- June 04, 2019
By Brent Purdy, PE, AutomationDirect
Recent additions to the National Electrical Code (NEC) mandate surge protection for safety interlock circuits to protect operators and machinery.
By Brent Purdy, PE, Product Manager, AutomationDirect
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.
Figure 1: This common power strip shows the results of a surge event, but any devices using solid-state electronics, even robust industrial devices such as sensors and controllers, are susceptible to damage from electrical surges.
Why Interlock Circuits are Susceptible
Any electrical device is subject to failure if subjected to a large enough electrical surge, also known as a transient over-voltage or spike. But many solid-state electronic components and microprocessors have small connections and electrical traces, making them more delicate and sensitive to surges than simple electro-mechanical devices like limit switches and relays.
There are many safety interlock device options now available, such as coded non-contact switches and light curtains, that incorporate electronic elements. These advanced components can offer enhanced safety by being difficult to defeat and by sensing operator presence before a person approaches a hazardous area.
The lynchpin for any safety system, whether on a machine or larger process, is the relay, safety controller, or programmable logic controller (PLC) operating to interrupt power when the safety circuit is broken. These devices constantly monitor the safety signals and perform the critical interlocking functions, and although they are designed and tested for safety service it is possible for them to be damaged by electrical surges. Therefore, they will operate even more reliably when protected by surge suppression devices. Signal wiring associated with safety circuits often routes throughout most of a machine or larger facility, making the safety system even more exposed and vulnerable to over-voltage transients.
Other Safety-Related Systems Also Impacted
Safety interlock circuits are not the only systems impacted by Code changes. In the 2014 NEC revision, emergency systems for power distribution switchboards and panelboards had a requirement for surge protection added.
Now for 2017, NEC 695.15 extends this required protection to fire pump controllers. The NEC is focused on protecting people and property from electrical hazards, primarily fire. Therefore, fire pumps used to fight fires are given special consideration.
Solid-state electronics and microprocessors play a prominent role in automating critical infrastructure subsystems such as switchgear and fire pumps. As with other areas of the industrial safety and control domain, the control of critical infrastructure can benefit by the performance and capabilities of microprocessor-based control, but by nature these controllers are susceptible to surges.
Based on NFPA survey results, more than 10% of participants in 2013-2014 reported some form of fire pump controller damage due to electrical surge issues. This raises a large enough concern to mandate the inclusion of surge suppression for these systems to reduce the chance of damage.
Surge Protective Devices in Action
The primary method of protecting equipment from electrical surges is by connecting properly sized and rated surge protective devices (SPDs) into the power lines supplying the target circuits. Surge protection devices should be compliant with Underwriter’s Laboratories (UL) 1449 standard for surge protective devices, especially if they are to be installed in UL 508A listed industrial control panels.
Industrial SPDs generally work by incorporating metal oxide varistors (MOVs) sized and configured to shunt a transient overvoltage spike to another conductor or ground so the electrical current is redirected away from protected components, thereby protecting them. Generally SPDs are connected in parallel with the circuit. SPDs are passive devices until a transient occurs then the SPD “takes” action shunting the energy safely away via the appropriate mode of protection, such as line-to-ground and line-to-neutral. Other technologies are possible, but they all work by conducting and redirecting current once a threshold voltage is exceeded. This threshold voltage is also known as the let-through or clamping voltage.
The UL 1449 standard classifies SPDs by type, with the two most common types for industrial control panel applications being Type 1 and Type 2. Both are permanently connected to protect the equipment, but Type 1 can be connected both upstream of the service equipment overcurrent device or downstream. Type 2 is only allowed to be connected downstream on the load side. The device specifications will indicate this rating, as well as the surge capacity in thousands of amps per phase. Since these devices are intended for use in UL508A control panels, they should also offer short-circuit current ratings so designers can perform the necessary short-circuit system calculations.
One downside of these devices is a limited operational lifetime, with degradation of the protection function after a certain number of surge events. The only way for a user to know the protection is active is if the SPD provides a trouble contact or indicating light. The indicating light is usually like that on a ground fault circuit interrupter, on and green if the protection is functioning, off or red otherwise.
How to Comply
SPDs are offered in a few form factors. One style is configured like a pass-through DIN rail terminal block. However, a convenient and commonly used style packages the surge suppression into a small enclosure with extended leads. This versatile style is suitable for use in and around switchgear, control panels and machinery wiring (Figure 2). Here are the features to look for:
- ANSI/UL Type 1 or 2 SPD protection
- Compact size
- NEMA 4X enclosure for indoor or outdoor use
- 0.75-inch NPT threaded nipple for mounting directly to conduit fittings or inside an industrial control panel using a mounting bracket.
- LED status indicator light
Figure 2: Surge protection devices, like this Mersen model offering UL Type 1 surge protection, are compact, flexible to install and provide an LED status indicator light.
Once the properly sized device is selected, designers should ensure all control voltages operating safety interlock circuits are protected by surge suppression to maintain compliance with NEC 670.6. Also, as noted earlier in this article, any power circuits supplying fire pump controls should have the same type of protection. In fact, the protection offered by SPDs is cheap insurance against equipment failure or outages due to electrical surges, and ideally should be applied to any power circuits supplying industrial controllers or instruments.
Prudent surge protection design calls for a layered defense strategy within the facility or equipment. This allows for cascaded levels of protection to dissipate the large amounts of energy that might be encountered from a lighting strike or large external switching event. Also, internally created surges, like the starting of a large motor or capacitor switching, may occur at multiple levels of an electrical distribution system. The layered defense is an effective method for mitigating risks in multiple applications.
Typically within a facility one might specify a Type 1 SPD at the main service entrance as the first line of protection. Additionally, Type 1 or 2 SPDs would be installed on distribution panels at several levels within the facility. Finally, as we have discussed here, the power supply to machines containing safety circuits require protection. Prudent engineers should specify and install surge protection on equipment such as:
- Fire Pumps
- Packaging machines
- Conveyor lines
- Metal working/forming equipment
- Industrial robots
- Woodworking machinery
Compliance Elevates Safety
Designers of industrial machinery and automated equipment will be required to comply with 2017 NEC 670.6 moving forward. This means installation of surge suppression for safety interlock circuits on all new machines and systems. Because this is specified in the NEC, electrical inspectors at new installations will be taking a hard look to ensure compliance.
Fortunately, the category is well-defined and there are readily-available products that are economical and easy to install for this service. Compliance comes with added benefits as surge suppression is realized for protecting electronics and controllers, so designers should consider adding these devices in all forms of industrial equipment.
These new requirements are only the beginning of the regulatory wave of interest in surge protection. Like ground fault protection and arc fault protection in residential homes, it is likely that future regulations will look at surge protection in other areas. Designers should consider surge protection to industrial datalines and control signals, residential services, or any applications where power supplies sensitive equipment. It is prudent and may be required one day.
About the Author
Brent Purdy, PE is the Product Manager for Power & Circuit Protection at AutomationDirect.com. Prior to his current position, Brent was a Product Engineer, and before joining AutomationDirect in 2013, he worked as an Electrical Lead and a Senior Engineer at Polytron, and as a Systems Engineer at Westinghouse Anniston. Brent holds a BSEE degree from Georgia Institute of Technology and is a certified Professional Engineer in the state of Georgia.Learn More
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