Surge Suppressor
Most Transient Damage Comes from
Routine Switching, Not Lightning.
Every start, stop, and stalled trip through a breaker or contactor can send a steep-front voltage surge into the motor, transformer, or generator terminals. These surges stress winding insulation in both new equipment and equipment already in service. Repeated exposure contributes to deterioration, and a severe surge can cause insulation breakdown. For critical motors, transformers and generators, that failure can interrupt production while equipment is repaired or a replacement is manufactured.
Insulation Fails Quietly, Then All at Once
Routine Switching Can Send Steep-Front Surges Into the System’s Most Vulnerable Insulation
During vacuum breaker or contactor operation, a pre-strike can occur before the contacts close, or a re-strike as they open. These events launch steep-front voltage waves along the cable toward the equipment.
The surge impedance of a motor, transformer or generator winding is typically much higher than the cable’s. This mismatch causes the wave to reflect at the equipment terminals, where the voltage can approach twice the incoming surge’s magnitude.
Motors and dry-type transformers have among the lowest surge-withstand capabilities of equipment connected to a three-phase system. Their withstand capability decreases further as the wave-front becomes steeper. A rapid voltage rise concentrates stress on the inter-turn insulation of the line-end coils, exposing some of the system’s most vulnerable insulation to severe electrical stress.
Routine motor switch-on can generate surges approaching four times the normal peak phase-to-ground voltage, while interrupting a stalled motor can produce five to six times that voltage. At sufficiently short rise times, these surges can exceed the motor insulation-withstand levels described in IEEE and CIGRé guidance.
How Transients Originate and Intensify
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Source or mechanism
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Effect on the equipment
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|---|---|
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Breaker or contactor switching |
Pre-strikes and re-strikes can launch steep-front voltage waves during starts, stops and stalled-motor trips. |
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Cable-to-winding surge-impedance mismatch |
Reflections at the equipment terminals can raise the voltage to nearly twice the incoming surge’s magnitude. |
|
Lightning or arcing ground faults |
These events can also generate steep-front transients that stress winding insulation. |
Protecting winding insulation helps preserve critical electrical assets and the production that depends on them.
A Failed Transformer or Generator
Can Take Months to Replace
The direct costs of an insulation failure are straightforward to add up: removing the failed unit, transporting it, repairing or replacing it, and returning it to service.
The larger cost often depends on how long the equipment is unavailable. A failed motor, transformer or generator may need more than a repair-shop appointment. It may need a place in a manufacturing queue. Building, testing and shipping a replacement can take months. Without a suitable spare or temporary alternative, the production that depends on it can remain stopped throughout that wait.
The cost of failure goes beyond the equipment. It includes every day of production waiting behind it.
What Would an Equipment Failure Cost You?
The cost depends on how long your equipment is out of service, the production affected, and the expense of repair or replacement. Enter your estimates below to see how those costs add up.
Illustrative only, based on the figures you enter.
ZORC®
Surge suppression for the winding insulation of motors, transformers and generators.
ZORC® connects from each phase to ground at or close to the equipment terminals. Its R-C-ZnO configuration combines resistors, capacitors and zinc oxide arresters to:
How ZORC® Works
A voltage- and frequency-dependent cable-terminating network of zinc oxide arresters, resistors and capacitors.
ZnO Arresters
Gapless zinc oxide elements conduct during high-magnitude, steep-front surges. Working with the capacitors, they limit the voltage excursion and reshape the wave-front reaching the winding insulation.
Resistors
Selected to suit the cable’s surge impedance (Zc ≤ R ≤ 3Zc, typically 30 Ω), the resistors minimize reflections and prevent the associated voltage-doubling effect at the equipment terminals.
Capacitors
High impedance at line frequency and low impedance during a fast transient. The capacitors act as a frequency-dependent switch, bringing the resistors into the circuit during the transient.
Cable surge impedance is independent of cable length and varies little between cable types and sizes. Standard ZORC® configurations therefore suit most installations, allowing catalogue selection without a detailed engineering study.
Why Add ZnO to an RC Snubber?
An RC snubber is a proven method for controlling switching transients. By matching the cable’s surge impedance, it reduces reflections, cancels voltage doubling at the equipment terminals, and reduces re-ignition surge voltages in vacuum switchgear.
The limitation is that an RC snubber does not provide voltage-dependent limiting during the highest magnitude transients. It controls how the surge travels through the system, but it does not actively clamp the peak voltage.
Adding gapless zinc oxide (ZnO) arresters creates an R-C-ZnO network. During large steep-front surges, the ZnO elements conduct to limit and reshape the transient, keeping the stress reaching the winding insulation within acceptable levels.
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Characteristic
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RC Snubber Alone
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ZORC® (R-C-ZnO)
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|---|---|---|
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Voltage doubling / reflection at the terminals |
Reduced by matching the cable’s surge impedance and cancelling the reflection effect. |
Reduced using the same cable-termination principle. |
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Re-strike / re-ignition escalation in vacuum switchgear |
Reduces re-ignition surge voltages. |
Eliminates sequential re-ignition events through combined wave-front control and ZnO limiting. |
|
Peak surge magnitude |
No voltage-dependent limiting during high-magnitude transients. |
ZnO elements conduct during large steep-front surges to limit and reshape the transient voltage. |
RC snubber solves the reflection problem. ZORC® adds transient limiting when the surge itself is too large.
Protects New Equipment.
Extends the Life of What’s Already Running.
ZORC® reduces the transient stress reaching motor, transformer and generator terminals by controlling surge magnitude, wave-front steepness and repeated switching events. Installed on new equipment, it helps protect winding insulation throughout its service life. Retrofitted to equipment already in service, it reduces further cumulative transient stress and helps extend the remaining insulation life.
The ZORC® Family
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Type
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Voltage Range
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Configuration
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|---|---|---|
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Standard ZORC® |
2.2 kV to 40 kV |
R-C-ZnO network; machine-mount and panel-mount versions |
|
Compact ZORC® |
3.3 kV and 6.6 kV |
R-C-ZnO with 1 m flexible leads, direct to the terminal box |
|
LVZ (Low-Voltage ZORC®) |
400 V to 1,100 V |
R-C network, for low-voltage three-phase systems |
|
RC ZORC® |
7.2 kV to 40 kV |
R-C network only, for dry-type and oil-immersed transformer applications |
|
Hazardous-location option |
All standard voltages |
Optional certification for Class I Division 2 (ExN, non-sparking), temperature classes T1 to T5 |
Available in single-phase and three-phase versions; single-phase units are suited to phase-segregated terminal enclosures.
Built to Recognized Capacitor and Insulation Standards
ZORC®‘s internal insulation levels and testing align with IEC 60871-1 for shunt capacitors rated 660 V and above, and with the general guidelines of ANSI, IEEE 18, NEMA CP-1, VDE 0560 part 410, and CIGRé 13.02.
Hazardous-area versions carry optional certification for Class I Division 2 (ExN, non-sparking) locations, temperature classes T1 to T5, to SABS 970-1971 (amended).
ZORC® is not a lightning protection device. It’s designed to shunt switching-generated transients; in areas with meaningful lightning incidence, I‑Gard recommends a dedicated surge arrester ahead of the ZORC® as well.
Typical Applications
FAQ
Frequently Asked Questions
Answers to our most commonly asked questions. Can’t find yours? Ask here
Specifying Surge Protection for New or Existing Equipment?
Tell us your system voltage and switching arrangement, and I‑Gard’s application engineers will help you select the right ZORC® configuration for the installation.