Over-voltage protection: critical differences between spark gaps and DC decouplers

Both isolating spark gaps and DC decouplers are commonly used for over-voltage protection on pipeline isolation joints and storage tanks where cathodic protection systems are applied.

Though spark gap devices are often applied because of their relatively low cost, they are not designed for many over-voltage conditions that commonly occur on pipelines.

Without fully understanding their limitations, pipeline operators may unwittingly put the mechanical integrity of their assets and operating personnel at risk from electrical events such as AC faults and lightning.

Isolation Joints Require Over-Voltage Protection

If an isolation joint is exposed to electrical disturbances that cause differential voltages beyond the isolator’s withstand voltage, an arc can form across or through the isolation material.

This arc can short out the joint and cause permanent damage to the isolation material and/or the joint itself. Figure 1 shows damage to the flange face of an unprotected isolation joint resulting from an AC fault on the pipeline.

The high energy from the fault not only burned through the isolating gasket, but also literally welded the two flange faces together.

Since the isolation joint was shorted, the cathodic protection (CP) system was compromised, and the affected pipeline was left without adequate protection until the short was identified and remedied.

In this case, the entire bolted flange connection had to be replaced at considerable expense.

Due to the obvious safety risks, especially in hazardous locations, and the potential for costly pipeline damage, effective over-voltage protection is essential for isolation joints.

The differential voltage across the joint must remain well below the isolator’s withstand voltage during lightning and AC fault events.

Common Solutions for Isolation Joint Protection

When properly protected, excess voltage across the isolation joint is limited by providing an alternate, lower-impedance path for current to flow around the joint during over-voltage conditions.

To maintain proper isolation for cathodic protection, the protective device must not conduct direct current (DC) under normal operating conditions.

Both solid-state protection devices and isolating spark gap devices are commonly used to protect isolation joints from such electrical disturbances.

Spark gaps are relatively low cost and effective for lightning protection. However, one of their key limitations is that typical AC fault current ratings are well below the fault levels commonly observed on pipelines.

What Happens When AC Faults Reach a Spark Gap?

As a rough approximation, AC fault currents of 3–10 kArms or greater can be expected to flow through an isolation joint protective device on pipelines located near HVAC lines.

For specific applications, numerical modeling tools should be used to provide precise predictions of fault current along the pipeline and determine the appropriate ratings required for protective devices.

Any device used on isolation joints for over-voltage protection should be properly rated to safely operate following exposure to such AC fault levels.

If the device is under-designed for AC fault current, it can eventually fail in a closed state. In this condition, the device becomes a permanent short circuit and defeats the purpose of the isolator.

It can also fail in an open state, where the device no longer conducts current during a fault or lightning event and is therefore unable to provide proper protection.

Both failure modes are undesirable. However, failure in an open state is particularly problematic because it presents a potential safety hazard from over-voltage and/or arcing and exposes the joint to possible permanent damage.

To better understand the relationship between electrical isolation, cathodic protection, and corrosion control, as well as the mitigation of alternating current and lightning effects addressed by NACE SP0177-2026, watch the Inspenet TV technical interview with specialists from Dairyland Electrical Industries during the AMPP conference in March 2026, focused on electrical decoupling and its role in corrosion protection systems.

Lightning vs. AC Faults: Why Energy Matters

Spark gap devices are effective for protection against lightning and low-level fault currents. However, the total energy from an AC fault is orders of magnitude greater than that from a typical lightning event, for which spark gaps are primarily designed.

This is because an AC fault can last much longer and therefore presents much higher total energy than lightning.

Spark gaps are not designed or rated to endure the high energy level associated with common AC faults, with typical ratings of only 500 Arms for 200 ms.

During conduction, the arc formed between the internal gapped electrodes can degrade the electrodes and, over time, result in either a failed-open or failed-shorted device.

When the device fails open, the electrodes can burn back, increasing the air gap between them and raising the spark-over voltage required for conduction during successive faults or lightning events.

In other fail-open scenarios, terminal components can break apart or the entire assembly can simply fall apart. In either case, the result is a permanent failed-open condition.

When the device fails shorted, high-energy dissipation overheats and deforms the internal components, resulting in a permanent short.

What Fault Current Testing Reveals

Figures 2–5 illustrate an example of a spark gap that failed open during fault current testing.

Figure 2 shows an oscillograph of a new spark gap under an initial fault current test at 2500 Arms / 745 Vrms at 60 Hz for 200 ms.

Once the voltage across the unit (blue trace) exceeds the spark-over voltage (-784 V in this test), the device goes into conduction and passes current (red trace).

Successive tests were performed on this sample, with time between tests to allow for cooling. Upon the sixth test at 2500 Arms, the device had failed open, as shown in Figure 3.

Although the voltage across the device (blue trace) reached 1043 Vpk, well above the spark-over voltage, no current (red trace) passed through the unit.

Figure 4 shows the internals of this failed spark gap test sample, while Figure 5 shows the internals of a similar sample prior to testing.

As shown in Figure 4, the electrode appears to have lost material, increasing the gap size.

To be fair, this unit was tested far above its published AC current rating. However, the failure mode observed reveals inherent weaknesses associated with the basic technology of a spark gap when exposed to fault current levels common to pipelines.

Like this tested sample, these devices can often fail in an open state, leaving the isolation joint, storage tank, or other protected asset exposed to potential arcing until the device is replaced.

Because the device fails open, this potentially dangerous condition is not easily detected in the field using a basic resistance check.

Dairyland Devices Are Built to Handle AC Faults

Unlike spark gaps, Dairyland’s AC voltage mitigation decouplers and overvoltage protection devices are designed and manufactured to protect isolation joints and pipelines against high-energy AC faults commonly encountered in these installations.

Popular Dairyland devices are rated and certified to 3.7 kArms at 0.6 s and are available with ratings up to 15 kArms at 0.6 s.

Dairyland devices also help protect personnel from over-voltage conditions and have clamping voltages far below those of spark gaps.

Unlike spark gap devices, Dairyland solid-state devices have shown to be so reliable that they have no need for periodic testing or replacement.

This reliability can result in lower operating and maintenance costs while supporting the long-term protection of pipeline assets.

Need to Upgrade Your Pipeline Protection?

Explore Dairyland’s range of DC decouplers and overvoltage protection devices, or contact Dairyland Electrical Industries through Inspenet Corporate to learn about the solutions available for your application.

Frequently Asked Questions (FAQs)

References

1. Dairyland Electrical Industries. Over-Voltage Protection: The Critical Differences Between Spark Gaps and DC Decouplers. Original technical article and primary source for this publication.
2. Dairyland Electrical Industries. Technical documentation and product specifications for DC decouplers and over-voltage protection devices.
3. AMPP — Association for Materials Protection and Performance. NACE SP0177-2026: Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems.
4. Inspenet TV. Electrical Decoupling and Corrosion Control. Technical interview with Dairyland Electrical Industries specialists at AMPP 2026.