by Nathan Xavier, Consulting Engineer, Rittal
More North American engineers than ever use IEC devices instead of NEMA components—especially with modular busbars to replace traditional wiring for controls.
Busbars and standardized IEC devices simplify integration and enhance safety, and now new combinations and kits are letting control-panel integrators complete systems less expensively than ever. Spurring this trend is global acceptance of IEC components and modular busbar systems with universal certifications for worldwide installation.
At their core, busbar systems are meant to replace all line-side wiring and associated accessories in an electrical panel. Usually after the main feed (and the main overcurrent-protection device) high-amperage cables run to power distribution blocks (PDBs). These PDBs split large cables into smaller low-amperage cables, which then run in trays to individual devices.
Busbars can replace these parts and more. Except for rare main-lug-only (MLO) applications, most control panels have a main overcurrent protective device (OCPD) within the cabinet. This can be a circuit breaker or a fused disconnect.
In most installations, the main OCPD mounts in the panel with the controls and control wiring (including line-side wiring, distribution blocks and wire duct) under it. But if the main OCPD is small enough (generally 600 A or less), it can often mount directly onto the bus (and be on the same row as the control devices).
If the OCPD is too large or otherwise incapable of mounting directly to the busbar (with an adapter), the design must energize the bus with feed cables. Such larger installations usually require multiple cables per phase. Insulated flexible busbar can replace these cables with a single conductor.
Here, flexible busbar carries all necessary certifications and ratings to facilitate an easy transition from standard round cable. Featuring a wire bend radius as little as 0.125 in., flexible busbar saves panel space and eliminates the need to cut and strip multiple cables. Elimination of line-side wiring, PDBs and wire tray can help offset most of the busbar’s cost. Smaller enclosure size or (for larger systems) total elimination of the enclosure also saves money—a benefit passed down from designer to integrator to end-user.
NEMA versus IEC
The National Electrical Manufacturers Association (NEMA) establishes standards for electrical components in North America. Industrial control and systems equipment must meet NEMA ICS 2, Standard for Industrial Control and Systems; they’re usually certified by a third party under UL 508, Industrial Control Equipment. The equivalent global standard is the International Electrotechnical Commission (IEC)—IEC 60947, Standards for Low-Voltage Switchgear and Controlgear. UL 508, the primary industry standard to which devices are certified in the U.S., is now harmonizing with IEC 60947 to become UL 60947. Effective 2017, UL 60947 will further cement IEC devices as the industry standard.
So within the U.S., IEC devices are becoming the first choice for many designers in automation. Here’s a comparison of IEC and NEMA equipment to show why.
NEMA standards are more general, so NEMA components form robust controls that address myriad applications. With just a few parameters to consider, design work is easier. NEMA devices meet standard size ratings (00, 0, 1, 2 … 9) so last longer, too. But NEMA devices are often overbuilt for their application. For example, a NEMA Size 0 contactor is rated 5 hp at 460 V for full-load current (FLC) of 8 A. This contactor has a continuous current rating of 18 A, more than double the full-load current.
In contrast, IEC standards are more specific, so components are more customizable. IEC components are rated by continuous-current ratings much closer to their switching capacity. So an IEC contactor equivalent to the NEMA size 0 is rated 5 hp at 460 V (with 8 A FLC) but with a continuous current rating of only 9 A. This tighter tolerance means IEC controls come in twice the number of sizes as NEMA counterparts.
IEC motor controls under 100 A (more than 80% of the market) are also 30 to 70% smaller and less costly than equivalent NEMA controls. That’s mostly because of compact arc suppression. Every time a contactor closes or opens, an arc forms that slowly degrades the metal contacts.
NEMA contactors have large conducting surfaces to address this; both the rated mechanical and electrical operations are higher than equivalent IEC units. But better arc suppression in IEC contactors means they need less material, so are smaller. IEC also classifies machines into different “usage categories” that define contactor and relay electrical load and duty cycles, so designers can pick units with just the right capacity.
Motor protection circuit breakers
Overload relays and contactors for motor control don’t protect against short circuits. In NEMA panels, branch short-circuit protection is through fuses or molded-case circuit breakers (MCCBs). In contrast, IEC panels use motor-protection circuit breakers (MPCBs) for disconnecting and short-circuit protection with instantaneous magnetic trips (plus manual motor control and overload protection with thermal trips).
There’s no NEMA equivalent to an IEC MPCB and the devices can’t replace a UL 489 MCCB or as a UL 98 disconnect switch per the NEC. So, UL simply classifies these products as manual motor controllers. Also known as manual motor protectors or motor-starter protectors, these often go in group-motor installations.
Such setups don’t need short-circuit protection for each individual motor circuit. There are restrictions, but NEC Article 430 (Motors, Motor Circuits, & Controllers) and UL 508A (Industrial Control Panels) allows several motor loads on one branch.
IEC motor-protection circuit breakers also satisfy UL 508 as combination motor controllers (CMCs). These work as branch circuit protectors for motor loads, eliminating the need for a UL 489 MCCB or fuses. These CMCs are Type E Manual Self-Protected Combination Motor Controllers—self-protected so they work after a short circuit, something even UL 489 MCCBs don’t do.
The contactor for remote motor control creates a Type F CMC that works in circuits similar to group-motor installations but without restrictions. Replacing single-function devices with manual motor protectors or CMCs can save panel space, wiring labor and money.
Standardized metric sizing and installation
There’s a bit more design work to configure IEC components, but it’s offset by how fast IEC panels set up. IEC components have standardized widths, so a two-pole miniature circuit breaker is twice the width of a one-pole; a three-pole version is triple the width. Most motor starters rated 37 A and below are 45 mm wide; those rated between 40 and 55 A are 54 mm wide; and 60 to 100 A are 72 mm wide. In addition, most accessories like auxiliary contacts work with devices regardless of rating or size. This standardization lets designers easily configure panel layouts, even with devices from different manufacturers.
Like NEMA devices, IEC controls can panel mount with two or four mounting holes. But unlike some NEMA devices, almost all IEC controls also mount to standard 35-mm DIN rail—so a panel integrator can install an entire row of IEC devices with just a few screws to mount the DIN rail.
About 30% of all motor failures are from overloads. Most IEC overload relays are Class 10 units with a 10-sec tripping time. Typical NEMA relays are Class 20 units with a 20-sec tripping time, overbuilt for general-purpose use. So one largely unfounded concern about IEC devices is the threat of nuisance tripping (when the relay trips the motor during peak current draw, usually startup). Most modern motors reach operating speed before the 10-sec tripping time, and special applications that require more time can incorporate Class 20 overload relays.
About 15% of all motor failures are from single phasing, usually when an upstream single-phase load causes imbalance in a downstream three-phase load. Here, bimetallic IEC overload relays can address phase-loss sensitivity. During electrical imbalance, a differential tripping mechanism makes the overload relay more sensitive by lowering the trip point. But damage can still happen, especially after extensive single phasing of a motor with a load not high enough to trip the relay. Engineers concerned with true phase-loss protection should invest in solid-state (electronic) overload relays or separate phase monitors.
Motor temperature depends on heat from magnetic and current effects of normal operation and ambient temperature. Overload relays must prevent motor overheating even under extreme heat and cold. But relays often have their own temperature-dependent characteristics different from that of the motor—so some overload relays trip more slowly when it’s cool, for example. Temperature compensation isn’t available for most NEMA equipment.
In contrast, IEC devices use mechanical means to reduce temperature effects and maintain consistent trip times. For safety, some IEC devices feature IP20-rated bodies, so objects bigger than 12.5 mm (0.5 in.) can’t contact the live terminals.
There’s no such NEMA standard, although some manufacturers design NEMA equipment with comparable features. In addition, IEC 60947 outlines differences between devices allowed to show damage after short circuits and those that aren’t. Type 1 coordination devices can show damage and aren’t for reuse. Type 2 coordination devices can’t show damage and are reusable after a short-circuit fault. Type 2 devices enhance safety by eliminating arc-flash risk, plus reduce downtime.
In the past, busbars only unified single-use systems for specific applications. Even today, only a few manufacturers make standardized busbar systems for control-panel use. Still, misperceptions persist due to lack of knowledge about these systems.
Misperception: Busbar systems are more dangerous due to difficulty making touch-safe installations. Traditional busbar installations such as load centers and MCCs offer touch safety with dead front panels that only let personnel access power connections from the enclosure’s rear. New modular systems for use with control panels incorporate touch-safe terminals and covers. They also eliminate terminal blocks so are safer than many typical power-distribution block-and-cable installations.
Misperception: Busbar systems involve complex fabrication and integration. In the past, many switchgear installations using busbar required bending, drilling and tapping of the copper bus. But newer busbar systems eliminate this, so installers can just cut it to length. Even cutting the bus to length is sometimes unnecessary. Connections to the bus are through adapters held in place with setscrews or spring clamps.
Easier integration with busbars
Much of the physical integration of controls is labor intensive. Here, busbars make it easier to terminate wires and connect equipment. Plus busbars reduce the amount of enclosure drilling and tapping for setup. It’s true that such work is easy for facilities with CNC machines, but it’s a monumental task for those without. Even plants with CNC machine access benefit from reduced tooling wear and turnaround time.
Power distribution blocks: PDBs usually install with at least two mounting screws per phase. This means a typical three-phase application needs at least six drilled and tapped holes in the mounting panel. Plus, each end of the feed cables must terminate into the block and OCPD. This constrains wire sizes and room for future expansion.
Line-side wiring: Completing a control panel’s line-side wiring takes considerable time. For example, a typical three-phase 300-A application with 12 motor starters could mean 36 cables for these devices, plus another three for the feed. Each of these cables must be measured, cut, stripped, routed, labeled (identified) and terminated on both the line and load sides.
Wire duct: All line and load wiring typically routs along wire duct. The wire duct must be measured and cut to size. The panel is then drilled and tapped to receive the wire duct fastening screws or just drilled to accept rivets. A typical mounting interval is 4 in., so each row in a 32-in.-wide enclosure with a 28-in.-wide mounting panel may need as many as eight holes. Here, installers must use extra caution, because the installation must accommodate the duct’s wire-fill capacity as well as the cable’s wire-bend radius. No busbar system can replace all wiring duct, but most can eliminate line-side wiring duct.
Main overcurrent protective device: This also requires mounting screws into the mounting panel. But some installations mount the main OCPD directly to the bus, to do away with screws and save panel space.
Miscellaneous control devices: Typical IEC control devices 100-A and less install on standard 35-mm DIN rail. This DIN rail must fasten to the panel with drilling and tapping of the mounting panel. Screw intervals are generally 4 to 8 in., so (as mentioned) as many as eight holes must be made for every row of controls in a typical enclosure. Each MCCB requires at least two of their four holes (three-pole) for direct panel mounting. Eliminating these holes reduces labor costs.
Note that busbar systems for control panels fall under the new standard IEC 61439 (Low Voltage Switchgear and Controlgear Assemblies). The components must pass requisite tests in a process known as design verification (formerly called type testing). Like the IEC devices with which they work, busbar assemblies come with requisite third-party listing (not recognition) to this standard. So, UL panel integrators can use all the parts without having to describe them (as they would need to do recognized products).
Busbar benefits for end users
Power distribution blocks aren’t touch-safe, and sometimes cause dangerous arc flashes. Modular busbar systems for control panels address this issue with myriad touch-safe covers for adapters, terminals and the bus itself. These make the busbar setup 100% touch-safe, even if stray fasteners, dropped tools or body parts contact the bus.
Traditional control panels constrain wire sizes, so there’s little or no room for future expansion. Even if expansion is possible, it’s often only with extensive drilling and tapping. In contrast, personnel can update modular busbars for machinery expansion without machining. Plus, control panels using busbars easily accept extensive retrofits. None of the devices screw into the panel, so all come out to make way for new hardware as needed. Even if the enclosure needs replacing, it’s possible to repurpose the busbar into the new enclosure—a change considerably easier than that for a traditional block and cable design.
Maintenance and troubleshooting busbar designs are also efficient. Line side wiring needs no tracing, and plant personnel can replace components from their adapters.