- By Aaron Henry
- Factory Automation
- Class 2 devices and wiring methods are a specific type of energy-limited control circuit defined in the NEC.
- Designs using Class 2 products and methods have improved protection performance, better safety, and other installation and operational benefits.
- Electronic circuit protection operates faster and more accurately than fuses and traditional circuit breakers while providing features for intelligent distribution of bulk DC power.
A new type of device provides designers of automated installations for North America with performance, cost, and safety benefits.
Industrial machinery and equipment system designs typically rely on proven materials and methods to ensure reliable and effective end results. This is especially true when it comes to electrical power branch circuit distribution and control circuits, where the products available on the market are mature and design practices are largely guided by codes. But every so often, a new option becomes available to improve performance, reduce cost, and increase safety.
Such is the case with respect to a relatively new type of power distribution device built to take advantage of a special National Electrical Code (NEC) classification called a “Class 2” circuit. For automated equipment and systems destined for the North American market, Class 2 has many benefits for low-voltage control circuits.
Traditional control circuit power distribution designs use fuses, circuit breakers, and conductors to meet standards and requirements. Although complying with all codes is essential, this classic circuit protection approach is inefficient in some ways for the relatively small amounts of power involved.
Class 2 recognizes the low energy levels associated with many control and signaling applications, and it provides an improved option for designers to use specific materials and methods for these situations. Implementing Class 2 principles with the right devices brings functional benefits, lowers the total installed cost, and improves safety for users and equipment.
Different global regions adopt specific directives, codes, and standards that must be observed for electrical design, products, and installations. Correspondingly, electrical products are typically made to comply with one or more of these requirements. Following are some relevant documents for power supplies within industrial equipment:
Europe and International:
- International Electrotechnical Commission (IEC)
- European Standards (EN)
- IEC/EN 61204-1: Low-voltage power supply devices
- IEC/EN 61439-1: Low-voltage switchgear and controlgear assemblies
North America (U.S., Canada):
- National Fire Protection Association (NFPA)
- NFPA 70: National Electrical Code (NEC)
- NFPA 79: Electrical Standard for Industrial Machinery
- Underwriters Laboratories (UL)
- UL 508A: Standard for Industrial Control Panels
- UL 1077: Standard for Supplementary Protectors for Use in Electrical Equipment
- UL 1310: Standard for Class 2 Power Units
- UL 2367: Standard for Solid State Overcurrent Protectors
Designers and engineers must be familiar with applicable guidance for the final location where equipment will be installed and inspected. European and North American standards are not necessarily harmonized, adding complexity.
Standards related to power supply and distribution are generally intended to protect personnel, and to prevent fires, or any other unsafe or damaging conditions. The design principles and applicable products are well understood, but sometimes the standards are updated, or newer products become available, providing options for improvement.
Class 2 considerations
In the simplest terms, a Class 2 circuit is of such low voltage and current, and therefore low power, that it does not present a fire hazard or a shock hazard to personnel. Designs that ensure available energy is limited have many engineering, regulatory, installation, and operational benefits.
Traditional hardwired devices like motor starters often draw too much power for Class 2 circuits to be practical. But low-power digital devices, which are increasingly prevalent—such as programmable logic controllers (PLCs), human-machine interfaces (HMIs), and other intelligent components—work well when powered by Class 2 circuits. Also, much of the associated communication, networking, and input/output signaling can be supplied by Class 2 circuits.
In North America, the NEC is the primary standard for electrical installations. Control panels and the circuits within them are addressed by UL 508A, while UL 1077 and 2367 are directed at branch circuit protection.
The NEC specifically defines a Class 2 energy-limited control circuit in article 725-121 as:
- not to exceed 60 VDC (although they are most commonly operated at 24 VDC or 24 VAC for industrial and commercial purposes)
- load side power potential must be limited to less than 100 VA (although adhering to these limits with standard fuses or circuit breakers is not acceptable)
- supplied by a Class 2 power source device specifically UL listed as such.
These conditions must be maintained even in the event of a short circuit or during fault conditions. There are also restrictions on what classes of circuits may be routed together. Generally, Class 2 circuits must be routed separately from other classes of circuits in control panels and raceways. Note that Class 2 is only applicable within North America.
Class 2 advantages
There are many benefits when product suppliers and designers comply with Class 2 requirements. Circuits and downstream devices operating under Class 2 require no further protection from shock or fire hazards. From a practical standpoint, this means Class 2 devices and conductors can be smaller, and circuitry is often simplified for these reasons. Field wiring methods are less demanding, because the materials are easier to procure, install, and maintain. These benefits combine to reduce the expense of design, materials, installation, and support.
Manufacturers submit their devices to UL for investigation to receive NEC Class 2 certification. Once a device is Class 2 certified, it is easier for designers to use it compared with using most other types of electrical components. For example, within UL 508A–listed control panels, any Class 2 circuits and the components connected to them need not be evaluated for UL compliance, greatly minimizing the panel design effort.
There are a few points to know for properly implementing Class 2 designs. Some downstream devices may be certified as requiring a Class 2 power source, so designers must be aware of this and make sure they provide the proper Class 2 source. Also, while Class 2 wiring methods are generally easier to use, the Class 2 circuits must be routed independently of other circuits, both within a control enclosure and in field raceways. Perhaps the biggest concern with Class 2 designs is the limited amount of power available for each circuit, constraining the number of downstream devices that can be supplied.
Class 2 implementations
There are a few options for implementing Class 2 power sources. Small transformers can be designed to convert 120 VAC to 24 VAC in a current-limited manner to meet Class 2 requirements. These are often used for residential and commercial signaling applications, e.g., heater and air conditioner thermostats or other building automation. However, typical industrial control systems commonly use 24 VDC.
Some component suppliers originally developed power supplies specifically for Class 2 applications. These power supplies were designed, tested, and certified to meet Class 2 requirements (figure 1). This is a workable approach, but these power supplies were often limited to a nominal 4 A or less output current at 24 VDC. Many control panels needed a greater amount of control circuit current, which therefore required multiple power supplies. Or perhaps several Class 2 power supplies could be used, but were not fully utilized. Both situations eroded some of the expected benefits.
Some suppliers responded to customer needs for a better way to deploy Class 2 designs by creating a new category of device called an electronic circuit protector (ECP). ECPs are a smarter family of Class 2 devices, performing intelligent power distribution and making it much easier to gain the benefits, while providing many standalone and modular options for distributing bulk 24 VDC power from standard power supplies (figures 2 and 3).
Using ECPs, it is possible to distribute up to 40 A of bulk upstream power to many individual Class 2 downstream circuits. Designers can therefore choose fewer, but larger, upstream power supplies as needed, perhaps even incorporating redundancy or an uninterruptible power supply. ECPs also have many other benefits.
More intelligent power distribution benefits
Because intelligent power distribution devices perform their ECP function using digital methods, the protection reacts faster than other physical methods, such as fuses or traditional circuit breakers. The response is more reliable than the trip curve of a standard breaker or fuse, and it is much closer to the desired protection level selected. For example, if 2 A protection is desired, the ECP trip curve is nearly 2.1–2.2 A, while it may be as high as 6 A for a breaker. This ECP responsiveness provides accurate overcurrent selectivity with other circuits, and it helps prevent voltage drops, and even cable fires.
An ECP monitors the current on each individual output channel, and it provides users with the ability to turn each output channel on and off individually, for example via remote signaling from control systems. Upon startup, the output channels are automatically started in a cascade manner to reduce the system inrush and minimize the chance of upstream overcurrent trips.
Some ECPs have a visual indication of channel loading, such as a steady green LED for less than 90 percent, blinking green for 90 percent to 100 percent, and red for greater than 100 percent. Also, a group alarm contact allows the ECP to be monitored by supervisory PLCs. Control actions can be taken if necessary, and HMI alarms can be triggered. The control and diagnostic capabilities far exceed what a tripped circuit breaker or blown fuse can provide, significantly easing startup, factory acceptance testing, and troubleshooting efforts.
Many ECPs offer compact form factors, and versions supporting higher channel numbers can take up less DIN rail space than fuses or circuit breakers. A line-side bridging system facilitates bulk power distribution, while compact form factors and clear labelling make it easier to design, fabricate, and maintain control panels.
Note that ECPs are not only for Class 2 installations. Some general ECP models may allow adjustable current limits per channel, while specific Class 2 models are necessary for true Class 2 installations. Therefore, ECPs can be used for many control circuit applications where Class 2 is not needed, but other intelligent power distribution benefits are desired.
Modern ECPs are the best choice for distributing bulk 24 VDC control panels to Class 2 circuits in control panels, and they also provide substantial benefits for distributing standard 24 VDC circuits. Implementing ECPs allows proactive monitoring, often precludes any need for personnel to open electrical cabinets, and unlike a fuse, does not require replacement after a trip.
Class 2 circuits are a very specific configuration, unique to the North American market and defined by the NEC. There are many safety, performance, regulatory, and economic benefits associated with implementing Class 2 control circuits.
Certain power supplies are rated to provide Class 2 circuits, but there are power constraints, so sometimes many such devices are needed. ECPs are the best and most flexible way to distribute multiple Class 2 circuits from the bulk 24 VDC power supplies typically used in control panels. ECPs also provide many benefits for distributing general power circuits.
For projects and equipment destined for North America, designers should familiarize themselves with the requirements, products, design methods, and installation practices associated with Class 2 control circuits, so they can take advantage of this compact, economical, and feature-rich solution.
Images courtesy of Murrelektronik
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