Regulations Require Surge Protection for Safety Circuits
Incorporating safety is a fundamental requirement for industrial machinery design. Safe practices and systems are the result of careful planning, driven by detailed codes and specifications which are continually updated as technology improves. In recent years, the bar has been raised many times, providing improved operator safety and protection for machinery. A recent example regards the added requirement of surge protection for safety interlock circuits and fire pump controllers to ensure personnel safety.
Emergency stop (e-stop) and safety interlock circuits have long been part of the applicable codes. For industrial machinery and associated systems, these safety devices take the form of emergency stop buttons, door limit switches, guard limit switches, light curtains and other components—all of which are connected directly to a safety-rated relay or controller to interrupt power to a machine in case of a problem. Therefore, machine builders are quite familiar with these safety provisions, and the need to integrate these crucial safety functions into equipment designs.
In recent years there has been an increased focus on power quality and surge protection throughout industry, especially as electronic devices are more commonly used, even for critical process and safety applications. If a safety interlock circuit of any type fails due to an electrical surge, this places operators at great risk. New codes recognize that safety interlock circuits should benefit from the additional protection offered by surge suppression devices. This article examines the background of the requirement and how to comply with updated codes.
By the Book
Several codes may impact machine builders. In the US, original equipment manufacturer (OEM) machine builders are most familiar with the National Fire Protection Association (NFPA) 70, also known as the National Electrical Code (NEC). Local municipalities generally adopt versions of the NEC within a few years of publishing, so OEMs must comply by that time. There is also NFPA 79 with specific guidance for industrial machinery, and other standards by the American National Standards Institute (ANSI), the International Electrotechnical Commission (IEC) and the International Organization for Standardization (ISO).
The primary item of interest added to the 2017 revision of NEC 670.6 states “industrial machinery with safety interlock circuits shall have surge protection installed.” NFPA 79 2018 adopted the same requirements in harmony with the NEC.
The reason behind this is that over a quarter of facility managers surveyed in 2013-2014 indicated their sites had experienced damage to safety interlock circuits attributable to electrical surge events. Since surge suppression is a well-understood and cost-effective technology, it only makes sense to apply it for critical applications.
Beyond the code requirements, it is best practice and a prudent investment to incorporate surge suppression for any application using solid-state electronic components or microprocessors (Figure 1). This is especially important for systems installed in any sort of challenging industrial environment, but the fact is that any location can be subject to an electrical surge.
