R.C. Snubbers

A Rex R.C. Snubber draws a small continuous capacitive charging current, with reactive power depending on system voltage, frequency, and capacitance.

At 60 Hz, typical examples include:

• 4.16 kV, 0.5 µF: 0.45 A per phase and 3.3 kVAR.
• 13.8 kV, 0.25 µF: 0.75 A per phase and 18 kVAR.
• 34.5 kV, 0.13 µF: 0.98 A per phase and 58 kVAR.

Continuous resistor losses are generally low. Proper capacitor sizing helps minimize reactive power demand while maintaining effective switching transient protection.

The capacitor absorbs high-frequency voltage transients and reduces the rate of voltage rise, while the resistor dampens oscillations and dissipates transient energy. Properly selected values help protect transformer insulation without introducing excessive current, losses, or unwanted resonance.

An R.C. Snubber does not directly change transformer efficiency, but it draws a small continuous capacitive current and consumes some power through its resistor. Operating costs are generally low when the snubber is correctly sized for the electrical system.

Yes. Vacuum circuit breakers can generate high-frequency switching transients that place excessive voltage stress on transformer insulation. A properly designed Rex R.C. Snubber helps dampen these oscillations and reduce the rate of voltage rise, protecting transformer windings and connected equipment.

An R.C. Snubber can help dampen high-frequency oscillations associated with lightning-induced transients, but it is not a replacement for a surge arrester. For comprehensive protection against lightning surges, an R.C. Snubber should be used alongside appropriately rated surge arresters.

Ideally, an RC snubber should be installed as close as possible to the transformer it is designed to protect. For new transformers, if space allows, the snubber can be integrated directly within the transformer enclosure, ensuring a minimal cable length between the snubber and the transformer. In retrofit applications or when a standalone snubber is required, careful attention should be given to minimizing the cable length between the transformer and the snubber to ensure optimal protection.

Since power systems are dynamic and each setup varies, there is no universal recommendation for the maximum acceptable cable length before the snubber’s effectiveness is compromised. A thorough analysis, including Transient Recovery Voltage (TRV) studies, is necessary to assess system-specific parameters and determine how the cable length between the RC snubber and the transformer will affect its performance.

A Rex R.C. Snubber should be installed as close as possible to the transformer terminals to effectively dampen switching transients and protect winding insulation.

Installing it in a switchgear cubicle may reduce its effectiveness due to cable length and voltage reflections. If transformer-side installation is not possible, Rex can evaluate the arrangement through a transient recovery voltage (TRV) study to determine suitable placement.

The ground path of a snubber is part of the damping circuit, so it must have low impedance at high frequency, not just at 60 Hz. A round conductor has roughly 1 µH of inductance per metre; at 100 kHz that is about 0.63 ohm per metre and at 1 MHz about 6.3 ohms per metre. Since the snubber capacitor itself presents only about 6 ohms at 100 kHz, a 5 m ground lead can add as much impedance as the capacitor and weaken the damping.

Rex therefore recommends:

• Bond the R.C. Snubber ground bus directly to the transformer ground bus or enclosure ground pad with the shortest practical conductor, ideally under 1 m, preferably a flat braid or strap because a wide flat conductor has lower inductance.
• Run the ground conductor alongside the phase cables so the loop area is small.
• Terminate cable shields at the snubber end on the same ground bus.

The transformer’s own connection to the station ground grid handles fault current and personnel safety; the snubber bond handles the transient. Both are required. The Knowledge Hub article Transformer Grounding: Understanding the Importance covers the safety side, and the IOMM gives torque values.

A retrofit follows five steps.

1. Gather the data a TRV study needs: transformer kVA, voltage, BIL and construction; breaker or contactor type and its chopping characteristics; cable type, length and surge impedance; typical load at switching; and any existing arresters. Rex engineering can review a study or advise on a conservative selection.
2. Confirm floor space beside the transformer for a floor-mounted Type 1 enclosure, or Type 3R, 4 or 4X outdoors, with door swing and cable entry clearances.
3. Plan the outage. The R.C. Snubber connects to the transformer primary terminals, so the transformer must be de-energized, tested dead and grounded while the new cable is landed.
4. Install the shielded medium-voltage cable with stress cones, bond the ground bus, and verify phase identification.
5. Commission: measure each branch’s capacitance and resistance, check fuse continuity, megger the assembly, and record the readings as a baseline.

A retrofit on a transformer that has already suffered switching-related partial discharge does not repair existing damage, so consider a partial discharge test on the transformer at the same outage. The Knowledge Hub article Transformer Failures: Causes, Faults and Prevention lists the symptoms that typically prompt a retrofit.

Commissioning a snubber is quick but should be recorded, because the readings become the baseline for later condition checks. Working from the IOMM, the checks are:

• Visual and mechanical. Confirm no shipping damage to capacitor bushings or resistor bodies, that insulators are intact, and that all connections are torqued to the values in the manual.
• Fuse continuity. Verify each 6 A fuse is intact and correctly seated; a fuse that opened in transit will leave one phase unprotected without any obvious sign.
• Resistance. Measure each resistor and confirm it is within ±10 percent of the nameplate value.
• Capacitance. Measure each surge capacitor with a capacitance meter and record the value; the three phases should match closely.
• Insulation resistance. Megger each phase to ground with the fuse in place and the far end disconnected, using a test voltage appropriate to the class.
• Ground bond. Check the bond to the transformer ground bus for low resistance and short length.
• Options. Test the ground current monitoring alarm contact and confirm glow tubes indicate when the system is energized.

Do these at the same outage as the transformer commissioning so the R.C. Snubber is in place for the very first energization, which is often the most severe switching event. The Knowledge Hub article Transformer Commissioning: Critical Checks covers the transformer side of the same outage.

Power systems are inherently dynamic, and each setup differs in terms of load characteristics, equipment ratings, manufacturer specifications, configurations, insulation levels, voltage ranges, and the interconnection of various components.

While standard benchmarks and guidelines exist for designing snubbers, a thorough analysis and Transient Recovery Voltage (TRV) studies are necessary to accurately predict the unique transient voltage levels and frequencies specific to each system. This is due to the fact that no two power systems are exactly alike.

By using the results from TRV studies, the ideal capacitor and resistor values can be sized to enhance the system’s protection.

BIL is verified with a 1.2/50 µs impulse: a wave that rises to crest in 1.2 µs and decays to half in 50 µs. A 13.8 kV dry-type transformer with 95 kV BIL withstands that wave shape at 95 kV, which is about eight times the 11.3 kV line-to-ground crest. A vacuum re-ignition transient, by contrast, can have a front of a fraction of a microsecond and repeat dozens of times in a single opening, and its peak may be only 2 to 4 per unit, well under BIL.

A surge capacitor reduces the rate of voltage rise (dV/dt) but may leave high-frequency oscillations that can stress transformer insulation and increase the risk of circuit breaker re-ignition.

The resistor in a Rex R.C. Snubber absorbs oscillation energy and dampens voltage transients, providing more effective protection against switching surges. A capacitor alone may be suitable in certain applications, but its effectiveness should be confirmed through a transient recovery voltage (TRV) study.

The capacitor sets how far the transient’s natural frequency is lowered and how much the chopping overvoltage is reduced; it also sets the continuous charging current and the transient energy the resistor must absorb. Larger capacitance means more protection and more burden.

As a general pattern, subject always to the TRV study:

• 0.5 µF is typical at 2.4 kV to 4.16 kV, where the charging current stays under 0.5 A and the larger value gives the most dV/dt reduction for small transformers whose own winding capacitance is very low.
• 0.25 µF is the common choice at 6.9 kV to 15 kV class, giving under 1 A of charging current at 13.8 kV and a natural frequency low enough to control re-ignition on typical cable lengths.
• 0.13 µF is usual at 25 kV to 34.5 kV, where a larger capacitor would draw several amperes and tens of kVAR per phase for little added benefit.

Within a class, choose the larger value when the cable is very short, the transformer is small, or the breaker is known to chop at high current; choose the smaller when the bus has a sensitive ground-fault scheme or many snubbers in parallel. Rex builds custom values when a study demands it. The R.C. Snubber product page lists the standard set, and the Product Configurator records the choice.

Yes. Rex Power Magnetics designs custom R.C. snubber assemblies to match equipment ratings and transient energy levels. Custom options include resistance and capacitance values, voltage ratings, mounting dimensions, and connection methods. Engineering teams use system transient analysis data to design optimized snubbers for maximum protection and efficiency. Each custom unit is built and tested to the same rigorous standards as catalog models.

A snubber has no moving parts and normally needs only inspection and measurement at the same interval as the transformer it protects, typically annually or at each planned outage. With the system de-energized, tested dead and the capacitors discharged, the checks are:

• Capacitance. Measure each surge capacitor and compare with the commissioning value. A drop of more than a few percent indicates internal element failure and the capacitor should be replaced before it fails short.
• Resistance. Confirm each resistor is within ±10 percent of nameplate. A rising value suggests thermal damage from repeated high-energy events and points to a switching problem worth investigating.
• Fuses. Verify continuity. An open fuse means one phase has been unprotected since it operated, and the cause, usually a failed capacitor, must be found.
• Insulation. Megger each phase to ground and record.
• Visual. Look for discoloration of resistor bodies, tracking on insulators, corrosion on the ground bus and evidence of moisture.
• Options. Test the ground current monitoring alarm and confirm glow tubes function.

Trend the capacitance and ground current readings; they give early warning that the transformer’s transient protection is degrading. The IOMM for the R.C. Snubber details the procedure, and the Knowledge Hub article Transformer Maintenance: Essential Checklist covers the transformer inspection done at the same time.

Each symptom points to a specific cause. A blown 6 A fuse on one phase almost always means the surge capacitor on that phase has failed short or partially short; measure its capacitance and insulation resistance, replace it if it fails, and fit the identical full-range current-limiting fuse. A fuse that blows repeatedly with a healthy capacitor suggests far more transient energy than designed for, which calls for a review of the breaker or cable configuration and possibly a repeat TRV study.

Overheating of a resistor, seen as discoloration or a smell at the enclosure, means it is absorbing more energy than its rating. Causes include very frequent switching, a breaker with unusually high chopping current, or a resistor value below the cable surge impedance that leaves the circuit ringing. Confirm the resistor value and discuss the switching duty with Rex engineering.

Audible noise or corona at the R.C. Snubber indicates a contaminated or damaged insulator and is urgent.

A snubber enclosure contains capacitors connected to medium voltage, and a capacitor stores charge after the circuit is de-energized. The precautions go beyond those for a transformer:

• Isolate and verify. Open and lock out the upstream breaker, and confirm the transformer and snubber are de-energized with a rated tester. The optional glow tubes on the R.C. Snubber give a first indication through the door, but they supplement rather than replace a live-dead-live test.
• Discharge. The series resistor does not discharge the capacitor, because the branch is open at the fuse end once the system is dead. Apply a grounding stick to each capacitor line terminal for the time stated in the manual, then apply working grounds and leave them in place.
• Re-check. Capacitors can recover a small voltage after grounds are removed, so re-apply the ground before touching terminals.
• PPE. Use the arc-rated PPE the site’s arc-flash study requires for the transformer terminal compartment, since the snubber is on the same bus.
• Fuses. Replace current-limiting fuses only with the identical type; a general-purpose fuse will not clear a capacitor failure fast enough.

Full procedures are in the English and French IOMM. The Knowledge Hub article Transformer Clearance Requirements covers the working space that must be kept in front of the enclosure door.

Rex offers all three classes as an option inside the R.C. Snubber enclosure, so that lightning and slow switching overvoltage protection sits at the same terminals as the transient damping. The three classes differ in energy capability, protective level and durability, and the choice follows the transformer’s exposure and value.

Distribution class arresters have the lowest energy rating and suit indoor transformers fed by cable from indoor switchgear, where lightning exposure is indirect. Intermediate class arresters offer higher energy capability and a lower protective level for the same voltage rating, and are the usual choice for dry-type transformers in industrial substations, particularly at 15 kV and above or where the feeder has some overhead exposure. Station class arresters have the highest energy rating and lowest protective level and are specified for large or critical transformers, direct overhead exposure, capacitor bank switching duty, and utility-interface substations.