Wednesday, 15 October 2025
Line Reactors vs. Transient Filters: Understanding Their Roles in VFD Systems
Variable Frequency Drives (VFDs) have become an essential part of modern power and ...
Reactors & Filters
Motor Guarding Transient Filters
Rex Power Magnetics Motor Guarding Transient Filters are engineered to protect motors and cables from voltage spikes, steep dv/dt waveforms, and reflected wave phenomena generated by variable frequency drives (VFDs). Installed between the drive and the motor, these filters reduce voltage stress, mitigate insulation damage, and improve overall motor reliability and longevity.
Each filter employs a carefully tuned combination of inductive and resistive elements to effectively limit high-frequency transients while maintaining low losses and minimal voltage drop. This R-L design provides superior damping of voltage overshoot and reduces electromagnetic interference (EMI) without affecting drive performance or motor efficiency.
Rugged dry-type construction, precision winding, and robust insulation systems ensure dependable operation in harsh industrial environments. Available in a wide range of voltage classes, current ratings, and enclosure options, every Rex Motor Guarding Transient Filter is custom-engineered to meet the electrical and mechanical requirements of the drive system — delivering proven protection and long-term system performance.
| Capacity | 2A to 320 A Three Phase |
| Voltage Class | 600V Max |
| Cooling | Air Cooled (ANN / AN) |
| Frequency | 60Hz (50/60Hz Optional) |
| Conductors | Copper (Cu) Windings |
| Insulation System | 180°C Insulation System (2 – 11A) (115 °C Temp Rise Max) 220°C Insulation System (17 – 320A) (150 °C Temp Rise Max) |
| Sound Level | Below 40dBA |
| Overload Capability | 150% of rated current for 1 min. |
| Saturation Level | Not less than 250% of rated current (min 50% of rated inductance at 350% rated current) |
| Enclosure Type | Type 1 Indoor |
| Warranty | 12 Months (See Warranty and Limitations) |
| Quality System | ISO 9001:2015 Quality Management System |
| Certifications | CSA Certified UL Listed |
| Reference Standards | IEEE C57.110 NEMA ST-20 |
| Open Motor Guarding Filter Reactors | |||
| Current Rating [A] |
Part No. | Dimensions Open Core & Coil A/B/C/D/E |
Weight (Lbs) |
| Open | |||
| 2 | 2C5LF | 8.25/6.50/7.00/7.25/2.00 | 5 |
| 4 | 4C5LF | 8.25/6.50/7.00/7.25/2.00 | 6 |
| 8 | 8C5LF | 8.25/6.50/7.00/7.25/2.00 | 9 |
| 11 | 11C5LF | 8.25/6.50/7.00/7.25/2.00 | 10 |
| 17 | 17C5LF | 8.25/6.50/7.00/7.25/2.00 | 11 |
| 27 | 27C5LF | 8.25/7.20/7.00/7.25/2.00 | 19 |
| 34 | 34C5LF | 8.25/7.20/7.00/7.25/2.00 | 25 |
| 45 | 45C5LF | 11.00/9.00/8.10/10.00/4.00 | 29 |
| 60 | 60C5LF | 11.00/9.00/8.10/10.00/4.00 | 30 |
| 80 | 80C5LF | 11.00/12.00/6.80/10.00/4.00 | 42 |
| 100 | 100C5LF | 11.00/12.00/6.80/10.00/4.00 | 47 |
| 130 | 130C5LF | 11.00/12.00/6.80/10.00/4.00 | 48 |
| 160 | 160C5LF | 11.00/13.00/8.20/10.00/4.00 | 49 |
| 200 | 200C5LF | 11.00/13.00/8.20/10.00/4.00 | 68 |
| 255 | 255C5LF | 11.00/13.00/8.20/10.00/4.00 | 84 |
| 320 | 320C5LF | 11.00/13.00/8.20/10.00/4.00 | 90 |
| 410 | 410C5LF | 14.00/14.50/14.00/13.00/6.00 | 95 |
| 500 | 500C5LF | 20.00/16.00/18.00/19.00/8.00 | 140 |
| 600 | 600C5LF | 20.00/17.50/18.00/19.00/8.00 | 148 |
Rex filters are current rated, therefore easy to specify. Simply use the total motor load current to select the appropriate filter rating. Filters are available enclosed or open style. Add the suffix /E at the end of the part number to denote enclosed style.
| Enclosed Motor Guarding Filter Reactors | ||||||
| Current Rating [A] |
Part No. | Enclosure Size | Enclosure Dimensions | Weight (Lbs) | ||
| length | Depth | Height | Enclosed | |||
| 2 | 2C5LF/E | E1-0 | 9.50 | 7.00 | 8.00 | 14 |
| 4 | 4C5LF/E | E1-0 | 9.50 | 7.00 | 8.00 | 15 |
| 8 | 8C5LF/E | E1-0 | 9.50 | 7.00 | 8.00 | 19 |
| 11 | 11C5LF/E | E1-0 | 9.50 | 7.00 | 8.00 | 20 |
| 17 | 17C5LF/E | E1-1 | 12.00 | 9.00 | 9.50 | 23 |
| 27 | 27C5LFE | E1-1 | 12.00 | 9.00 | 9.50 | 32 |
| 34 | 34C5LF/E | E1-1 | 12.00 | 9.00 | 9.50 | 37 |
| 45 | 45C5LF/E | E1-3 | 15.10 | 11.00 | 14.00 | 48 |
| 60 | 60C5LF/E | E1-3 | 15.10 | 11.00 | 14.00 | 50 |
| 80 | 80C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 68 |
| 100 | 100C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 78 |
| 130 | 130C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 79 |
| 160 | 160C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 88 |
| 200 | 200C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 99 |
| 255 | 255C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 115 |
| 320 | 320C5LF/E | E3R-6 | 20.50 | 20.75 | 26.50 | 150 |
| 410 | 410C5LF/E | E3R-6 | 20.50 | 20.75 | 26.50 | 158 |
| 500 | 500C5LF/E | E3R-6 | 20.50 | 20.75 | 26.50 | 200 |
| 600 | 600C5LF/E | E3R-6 | 20.50 | 20.75 | 26.50 | 210 |
Note: All Rex enclosures larger than #3 are standard NEMA 3R sprinkler proof. Boxes #7 and smaller can be floor or wall mounted. Consult our engineering department for the most up-to-date information on enclosures. See standard enclosures page for detailed drawings.
Product Catalogs
Line Reactor Catalog (English)
Instruction Sheets & Manuals
Rex Power Magnetics Motor Guarding Transient Filters are engineered to protect motors and cables from voltage spikes, steep dv/dt waveforms, and reflected wave phenomena generated by variable frequency drives (VFDs). Installed between the drive and the motor, these filters reduce voltage stress, mitigate insulation damage, and improve overall motor reliability and longevity.
Each filter employs a carefully tuned combination of inductive and resistive elements to effectively limit high-frequency transients while maintaining low losses and minimal voltage drop. This R-L design provides superior damping of voltage overshoot and reduces electromagnetic interference (EMI) without affecting drive performance or motor efficiency.
Rugged dry-type construction, precision winding, and robust insulation systems ensure dependable operation in harsh industrial environments. Available in a wide range of voltage classes, current ratings, and enclosure options, every Rex Motor Guarding Transient Filter is custom-engineered to meet the electrical and mechanical requirements of the drive system — delivering proven protection and long-term system performance.
| Capacity | 2A to 320 A Three Phase |
| Voltage Class | 600V Max |
| Cooling | Air Cooled (ANN / AN) |
| Frequency | 60Hz (50/60Hz Optional) |
| Conductors | Copper (Cu) Windings |
| Insulation System | 180°C Insulation System (2 – 11A) (115 °C Temp Rise Max) 220°C Insulation System (17 – 320A) (150 °C Temp Rise Max) |
| Sound Level | Below 40dBA |
| Overload Capability | 150% of rated current for 1 min. |
| Saturation Level | Not less than 250% of rated current (min 50% of rated inductance at 350% rated current) |
| Enclosure Type | Type 1 Indoor |
| Warranty | 12 Months (See Warranty and Limitations) |
| Quality System | ISO 9001:2015 Quality Management System |
| Certifications | CSA Certified UL Listed |
| Reference Standards | IEEE C57.110 NEMA ST-20 |
| Open Motor Guarding Filter Reactors | |||
| Current Rating [A] |
Part No. | Dimensions Open Core & Coil A/B/C/D/E |
Weight (Lbs) |
| Open | |||
| 2 | 2C5LF | 8.25/6.50/7.00/7.25/2.00 | 5 |
| 4 | 4C5LF | 8.25/6.50/7.00/7.25/2.00 | 6 |
| 8 | 8C5LF | 8.25/6.50/7.00/7.25/2.00 | 9 |
| 11 | 11C5LF | 8.25/6.50/7.00/7.25/2.00 | 10 |
| 17 | 17C5LF | 8.25/6.50/7.00/7.25/2.00 | 11 |
| 27 | 27C5LF | 8.25/7.20/7.00/7.25/2.00 | 19 |
| 34 | 34C5LF | 8.25/7.20/7.00/7.25/2.00 | 25 |
| 45 | 45C5LF | 11.00/9.00/8.10/10.00/4.00 | 29 |
| 60 | 60C5LF | 11.00/9.00/8.10/10.00/4.00 | 30 |
| 80 | 80C5LF | 11.00/12.00/6.80/10.00/4.00 | 42 |
| 100 | 100C5LF | 11.00/12.00/6.80/10.00/4.00 | 47 |
| 130 | 130C5LF | 11.00/12.00/6.80/10.00/4.00 | 48 |
| 160 | 160C5LF | 11.00/13.00/8.20/10.00/4.00 | 49 |
| 200 | 200C5LF | 11.00/13.00/8.20/10.00/4.00 | 68 |
| 255 | 255C5LF | 11.00/13.00/8.20/10.00/4.00 | 84 |
| 320 | 320C5LF | 11.00/13.00/8.20/10.00/4.00 | 90 |
| 410 | 410C5LF | 14.00/14.50/14.00/13.00/6.00 | 95 |
| 500 | 500C5LF | 20.00/16.00/18.00/19.00/8.00 | 140 |
| 600 | 600C5LF | 20.00/17.50/18.00/19.00/8.00 | 148 |
Rex filters are current rated, therefore easy to specify. Simply use the total motor load current to select the appropriate filter rating. Filters are available enclosed or open style. Add the suffix /E at the end of the part number to denote enclosed style.
| Enclosed Motor Guarding Filter Reactors | ||||||
| Current Rating [A] |
Part No. | Enclosure Size | Enclosure Dimensions | Weight (Lbs) | ||
| length | Depth | Height | Enclosed | |||
| 2 | 2C5LF/E | E1-0 | 9.50 | 7.00 | 8.00 | 14 |
| 4 | 4C5LF/E | E1-0 | 9.50 | 7.00 | 8.00 | 15 |
| 8 | 8C5LF/E | E1-0 | 9.50 | 7.00 | 8.00 | 19 |
| 11 | 11C5LF/E | E1-0 | 9.50 | 7.00 | 8.00 | 20 |
| 17 | 17C5LF/E | E1-1 | 12.00 | 9.00 | 9.50 | 23 |
| 27 | 27C5LFE | E1-1 | 12.00 | 9.00 | 9.50 | 32 |
| 34 | 34C5LF/E | E1-1 | 12.00 | 9.00 | 9.50 | 37 |
| 45 | 45C5LF/E | E1-3 | 15.10 | 11.00 | 14.00 | 48 |
| 60 | 60C5LF/E | E1-3 | 15.10 | 11.00 | 14.00 | 50 |
| 80 | 80C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 68 |
| 100 | 100C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 78 |
| 130 | 130C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 79 |
| 160 | 160C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 88 |
| 200 | 200C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 99 |
| 255 | 255C5LF/E | E3R-4 | 15.75 | 16.00 | 21.00 | 115 |
| 320 | 320C5LF/E | E3R-6 | 20.50 | 20.75 | 26.50 | 150 |
| 410 | 410C5LF/E | E3R-6 | 20.50 | 20.75 | 26.50 | 158 |
| 500 | 500C5LF/E | E3R-6 | 20.50 | 20.75 | 26.50 | 200 |
| 600 | 600C5LF/E | E3R-6 | 20.50 | 20.75 | 26.50 | 210 |
Note: All Rex enclosures larger than #3 are standard NEMA 3R sprinkler proof. Boxes #7 and smaller can be floor or wall mounted. Consult our engineering department for the most up-to-date information on enclosures. See standard enclosures page for detailed drawings.
Product Catalogs
Line Reactor Catalog (English)
Instruction Sheets & Manuals
Faqs
- General FAQs
- Technical Specification FAQs
- Installation & Application FAQs
- Performance & Product Selection FAQs
- Protection, Troubleshooting & Ordering FAQs
A motor guarding transient filter is an R-L network that protects motors and drives from voltage transients, reflected waves, and rapid voltage rise (dV/dt) on the output of variable frequency drives (VFDs). It combines a reactor (inductive element) with damping resistors to smooth voltage pulses and absorb switching energy. Rex Power Magnetics motor guarding transient filters are engineered to protect motor insulation, minimize cable stress, and extend equipment life in demanding industrial drive applications.
The reactor in the filter slows the rate of voltage change (dV/dt), while the resistors dissipate transient energy and damp high-frequency oscillations that can develop between the drive and motor. The result is a smoother, more sinusoidal voltage waveform at the motor terminals with reduced overshoot. Rex Power Magnetics designs each filter to provide optimal impedance characteristics for the drive’s switching frequency, motor cable length, and load conditions, ensuring balanced performance and low losses.
Modern PWM drives generate steep, high-frequency voltage pulses that can cause reflected wave transients along motor cables. These reflections can produce peak voltages up to twice the DC bus level, stressing motor insulation and bearings. R-L filters reduce dV/dt by limiting current surge through the reactor and dissipating transient energy in the resistors. This combination protects both the drive and motor, ensuring compliance with motor insulation voltage limits and preventing premature failure.
A Rex Motor Guarding Transient Filter uses a combination of inductive and resistive elements to reduce voltage spikes and high-frequency oscillations generated by VFD switching.
The reactor slows the rate of voltage rise, while the resistors dampen electrical oscillations and reduce reflected-wave overvoltage at the motor terminals.
This helps protect motor winding insulation, reduce electrical stress on motor cables, and improve reliability, especially in installations with long cable runs between the VFD and motor.
Both devices help protect VFD-driven motors from rapid voltage changes and insulation stress.
A conventional dV/dt filter primarily slows the rise of voltage pulses reaching the motor. Rex Motor Guarding Transient Filters use a tuned resistor-inductor (R-L) design that also dampens high-frequency oscillations and reflected-wave voltage spikes.
The appropriate filter depends on motor cable length, drive switching characteristics, motor insulation requirements, and the level of transient protection needed.
Yes. Repeated voltage spikes from VFD switching can gradually damage motor winding insulation, increasing the risk of insulation breakdown and unexpected motor failure.
A properly selected Motor Guarding Transient Filter reduces voltage overshoot and electrical stress, helping protect motor windings and extend equipment service life.
This can reduce unplanned downtime, maintenance expenses, and costly motor replacements, particularly in facilities where motors operate continuously or are connected to VFDs through long cables.
A Motor Guarding Transient Filter is a three-phase iron-core reactor built on the same 5% class magnetic design as a Rex load reactor, with a damping resistor network added to each phase. The reactor is wound with copper conductor on a gapped laminated-steel core so that inductance stays linear well beyond rated current, and the assembly is resin-dipped for mechanical rigidity and moisture resistance. The resistors are mounted on the same frame and are sized to absorb the energy of each switching edge without limiting fundamental current.
The two elements do different jobs. The inductance slows the rate of voltage rise leaving the drive, which stretches the pulse rise time. The resistance damps the resonance formed between that inductance and the distributed capacitance of the motor cable, so the reflected wave decays within a cycle or two instead of ringing. Rex describes this as a “carefully tuned combination of inductive and resistive elements”, and the tuning is set at the design stage for the current rating, switching frequency range and cable lengths typical of that size. The Knowledge Hub article on line reactors versus transient filters shows the arrangement diagrammatically, and the Line Reactor Catalog gives the outline drawings.
Your motor may need a transient filter if it operates with a variable frequency drive (VFD) and experiences repeated insulation failures, unexpected shutdowns, or excessive voltage spikes.
Long motor cables, high VFD switching frequencies, and older motor insulation can increase electrical stress. Measuring voltage at the motor terminals can help determine whether transient protection is necessary.
A properly selected transient filter helps reduce damaging voltage spikes, protect motor windings, and minimize the risk of premature failure and costly downtime.
For more information on protection options, see Rex’s Motor Guarding Transient Filters and Line Reactors vs. Transient Filters .
Yes. Variable frequency drives generate fast-switching voltage pulses that can produce high-frequency transients and reflected-wave overvoltage, particularly in installations with long motor cables.
A Rex Motor Guarding Transient Filter combines inductive and resistive elements to reduce voltage spikes and dampen electrical oscillations before they reach the motor. This helps protect winding insulation, reduce electrical stress, and minimize the risk of premature motor failure.
Proper filter selection depends on the VFD’s switching characteristics, motor cable length, and insulation requirements.
Long motor cables increase the risk of reflected-wave voltage spikes in VFD-driven systems. When fast-rising voltage pulses reach the motor terminals, differences in electrical impedance can cause voltage reflections that increase the peak voltage experienced by the motor.
These repeated spikes can damage winding insulation and contribute to premature motor failure.
A Rex Motor Guarding Transient Filter helps reduce voltage overshoot and dampen reflected-wave oscillations, making it particularly useful for installations with long cable runs.
The need for a filter depends on cable length, VFD switching characteristics, and motor insulation ratings. For additional guidance, read Rex’s Line Reactors vs. Transient Filters .
Yes. Older motors may have insulation systems that were not designed to withstand the rapid voltage pulses generated by modern VFDs.
Repeated exposure to voltage spikes can gradually weaken winding insulation, increasing the risk of electrical breakdown and unexpected motor failure.
A Rex Motor Guarding Transient Filter reduces voltage overshoot and high-frequency oscillations, helping protect motor windings and extend equipment service life.
This is particularly beneficial when upgrading an existing motor installation to VFD operation. However, the motor’s insulation condition and compatibility with the drive should also be assessed before installation.
A Motor Guarding Transient Filter primarily protects motor windings from voltage spikes, reflected-wave overvoltage, and high-frequency electrical stress generated by VFD switching.
By reducing these transients, a properly selected filter helps protect insulation and may reduce associated electrical heating and winding damage.
However, it does not eliminate overheating caused by excessive mechanical loads, inadequate cooling, incorrect VFD settings, or other operating problems.
If your motor is overheating, check its operating current, cooling conditions, and drive configuration. Rex’s Motor Guarding Transient Filters can provide additional protection against switching-related insulation stress, while proper motor sizing and thermal protection remain essential.
Rex rates the Motor Guarding Transient Filter at below 40 dBA, the same figure as the 3PR reactor, and that is the level to expect from the core at fundamental frequency. In service, however, an output filter carries the drive’s PWM waveform rather than a sinusoidal supply, so the magnetic forces in the core and windings contain components at the carrier frequency and its harmonics as well as at twice the fundamental. A load-side filter can therefore emit a faint tone at the carrier frequency, most noticeable at carrier settings of 2 kHz to 5 kHz where human hearing is sensitive, and audible in a quiet mechanical room even when the dBA figure is met.
R-L transient filters are used in systems where VFDs drive motors over long cables or in installations with sensitive insulation systems. They are common in HVAC equipment, pumping stations, conveyors, compressors, and mining or water treatment facilities. They are also used when multiple motors are powered by a single drive or where the installation environment introduces significant switching noise. Rex Power Magnetics filters ensure reliable operation under diverse industrial load conditions.
Yes. Unlike a plain reactor, which is electrically symmetrical, the Motor Guarding Transient Filter has a defined drive side and motor side because the damping network is placed relative to the inductance to control the reflection returning from the motor. Installing it reversed will not damage the filter and the motor will run, but the damping will not be matched to the circuit and overshoot at the motor terminals will be higher than the design value.
By limiting voltage spikes and smoothing pulse edges, R-L filters drastically reduce dV/dt stress and prevent overvoltage conditions at motor terminals. This protects winding insulation, reduces partial discharge, and eliminates the bearing currents that can occur with fast-switching drives. The improved waveform lowers motor heating and torque pulsations, resulting in quieter operation, longer motor life, and fewer drive trips due to transient interference.
Rex’s guidance is to select the Motor Guarding Transient Filter on the total motor load current, meaning the current the motor will actually draw continuously. For a motor never loaded beyond nameplate, that is the nameplate full-load current. For a motor whose 1.15 service factor will be used, or one that runs in field weakening where current rises for a given torque, use the service-factor or expected continuous current, whichever is higher. The drive’s rated output current is not the right basis, because drives are commonly a frame size or two larger than the motor and a filter sized to the drive would be heavier and have proportionally lower inductance than the motor needs.
No. A Drive Isolation Transformer or input line reactor addresses supply-side issues but does not reduce inverter output voltage spikes or reflected waves.
A Motor Guarding Transient Filter protects the motor from output-side transients, particularly with long motor cables. Whether both input and output protection are needed depends on supply conditions, cable length, and motor specifications
Primarily it protects insulation, but there are secondary performance benefits. The steep edges and ringing on an unfiltered motor cable contain energy at frequencies far above the fundamental. Some of that energy is dissipated in the motor as additional iron and stray losses, and the ringing current adds to bearing discharge activity. By removing most of the overshoot and the ringing, the Motor Guarding Transient Filter shifts that dissipation from the motor to its own damping resistors, where it is designed to occur. The result is a slightly cooler motor at the same load, which is meaningful for totally enclosed motors at the top of their thermal rating or for motors running at reduced speed on a shaft-mounted fan.
No separate overcurrent device is required for the filter itself. The Motor Guarding Transient Filter is a series component in the drive’s output circuit, and that circuit is protected by the drive’s own electronic current limit and fault detection, which respond far faster than any fuse. Under the Canadian Electrical Code and the NEC the motor branch circuit’s short-circuit and ground-fault protection sits ahead of the drive, and the motor overload protection is provided by the drive’s electronic thermal model or by a separate overload relay; the filter sits inside that protected zone in the same way that the motor cable does.
Trips immediately after commissioning a Motor Guarding Transient Filter almost always come from the installation rather than the filter. Work through the following in order:
• Phase continuity and sequence. Confirm each phase passes through the filter to the corresponding motor terminal with none open or crossed; an open phase produces a single-phasing trip.
• Drive side versus motor side. Check the terminal designations; a reversed filter runs with degraded damping and, on a marginal cable, can raise ringing current enough to trigger an output fault.
• Cable shield and ground path. A ground-fault trip usually means the shield is terminated to the filter enclosure only or left floating at one end, so common-mode current returns through an unintended path. Bond at the drive and the motor.
• Motor autotune. Re-run the stationary autotune with the filter in circuit so the drive’s motor model includes its inductance and resistance; a model tuned without the filter can trip on acceleration in vector modes.
• Carrier frequency. At very high carrier settings the resistor current at each edge can register as excess output current on some drives; try the manufacturer’s default.
If the reactor or resistors show heat damage after a trip, take the unit out of service and contact Rex. The article on commissioning checks gives a checklist that adapts well to an output filter.
Warm resistors are normal; resistors that are discolouring, that trip a fitted thermostat, or whose temperature has risen at constant load are not. The resistor duty on a Motor Guarding Transient Filter depends on carrier frequency, cable length and capacitance, and DC bus voltage, so the first questions are whether any of those has changed since the filter was selected.
• Carrier frequency raised. Doubling the carrier frequency roughly doubles resistor loss. Compare the drive’s setting with the value the filter was ordered against.
• Cable extended or changed. A longer run, a higher-capacitance cable, or cable now lying in water all raise the energy per edge.
• Supply voltage higher. A drive moved from 480 V to 600 V, or a supply well above nominal, raises resistor loss with the square of bus voltage.
• Ventilation restricted. Clogged panel filter mats or obstructed enclosure louvres starve the unit of cooling air; check the ambient inside the enclosure against the rise rating.
• Reversed connection. A reversed filter can force the resistors to handle more ringing energy than the design assumes.
If none of these applies and the reactor element is also hot, measure the fundamental current against the rating; an overloaded reactor and hot resistors together indicate the filter is undersized. Rex’s guide to maintenance and inspection includes the infrared survey technique that best catches resistor problems early.