It is interesting seeing the changes that happen over time with the National Electric Code (NEC). In the 2014 code, we saw the addition of Surge Protective Devices (SPD) combined with the word  “shall” be used on emergency systems. In 2017, the required usage moved to several other critical areas. The one that stands out to me is Article 670.6, which addresses industrial equipment with safety interlock circuits. It states that “industrial machinery with safety interlock circuits shall have surge protection installed.” No ambiguity about that.

Why? The main concern is that electrical surges may cause the interlocks to fail independent of the machine operation. The purpose of safety is to prevent folks from getting to the hazard. Imagine if a surge damages a redundant safety sensor, light curtain, Safety PLC, or the Safe Torque Off (STO) of a drive.

We have seen this happen in the real world and in a lab environment. Take a look at our video showing a surge of 28,000 volts destroying an ABB ACS550.


The NEC guidelines say that the following must have SPDs installed:

  • Article 501.35 A Class I Locations
    Surge arresters, surge-protective devices, and capacitors shall be installed in enclosures identified for Class I, Division 1 locations.
  • Article 501.35 B Class II Locations
    Surge arresters and surge-protective devices shall be nonarcing, such as metal-oxide varistor (MOV) sealed type, and surge-protective capacitors shall be of a type designed for specific duty. Enclosures shall be permitted to be of the general-purpose type. Surge protection of types other than described in this paragraph shall be installed in enclosures identified for Class I, Division 1 locations.
  • Article 620.51 Elevators, Dumbwaiters, Escalators, Moving Walks, Platform Lifts and Stairway Chairlifts
    Where any of the disconnecting means 620.51 has been designated as supplying an emergency standby system load, surge protection shall be provided
  • Article 645.18 Critical Operation Data Systems
    Surge Protection shall be provided for all critical operations data systems
  • Article 646.3 Modular Data Centers
    Where provided, surge-protective devices shall be listed and labeled and installed in accordance with Article 285.
  • Article 670.6 Industrial Machinery
    Industrial machinery with safety inter-lock circuits shall have surge protection installed
  • Article 694.7, Wind Electric System
    A surge protective device shall be installed between a wind electric system and any loads served by the premises electrical system.
  • Article 695.15 Fire Pumps
    A listed surge protective device shall be installed in or on the fire pump controller.
  • Article 700.8, Emergency
    A listed SPD shall be installed in or on all emergency system switchboards and panelboards
  • Article 708.20, Critical Operations Power Systems
    Surge protection devices shall be provided at all facility distribution voltage levels.
  • Article 810.6 Radio & Television Equipment
    Where an antenna lead in surge protector is installed, it shall be UL Listed.
  • Informative Annex G SCADA
    The power supply shall be provided with a properly installed surge –protective device (TVSS) at its terminals with a direct low-impedance path to ground. Protected and unprotected circuits shall be physically separated to prevent coupling.


So, all safety systems, wireless systems and HMI/SCADA systems require SPDs in a nutshell for the more common applications that we see.

Florida is the lighting capital of the United States with Georgia and South Carolina not too far behind. B&D Technologies started putting SPDs in all of our systems a few years ago. Please refer to the accompanying pictures. Why would a designer not use a simple device as an insurance policy against failures that no one can control? I still get amazed at the resistance from some folks.

I used to think they were snake oil, until we actually tested them live. My previous belief came from the vintage oil pot arrestors that I got tired of replacing many years ago at the steel mills. But, the newer designs are far better and more reactive. To my initial surprise, they really worked!

Last Words

Citel makes great SPDs and were the first folks to actually work with B&D Technologies as a partner. They are based in Florida and not too far from our Fort Lauderdale branch. After many discussions over the last few years, Citel and B&D Technologies believe that it is just a matter of time before the NEC requires all industrial applications to use SPDs.

C. Tolbert