Make Circuit Protection Easier

Aaron Hand is the managing editor for Control Design and for Industrial Networking. Email him at [email protected] or check out his Google+ profile.Besides complying with the UL 508A safety standard, which outlines circuit protection for industrial control panels, circuit protection is essential for reducing downtime and finding the root cause of failures in order to maintain consistent production flow. After decades of manufacturing, all industries still struggle to troubleshoot electrical equipment effectively, according to Stephen Starr, automation product marketing and sales specialist at E-T-A Circuit Breakers.In some ways, this is truer today than ever. "With the most experienced workforce in the last 50 years beginning to retire, a lot of experience will be walking out the door with very little young talent to backfill the needs on the plant floor," Starr says.Tony Locker, manager, product management for Littelfuse, agrees. "As plant staffs continue to shrink, younger workers often don't have the benefit of working side by side with experienced workers who truly understand electrical protection," he says.Circuit protection suppliers are doing more with the technology to not only make it work faster and more effectively, but also make it easier for plant personnel to detect and resolve electrical faults.For example, current-limiting fuses and circuit breakers will open at least 12 times faster than a typical non-current-limiting circuit breaker, which can take up to six ac cycles to open under short-circuit conditions, according to Locker. Current-limiting devices, on the other hand, can open within the first half cycle of a fault (8.3 ms). "Its fast action limits the amount of current that can flow through the circuit and reduces the destructive energy of a short or arc-flash," Locker says.Once a fault occurs, finding which fuse has opened typically requires poking around an energized electrical panel with a tester. "Besides the danger, this process prolongs downtime," Locker points out. Indicating fuses, on the other hand, have a dark spot that appears when the fuse opens; and indicating fuse holders point out open fuses with a bright neon lamp or with a signal to a PLC.

(Fuse Holders Leverage Protection)

UL 508A requires a short circuit current rating (SCCR) on any components in the electrical power distribution path. The required SCCR, which is the maximum short circuit a component, assembly or equipment can safely withstand with a specific overcurrent protective device, is changing the way machine builders and end users are looking at circuit protection. "I would say that we see customers moving toward using branch circuit protection devices that make accomplishing the overall panel SCCR easier," says Michelle Goeman, product manager, terminal blocks and electronic interface for Wago.

Using fuses rather than circuit breakers makes accomplishing panel SCCR more convenient, according to Goeman. "Because fuses all have a standard trip curve, manufacturers can test their components with a particular fuse and achieve a short circuit current rating with all fuses of equal or lower ampacity rating that meet or exceed the trip curve. However, a component that is tested with a circuit breaker to achieve an SCCR may only be used with that exact circuit breaker — it is much more limiting."

The need to change current levels in the field during the commissioning or installation phase has become a new parameter at the design level.  "Breakers need to move towards the flexibility of a fuse, while maintaining the integrity of a breaker," says E-T-A's Starr, who adds that the need to change current levels in the field during the commissioning or installation phase has become a new parameter at the design level.

Starr maintains that solid-state electronic circuit breakers, which detect both overcurrent and short circuits with alarm outputs and remote reset inputs, offer the best protection for field I/O devices when a switch-mode power supply is used. Flexibility and interchangeability will make those circuit breakers even more useful in the field.

"We have developed a breaker combination that will allow for the current rating to be field-modified and allow for a technology change to suit the needs of the load protection requirements, from thermal magnetic to electronic and vice-versa," Starr says. E-T-A plans to release the product later this year.