Article
Surge protective devices (SPDs) are critical components of any 
commercial or industrial electrical system. SPDs guard equipment from 
the over-voltages that can result from lighting strikes or equipment 
operation. Without this protection, these short-term events can cause 
long-term – and expensive – damage. Choosing the right SPD requires 
knowing where the device will be applied, along with the appropriate 
size to meet application needs.

Surge Protective Devices: Know What You’re Specifying

Chuck Ross
Surge protective devices (SPDs) are critical components of any commercial or industrial electrical system. SPDs guard equipment from the over-voltages that can result from lighting strikes or equipment operation. Without this protection, these short-term events can cause long-term – and expensive – damage. Choosing the right SPD requires knowing where the device will be applied, along with the appropriate size to meet application needs.

What’s your type?
Up until 2006, SPDs were generally classed as either surge arrestors or transient voltage surge suppressors (TVSSs). Surge arrestors were defined as devices installed on the line side of a power distribution system to guard the overall distribution-system structure. TVSSs were defined as being installed on the load side of the main service disconnect, imposing a tighter limit on transient voltages to protect connected electronics.

Underwriters Laboratories (UL) introduced the third edition of its UL 1449, Standard for Surge Protective Devices in 2006, and that document – along with the National Electrical Code’s (NEC) corresponding Article 285 – threw out these two terms. The more generic term “surge protective device” now covers both product categories. SPDs now are designated by type:

  • Type 1 SPDs replace surge arrestors. While the term “surge arrestor” still exists, it now refers only to devices over 1000 V. Those products are covered by the NEC’s Article 280.
  • Type 2 SPDs replace TVSSs on the load side of the panel. 
  • Type 3 SPDs refer to receptacle surge protectors. These are products like power strips that add surge protection at the individual load level.

Size matters
Beyond the appropriate type of SPD, specifiers also need to determine the most appropriate size of the device. Maximum continuous operating voltage (MCOV) is a key attribute to consider in this decision. Simply sizing up to higher MCOVs as system size increases isn’t the best approach, according to Richard Dale, a product manager with SPD manufacturer Littelfuse.

“Many people think bigger systems need bigger SPDs, which isn’t accurate,“ he says. “An MCOV that’s too high will damage equipment, too low and it won’t provide complete protection. “It’s best to use the lowest available MCOV that is above the system’s reference to ground, and the way to determine reference to ground depends on the type of system.”
Photo courtesy of Littelfuse
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