2024 Beyond Code: The Case for Surge Protective Devices in Electrical Safety

Submitted by Mike Miller, Curriculum & Training Manager

Surge protective devices (SPDs) were first introduced into the National Electrical Code (NEC) in 2002, specifically in Section 285, where they were identified as Transient Voltage Surge Suppressors (TVSS). Notably, at that time, their use was restricted to circuits below 600 volts.

Over the years, the electrical workers has debated the effectiveness of surge protective devices and their place within the NEC, which is guided by the National Fire Protection Association (NFPA). In previous articles for The News, I've explored various devices introduced into the code, questioning whether we are enhancing safety beyond what is practical.

Today, I aim to provide compelling evidence that SPD's are a worthwhile investment for our clients.

In essence, a surge protective device safeguards against fires by diverting excess voltage to the grounding portion of a circuit. This process prevents harmful voltage from affecting resistive or inductive loads, which could overheat and ignite combustible materials or even cause the device itself to catch fire. While the equipment grounding conductor directs objectionable current to the earth, it also serves as a pathway for voltage surges.

So, how does a TVSS effectively redirect excess voltage away from connected electrical systems? This question can be answered by examining the components of a surge protective device. Depending on the type of surge protective device, capacitors, diodes, or inductors, or a combination of these components serve to clamp voltage at a much lower level than it arrived from the supply source.

Surprisingly, power surges—those sudden "rushes of energy"—are quite common. For instance, a voltage surge as high as 900 volts can occur on a 120-volt supply circuit in commercial settings.

But where do these significant surges originate, and why don't they damage every device connected to the circuit? Surges can arise from various sources, including lightning strikes, power outages and restorations, and the starting or stopping of large machinery. Think of it like a sudden rush of water through a pipe: if the pipe (or wire) isn't prepared for that volume, it can create a pressure spike (voltage surge). Some or all of the same principles may apply in electrical circuits:

Inductive Reactance: When motors or other inductive loads suddenly turn off, the collapsing magnetic field generates a voltage spike.

Capacitive Discharge: Rapid discharges from capacitive loads release stored energy, causing spikes in voltage.

Load Switching: Sudden changes in demand, such as turning heavy machinery on or off, can lead to brief surges as the system adjusts.

In all these scenarios, the electrical system struggles momentarily to maintain stable voltage, resulting in spikes or surges. Surge protective devices are designed to safely redirect this excess voltage, usually to ground, thus preventing damage or fire.

The reason not every device on a circuit is destroyed during a surge is that these surges typically last only a fraction of a second. However, repeated exposure to even brief surges can compromise sensitive electronics, such as smoke detectors, underscoring the importance of employing SPDs to mitigate their effects.

As this is a precise science, there is undoubtedly more to learn. However, we can now appreciate the necessity of SPD''s as mandated by the NEC.