(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.
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