the harmless capacitive leakage as a fault and nuisance-trip the motor. In other words, the relay isn’t “VFD-aware” and reacts to normal VFD-induced currents that wouldn’t exist in a DOL setup.

Fix:

• Use VFD-compatible earth-leakage relays or settings. Select relays designed to filter out high-frequency and DC leakage (e.g. Type B or harmonic-filtering EL relays) so that normal VFD leakage doesn’t trigger them. NewElec’s MA series, for instance, is engineered to accommodate VFD-induced leakage without false trips.
• Coordinate EL zones and delays. Implement a zoned earth-leakage scheme with slight time delays so that downstream (motor-level) EL devices trip before the upstream feeder trips the whole panel. A small delay or higher threshold upstream ensures only the affected drive is isolated while the rest of the MCC stays live.
• Improve cable shielding and grounding. For long motor feeds, use properly shielded cables, segregate VFD output cables from control wiring, and ensure solid grounding at multiple points. Good cable practices reduce stray capacitive leakage and electromagnetic interference that can mimic EL faults.

Further reading: NewElec’s MA Relay Deep Dive guide covers VFD leakage considerations, and the Knowledge Hub Q&A discusses practical VFD-EL coordination techniques.

Pitfall 2: Stall/Locked-Rotor Timer Set Too Tight for High-Inertia Loads

Symptom: 

Overload or stall trips occur right when heavy machines (e.g. crushers or long conveyors) start up.

Cause:

High-inertia loads like large conveyors, mills, or crushers naturally take longer to reach full speed, drawing higher currents for longer. An overly short stall timer (the delay before a relay decides a motor is not accelerating and trips) will mistake a normal slow start for a stalled rotor. In effect, the relay trips “early,” thinking the motor is jammed when it’s actually just overcoming the load’s inertia.

Fix:

Base stall time on real data. Set the locked-rotor/stall protection delay using the motor’s datasheet and the load’s characteristics. For example, if the motor’s start-up curve or site testing shows it needs 5 seconds to accelerate a loaded conveyor, don’t use a 3-second stall trip! Tailor the stall threshold and time to the actual measured start profile of the machine.

Use start supervision features. Many advanced relays (e.g. NewElec MA or NewCode) include start supervision logic that differentiates a genuine stall from a slow but successful start. This allows the relay to permit longer start times for heavy belts or crushers, while still tripping quickly if the motor truly stalls and current remains high abnormally long.

Leverage event logs for tuning. After commissioning, review the relay’s start event logs or motor traces to see how long each start actually takes. If you find that normal start-ups are nearing your stall trip limit, adjust the timer or threshold accordingly. Using real site data ensures you’re not flying blind.

Further reading: The MA Relay Technical Guide details start supervision settings and how to adjust stall curves for different duty classes.

Pitfall 3: Underload (Dry-Run/Jam) Settings Copied from Other Sites

Symptom: 

You either get frequent false “dry-run” trips on pumps/fans that are actually fine, or you completely miss true underload conditions (like a pump running dry) because the relay never trips.

Cause: 

Underload or dry-run protection is highly specific, each pump, fan, or conveyor has a unique normal load profile. Simply copying trip settings from a different plant or a generic template means the thresholds likely don’t match your equipment. Without on-site tuning, the relay may trip too early or not at all when needed.

Fix:

• Establish a baseline on site. Run the pump or feeder under normal load and record its stable running current (or power). This baseline is your reference. For example, a certain slurry pump might normally draw 50 A when pumping liquid; if that drops significantly, it could indicate a dry-run. Knowing the normal makes it easier to set an accurate trip point.
• Set a percentage drop and add a delay. Instead of a fixed amperage, configure the underload relay to trip if current falls say 20–30% below the baseline for a sustained few seconds. The delay prevents brief dips (e.g. due to pipeline air or fluctuating feed) from causing nuisance trips. This way, a true loss-of-load (like a broken belt or loss of suction) triggers a trip, but minor fluctuations do not.
• Enable anti-cycling (restart delay) timers. Underload trips often indicate a process issue (empty bin, no flow) that might take time to resolve. If the relay instantly allows an automatic restart, the motor could surge on and off repeatedly. To avoid this, use anti-cycling logic to enforce a minimum off-time or a manual reset after an underload trip. This prevents rapid-fire restarts that can damage equipment and draw inrush currents.

Further reading: Check out the related blog “Pump Jam and Dry-Run Detection in Mining” for in-depth strategies on tuning underload relays in slurry pumps and feeders.

Pitfall 4: Ignoring Upstream Power Quality (and Blaming the Relay)

Symptom: 

The motor relay trips at random times that don’t obviously align with motor conditions, yet upon investigation, those trips coincide with disturbances on the incoming feeder or utility supply.

Cause: 

Poor power quality (PQ) upstream can masquerade as motor faults. Voltage sags, spikes, phase unbalances, or transient dips on the mine’s power bus can trigger a motor relay’s protective functions unnecessarily. For instance, a brief voltage dip might cause a momentary current surge or torque reduction that the relay sees as an overload or under-voltage condition. The relay trips to protect the motor, but the real culprit was the supply, not the motor itself. It’s easy to mistakenly “blame the relay” for being too sensitive, when in fact it reacted to real (but upstream) electrical events.

Fix:

• Monitor feeder power quality. Install power quality monitors or relays with PQ capabilities (like NewElec’s KD Series) on main feeders. These devices log events like voltage dips, frequency deviations, or harmonics. By comparing the timestamp of a nuisance trip with the PQ log, you can confirm if a supply issue triggered the relay. For example, if the KD relay shows a 20% voltage sag at the exact time your MA motor relay tripped, you’ve pinpointed the cause.
• Address the upstream issue first. Before you rush to widen a relay’s trip settings (or disable a protection!), investigate feeder and bus reliability. Check for loose connections, transformer tap settings, or heavy equipment switching that could cause transients. It may be wiser to fix a weak bus or add a stabilizing capacitor bank than to desensitize the motor protection.
• Log and trend PQ over time. Power quality can vary throughout a shift or with seasonal changes in the grid. Gather PQ data over several days or weeks before making major relay setting changes. Consistent logs might reveal, for example, that every day at 3pm a large shovel energizes and causes a dip, valuable insight that helps coordinate scheduling or further mitigation.

Further reading: See the Mining Relay Comparison Matrix (Power Quality column) to identify which protection relays offer built-in PQ monitoring and how they can help distinguish supply issues from motor faults.

Pitfall 5: No Selective Coordination of Earth-Leakage Zones

Symptom: 

A minor earth fault on one machine (say a drill or a conveyor) knocks out an entire MCC section, cutting power to multiple healthy motors. In other words, one leakage trip took down everything upstream.

Cause:

Lack of selective coordination in earth-leakage (EL) protection. If all circuits share a single EL relay or if upstream and downstream EL relays have identical settings, a small ground fault doesn’t stay isolated. The upstream device (feeding many loads) will trip as soon as the leakage exceeds its threshold, even if the fault was on one motor. Without time grading or zoned trip levels, you lose the discrimination that would confine the outage to just the faulted circuit.

Fix:

• Implement zoned, time-graded EL protection. Divide your system into leakage zones – for example, each motor or group of motors gets a local EL relay, and the incomer has a higher-level EL protection set slightly less sensitive or slower. By giving downstream relays a faster trip and upstream relays a short intentional delay (and/or higher threshold), only the downstream device nearest the fault should trip first. This selective tripping ensures the rest of the plant stays powered.
• Differentiate personnel vs. equipment protection. Use appropriate trip settings for safety vs. equipment protection. For instance, an individual machine’s EL relay might be set at 30 mA instant trip to protect personnel from shock, while the upstream feeder EL is set at 500 mA with a 300 ms delay to cover fire protection without usurping the local device. Clearly separate these layers so that nuisance leakage from VFDs or long cables is tolerated at the feeder but cleared at the branch.
• Calibrate using site profiles. When setting the thresholds and delays, consider actual site leakage levels and fault currents. If long cables inherently leak, you might set a slightly higher trip level on those circuits or use adjustable filtering. NewElec’s GA Series earth-leakage relays, for example, are VFD-ready and support selective zone coordination, making them suitable for plant-wide EL schemes.

Further reading: Refer to the GA Series Earth-Leakage Relay application notes for examples of zone coordination in mines, and how to implement time-delay discrimination in practice.

Pitfall 6: Unbalance and Phase-Loss Settings Not Adapted to Mine Conditions

Symptom: 

Motors run hot or give intermittent unbalance alarms, yet no single phase has obviously failed. You might also notice more frequent winding failures or tripped overloads on certain motors fed from long lines or distant substations.

Cause: 

Voltage unbalance or phase loss conditions in mines can be subtle but severe. Long cable runs, deteriorated joints, or uneven single-phase loads (like drills or lighting) can create a few percent voltage imbalance between phases. Even a small voltage asymmetry (e.g. 3–4%) can induce a much larger current imbalance and heating in a motor. If the relay’s unbalance trip is set too wide (or phase-loss not detected until very low voltage), motors might slog along in unhealthy conditions. Conversely, overly tight settings without considering normal mine voltage fluctuations can cause nuisance trips. It’s a balance often not tuned to actual mine power conditions.

Fix:

• Tighten thresholds based on real data. Measure the typical voltage balance at your motor control center (MCC) during normal operation. If you find, say, only ~1% imbalance normally, you can set the unbalance trip maybe at 4–5%. But if your site often sees 2–3% swings, set the relay a bit higher (e.g. 6–8%) to avoid false trips while still catching significant deviations. Aim to keep voltage imbalance under ~2% for critical motors, and adjust relay settings accordingly.
• Correlate with PQ logs and events. Use your PQ monitor or relay event records to see if unbalance alarms correlate with any external events (like a heavy shovel starting on one phase or a capacitor bank switching). If unbalance trips happen concurrent with other known issues, it might not be the relay’s threshold at fault but an upstream distribution problem to fix. Conversely, if a specific motor frequently shows unbalance, inspect that circuit.
• Inspect connections and components. Many unbalance issues trace back to high-resistance connections, partial phase failures, or a blown fuse in one phase of a capacitor bank. Regularly thermoscan and tighten terminations, check fuse continuity on all three phases of starters, and ensure backup generators or transformer taps are configured correctly. The relay can only alert you – it’s on the engineers to find and remedy the physical cause.

(Reminder: Phase-loss protection is equally critical – ensure the relay trips promptly (without intentional delay) on a completely lost phase to save the motor.)

Pitfall 7: Forgetting Differences in Start Method (DOL vs. Soft Starter vs. VFD Control)

Symptom: 

The protection settings work perfectly for motors started direct-on-line (DOL), but those same settings behave poorly under soft starters or VFDs, perhaps tripping too early or not protecting enough. For example, a relay that never trips on a DOL pump might nuisance-trip on the identical pump when a VFD is used, or vice versa.

Cause: 

Different motor starting methods dramatically change the motor’s electrical profile. A DOL start has a very high inrush current and a rapid ramp-up. A soft starter limits inrush and lengthens the acceleration time. A VFD can do a slow, current-limited ramp and even vary the speed during operation. These differences mean the relay’s settings must account for the control method. If you “set and forget” assuming all starts are alike, you’ll likely mis-tune features like start delays, thermal curves, and stall detection. One size does not fit all here.

Fix:

• Use separate profiles or setting groups. If your relay supports it, maintain distinct setting files or profiles for each start method. For instance, a “DOL profile” might have a shorter start timer and more aggressive stall trip, whereas a “VFD profile” uses a longer start delay and perhaps disables instantaneous overcurrent during ramp-up. Some advanced relays allow on-the-fly switching of profiles or have adaptive algorithms for this. NewElec’s relays are designed for stable operation with VSDs, soft starters, and DOL alike, but you still need to configure them appropriately for each case.
• Validate with real start data. After installing a new soft starter or VFD, capture the motor’s start waveform (many relays can record the current vs. time during start). Use this to adjust the thermal model and stall settings. For example, a crusher on a soft starter might take 15 seconds to reach speed with 300% FLA current. Your relay should be set to accommodate that, while still tripping if it takes, say, 30 seconds (indicating a problem).
• Leverage adaptive features. Some modern motor protection units (like the MA Series with BBRTU/Ethernet) offer adaptive control profiles that automatically modify protection settings based on detected starting mode or external inputs. Where available, use these features for multi-drive setups to avoid manual reconfiguration errors. At minimum, document on the relay which start method each motor uses and review the protective settings any time a control method is changed.

Further reading: The Relay Comparison Table on NewElec’s site has a “Control Compatibility” column showing which relays handle VFD/soft-starter applications best.

Pitfall 8: Missing Anti-Cycling or Permissive Logic

Symptom: 

Certain motors experience frequent stop-start cycles in short succession, leading to excessive mechanical wear or overheating. For instance, a conveyor might trip on overload, get immediately reset, start again, trip again or an operator might rapidly toggle a feeder trying to clear a jam. In extreme cases, the motor never gets a chance to cool and ancillary equipment (gearboxes, couplings) take a beating.

 

Cause: 

Lack of anti-cycling control and permissive interlocks. This often happens if control logic is left entirely to a PLC or operators, without using the relay’s built-in capabilities. A relay or starter configured with no minimum off-time or start inhibit will allow restarts as soon as the start command is re-given. Similarly, if there are no permissive signals (e.g. “don’t start this conveyor unless the downstream conveyor is running”), motors can be started under unfavorable conditions and trip again. Relying solely on external PLC logic for all these protections can be risky communications delays or programming oversights may not react as fast as a relay’s internal logic.

Fix:

 

• Use the relay’s built-in logic blocks. Take advantage of programmable logic in modern relays (for example, the NewCode relay offers configurable logic gates and timers). Implement start permissives at the relay level – e.g. wire an input from a downstream device or a bin level sensor, so the relay itself blocks a start if conditions aren’t right. This provides a second layer of protection independent of the PLC.
• Enforce anti-cycling delays. Configure a minimum off-time after each trip or stop command. If a motor stops, the relay’s logic can impose (say) a 30-second delay before allowing a restart, even if an operator or PLC tries sooner. This gives time for mechanical stress relief and prevents rapid heating. Likewise, set a max starts-per-hour limit if the relay supports it.
• Add fault-specific lockouts. For sensitive scenarios, use a “restart inhibit” timer after certain faults. For example, after an overload trip, you might require a manual reset or a longer delay, ensuring someone checks the equipment before restarting. NewElec’s NewCode relay proved this concept at a Mpumalanga coal plant by using internal logic for anti-cycling and permissive interlocks, greatly reducing nuisance restarts.

By embedding these controls at the relay level, you create a faster and more foolproof safety net that complements the PLC logic.

Pitfall 9: Retrofit Oversights (CT/VT Ratios, Space Constraints, Wiring)

Symptom: 

After retrofitting a new relay into an old Motor Control Center (MCC), readings don’t match actual currents, or the relay runs hot / the panel layout is messy. You might find that the motor’s measured current is off by a factor (e.g. reading 5 A instead of actual 50 A), or the cabinet doors barely close because the new device and its wiring loom are crammed in.

 

Cause: 

Retrofitting new relays into legacy MCCs can introduce issues if details are overlooked. Common pitfalls include: using existing Current Transformers (CTs) or Voltage Transformers (VTs) with incompatible ratios for the new relay, mis-wiring CT polarities, not accounting for the larger size or different form-factor of the modern relay (leading to tight fits and poor ventilation), and improper cable shielding or routing in the new scheme. Each of these oversights can affect performance or even cause the relay to behave incorrectly.

Fix:

• Verify CT/VT details before commissioning. Always check that the CT secondary ratio matches the relay’s expected input (e.g. 500:1 CT feeding a relay set for 5 A input). If the relay displays primary current, ensure its scaling is set properly (or use the correct plug-in module). Also double-check CT polarity markings and wiring, reversed CT connections can make measured currents subtract or show as zero. It’s good practice to perform a primary injection test or a simple load test to confirm readings align with reality before full startup.
• Mind the space and thermal requirements. Modern intelligent relays often need more panel depth or width than old electromechanical ones. Use available mounting adapters or kits to achieve a neat fit, and avoid sandwiching heat-producing devices too tightly. Provide adequate ventilation or spacing as recommended in the relay manual. For instance, if replacing a simple overload relay with a larger multi-function unit, you might need to redistribute other components or use extension wires to mount the relay where there is space. The goal is a clean installation that dissipates heat and is serviceable.
• Ensure proper shielding and wiring practices. In retrofit scenarios, you might be adding communications wiring (Ethernet/serial cables) or connecting to existing core-balance CTs, etc. Follow the spec for cable types (shielded vs unshielded, twisted pair for comms, etc.) and segregate low-level signal wires from high-power cables to avoid noise coupling. Ground shields at a single point as directed. Essentially, bring the wiring up to modern standards along with the relay to guarantee it operates as intended.

Further reading: The Mining Relay Comparison Matrix includes a “Mounting & Retrofit” column that notes physical sizing and retrofit-friendly features of each relay (like NewElec’s LA/MA Series which have integrated CTs for easy drop-in).

Pitfall 10: No Commissioning Record (and No Trending Over Time)

Symptom: 

Six months after startup, the motor protection settings have been tweaked multiple times and no one is quite sure what’s in the relay now. When a problem arises, there’s confusion, “Did we disable that alarm last outage? What was the original trip setpoint?” Inconsistent record-keeping means over time the relay’s configuration may drift or critical protections might be turned down without a clear history. This lack of a “single source of truth” leads to guesswork during troubleshooting and can compromise safety.

 

Cause: 

Lack of documentation and trending. Busy maintenance schedules and turnover can result in changes not being written down. Without saving setting files or logging events, the tribal knowledge gets lost. Additionally, if you’re not regularly reviewing event logs or trend data, you might miss early warning signs. Over years, a relay’s trip history (e.g. increasing frequency of over-temperature alarms) is invaluable for predictive maintenance – but if nobody enables logging or exports the data, that insight is wasted.

Fix:

• Capture settings “as-found” and “as-left.” Every time you commission or modify a relay, save a copy of the settings (most digital relays allow you to export to a file or printout). Label it with date and motor ID. This provides a baseline to revert to if needed and a reference to compare against during audits.
• Maintain a change log. Even a simple logbook or spreadsheet where you note any setting changes (date, who, what changed, why) can hugely improve clarity. It helps new engineers understand the rationale and ensures accountability.
• Enable event trending and export logs. Make use of the relay’s memory: for example, NewElec’s KD and NewCode relays can log thousands of events and faults. Download these logs periodically and review them. Trends like a steady rise in motor running current or repeated under-voltage alarms will jump out, allowing proactive fixes. Some sites even integrate relay data into a SCADA historian or condition monitoring system – if available, definitely do this to leverage the full predictive potential. As a rule, don’t rely on memory alone – rely on data.

Lastly, perform periodic “sanity checks” on each protection relay, a quick audit to ensure settings are as expected (compare to your last saved file, etc.) and test key functions. This practice catches any inadvertent changes or malfunctions before they cause an incident.

Quick Commissioning & Sanity Checklist

Keep this checklist handy for fast verification before a motor start or after any relay setting change:

• CT/VT configuration: Verify all CT and VT connections. Correct polarity and ratio settings so the relay measures true values.
• Baseline readings: Record the baseline full-load current and voltage for each motor once running normally. This provides a reference for underload or unbalance settings.
• Earth-leakage coordination: Double-check that earth-leakage trip levels and delays are properly graded (downstream vs upstream) to ensure only the intended device will trip on a fault.
• Start and stall protection: Test the start supervision and stall timer by observing an actual start. Confirm the relay allows the motor to reach speed and trips if a genuine stall occurs.
• Unbalance and PQ settings: Confirm phase-unbalance trip settings are in line with actual site voltage conditions, and that any power quality monitor upstream is active and logging.
• Final settings backup: After commissioning, export the final relay settings file or write them down. Also ensure event logging is enabled and time-synchronized. (This will aid future troubleshooting.)