Many of today's automation components — including motor controllers — are now available with their own protective enclosures. Given this trend, many OEMs and manufacturers are adding cable receptacles to such enclosures to allow installers to connect to them with factory-molded connectors during field installations of distributed hardware. By connecting enclosures in a “wirewayless” way, you can simplify installation and significantly reduce wiring requirements over the traditional method involving wireway (or raceway), wire, and field terminations.
You instantly reduce costs in such areas as engineering, documentation, verification, and troubleshooting. You can enjoy further savings by reducing the time involved to install wires, terminate and label each connection, and route wires to the control panel. Plus you eliminate the need to design and document motor controller panels. As a result, you significantly reduce the task of verifying that the sensor(s) and associated wiring are correct for a given motor controller, verifying correct motor rotation, and troubleshooting miswired connections.
For the end user, the benefits include floor-space savings, improved Mean Time to Repair (MTTR), enhanced control system reliability, increased productivity, and greater flexibility. Flexibility can be important where the physical arrangement and orientation of physical assets change frequently.
Sounds good, doesn't it? Yet, some people are concerned that this new approach may sacrifice safety and reliability. To address this concern, we must first look more closely at what we achieve with traditional methods.
Absolute really isn't. Traditional wiring methods and standards are not foolproof. What we've accepted as the “absolute” in achieving safe, effective, and consistent distribution of power and control signals has inherent weaknesses.
For example, wireways and raceways may fill with contaminants or separate at the joints. Based on experience, you can probably make a decent-sized list of failures you've noted in these systems. You'd probably also agree that any wiring system requires maintenance, as no wiring system or method inherently guarantees safety and reliability.
It's also important to remember that requirements vary by situation and application. Just compare the requirements of a general-purpose product distribution warehouse to those of a heavy industrial manufacturing plant. Before applying any type of electromechanical control design approach, you need to look at the application and understand the specific regulatory, ergonomic, and operational requirements. In doing so, you may be able to save considerable time and money.
Safe exposure. With wirewayless wiring, the most notable change is that wires previously running into the control panel via wireways are now visible as cables running on equipment surfaces. A common concern is the potential shock hazard this design might present. Is the tradeoff in reduced wiring and installation costs via exposed cables too high of a safety risk?
To answer that question, we need look no further than the NEC. You can use exposed cable in industrial environments to extend from a cable tray to a piece of equipment without wireway, if the tray cable meets the crush and impact test requirements of Type MC metal-clad cable [336.10 (7)].
The FPN in 670.1 refers to NFPA 79, “Electrical Standard for Industrial Machinery”. This standard allows you to install exposed cables along the structures of equipment. Those cables can follow the surface and structural members of the machinery, provided they fit the requirements for the operating conditions and external influences of that application [336.10 (7) and NFPA 79 18.104.22.168].
What if someone inadvertently pulls a plug apart — does this elevate the risk of electric shock? No. The design philosophy for plugs for exposed wiring is similar to that used for home electrical appliances. For example, if you unplug a lamp from an electrical socket, you can touch the prongs of the removed plug. The female connector is also hidden behind the electrical socket so your fingers can't touch it. This is equivalent to the IP20 enclosure standard. Similarly, all components ideally suited for distributed hardware solutions are designed so an exposed male pin is not live, and the female connector is always IP20-protected.
To provide additional shock protection, the NEC and NFPA 79 require the grounding pin to be the longest pin. This way, it mates first during plugging and separates last during unplugging. That means the power pin mates or unmates only when a ground connection is present.
To avoid electrical shock and provide safe operation of equipment, devices must be marked with their interrupting rating [240.86(B)]. Since cabling connectors used in distributed hardware solutions are not designed to interrupt current, qualified electrical testing organizations require the connectors be labeled “not for interrupting current.” This means you must shut off power to the device and implement lockout tag-out procedures when you want to disconnect a device by unplugging the connector and receptacle.
Locks on the connectors provide another safeguard for exposed run wiring. These allow you to latch and lock the cable into place when you plug the cable into the wall. With these locks, you can't inadvertently pull the connector apart and accidentally create a hazard. One method of locking involves provision of a snap-fit locking enclosure over the connector.
A third safety measure is to use connectors that have shorter pins for the control circuit power to the contactor. With this arrangement, pulling the connector apart will cause the contactor power to drop out before the pins carrying the power current separate.
Corrosion conquered. Wash-downs raise concerns that cutting fluids or washing materials might seep into a connector — causing rust, chemical corrosion, or electrical shock. But manufacturers and standards making bodies have already thought of this. For example, an enclosure or system rated IP67 (per IEC 60529, “Degrees of Protection Provided by Enclosures”) is “dust tight” and provides “temporary protection during water emersion for 30 minutes.”
Controls and cable connectors rated IP67 have a level of environmental protection that provides complete protection against dust ingress and temporary immersion in liquids. The plating or paint on the exposed parts protects the controls and connectors from corrosion. Furthermore, the metal parts of connectors are anodized aluminum (or e-coated zinc die cast) — the exposed parts must pass extreme wash-down and salt spray-type tests.
Motor branch-circuit requirements. The cables and connectors of distributed hardware systems are part of a motor branch circuit and must be able to withstand short-circuit currents per UL 1682. You must protect these with short-circuit and overload protection devices per Art. 430. If you're designing the system, you need to know the available fault current and determine if the controllers and cabling are adequately rated.
The substation transformers of manufacturing facilities are typically anywhere from 1,000 to 2,500 kVA in size and have a short-circuit current capability of 38,000A to 65,000A, respectively. Unless another transformer is between the facility's substation transformer and the wirewayless wire system, this is the available fault current the distributed hardware system may encounter if a short circuit occurs.
Wirewayless wiring and its components must pass testing by a qualified electrical testing organization and be listed to withstand these high short-circuit currents for the time it takes their supply circuit breaker to interrupt the power. Therefore, it's important for the system of cables, connectors, and controllers to be listed for use on motor branch circuits when protected by the largest circuit breaker allowed by Code.
Electrical inspections. Some people are concerned that their installation may not pass an electrical inspection if they use a wirewayless wiring approach. In an industrial facility, you can install tray cables between the cable tray and the connected equipment [336.10]. You must install this cable with good workmanship and support it properly every six feet or less (Sidebar on page 46).
Unfortunately, not all municipal electrical inspectors are sufficiently knowledgeable to approve electrical equipment that uses exposed wiring. The installer or end user may need to have:
A third party (e.g., a field engineer from a qualified electrical testing laboratory) certify the installation and equipment to their standards.
A registered professional engineer evaluate the installation for compliance with applicable NEC and NFPA 79 requirements.
The manufacturer document or demonstrate the equipment uses listed components and conforms to NEC and NFPA 79 requirements.
Or the inspector may elect to carry out an evaluation on the equipment before approving the installation as compliant with the appropriate codes. This latter method is likely in states and municipalities that have made NFPA 79 mandatory and enforceable.
You've now seen that the wirewayless wiring approach doesn't sacrifice safety or reliability. You've also seen a basic outline of the requirements for implementing it. The question that remains is this. Are you ready to reap the benefits?
Wielebski is a principal engineer with Rockwell Automation in Milwaukee.
Sidebar: Inspector's Tips
Following the NEC guidelines, install electrical wiring and equipment in a neat and workmanship-like manner.
Use control gear listed to UL 508, as they meet the regulations spelled out in NFPA 70 and NFPA 79, respectively.
Have documentation to show your machine was built and tested for its particular environment.
Ensure the conductors and cords are electrically protected within their limitations, voltage, and currents.
If using transient voltage surge suppressors, install them downstream from the circuit breakers or fuses so they don't become a fire hazard.
Do not use supplemental protectors as a substitute for a branch-circuit-rated overcurrent protective device.