Electrical Safety

Series vs. Parallel Surge Protective Devices (SPDs) – Which is right for you?

When designing and installing power distribution systems, engineers and contractors select surge protective devices (SPDs) to protect electrical equipment and systems from transient overvoltages. To do so, specifiers must understand the differences between parallel-connected and series-connected SPDs, and where to best apply each type of device. The following narrative summarizes the design and function of parallel-connected and series-connected SPDs and describes some common applications for each.


In general, a parallel-connected device is installed on an electrical switchboard or panelboard, typically to a dedicated service disconnect, such as a circuit breaker. Alternatively, standalone series-connected filters are connected in-line near protected load equipment.

Parallel-Connected SPDs

Parallel-connected SPDs most commonly use Metal Oxide Varistors (MOVs) to divert transient overvoltages from loads. These voltage-sensitive components conduct only when the surge voltage exceeds the varistor’s clamping level, sending excess voltage to ground. Remnant voltage found below the clamping levels, but above the nominal voltage, is known as let-through voltage.

In parallel-connected SPDs, MOVs are attached across each mode of the electrical system. The lengths of the resulting surge pathways depend on the lengths of the conductors installed to connect the unit to its circuit. The length of leads has a direct effect on device performance, with greater lead lengths equating to greater let-through voltages. Consequently, parallel-connected SPDs should be mounted as close as possible to the panelboards they protect.

Parallel-connected SPDs are capable of clamping high-energy low-frequency transients. Their benefits include lower cost and smaller size than series-connected devices, and installation procedures that do not require interruption of power to loads. Their disadvantages include performance differences according to the length of installed leads, instantaneous let-through voltages that vary by phase angle, and lack of attenuation of high-frequency noise.

Series-Connected SPDs

Series-connected devices fall into the two broad categories of series-connected SPDs and series-connected filters. Typically, series-connected SPD designs are wired in series, but internal suppression components are connected in parallel to the circuit. The benefit of this configuration is that lead length is not added during installation, allowing the device to clamp impulses at tighter levels than a parallel-connect unit. The ampere ratings of series-connected units are limited by their lug or terminal block characteristics.

Series-connected filters are designed to protect sensitive equipment, such as computers and digitally controlled equipment, from high-frequency noise that could disrupt reliable operation. Manufacturers offer these products to provide protection from high-frequency, low-energy transient overvoltages as well as noise filtering. These devices mitigate voltage whenever transient voltages exceed the instantaneous nominal voltage by a specified amount, regardless of phase angle.

Series-connected filters use a low-pass circuit (consisting of series-connected inductors, capacitors, and resistors) to eliminate high-frequency noise. Because the entire load current passes through the components, the device must be designed to pass all of the current carried by the circuit it serves. As a result, series-connected filters are typically larger and more costly than parallel-connected devices.

Benefits of series-connected devices include mitigation of potentially damaging high-frequency noise, a tighter clamping voltage, and performance that is independent of installation practices.

Which to Choose

During the design process, specifiers decide where to use parallel-connected SPDs and where to use series-connected filters. The appropriate selection could depend on whether protection is needed against damage to equipment, disruption of operations, or both. It also depends on the location in the power distribution where the device will be installed, as differing types of transients are likely to occur at different locations within a power distribution system. Studies show that 20-30 percent of transient overvoltages originate externally, from environmental and utility sources, and are more likely to consist of high-energy, low-frequency impulses. That means 70-80 percent originate from equipment and operations inside of a facility and are more likely to consist of low-energy, high-frequency noise, such as ringwaves.

If destructive high-energy transients pose the greatest risks, then a parallel-connected device is likely to be the most cost-effective solution. Consequently, parallel-connected devices are most often used at or near a building’s service entrance and on panels and equipment that manage large amounts of load. If sensitive equipment could be disrupted by high-frequency noise, then a series-connected filter may be the best choice. Where multiple types of transients may occur or where the characteristics of any power disturbance are unknown, then a staged strategy involving both parallel-connected and series-connected devices should be used. Effective protection is often provided when parallel-connected SPDs are installed at service entrances and distribution panels, and series-connected devices are installed near load equipment.


Parallel-connected SPDs typically use voltage-sensitive MOVs that conduct current only when line voltage exceeds their maximum continuous operating voltage. Parallel-connected SPDs can clamp high-energy, low-frequency transients. These are the most common type of SPDs and are generally cascaded throughout a facility.

Series-connected filters provide protection from low-energy transient overvoltages as well as noise filtering. These devices provide consistent clamping of let-through voltages regardless of phase angle and are usually installed close to critical loads. These are used to protect specific circuits that may have sensitive equipment.

Cascading parallel-connected devices at service entrances and power distribution panels, coupled with series-connected devices at both load equipment and internal sources of transients and noise, can provide comprehensive protection from a range of potentially damaging and disruptive surge events.

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