The Raritan Blog

Keep it Safe: Overload Protection at the Rack

Jessica Ciesla
September 20, 2021

This article from Raritan provides an overview of overload protection standards for rackmount PDUs.

Lost in most discussions about rackmount PDUs and intelligent PDUs is a simple premise: their job is to keep you and your equipment safe. Newly certified ITE PDUs of any type and with more than 20 amps are required to use overcurrent protection that meets branch circuit protection requirements in accordance with the National Electrical Code, ANSI/NFPA 70. In effect, this means these products are required to have branch circuit breakers listed under UL 489, “Standard for Molded-Case Circuit Breakers, Molded-Case Switches and Circuit Breaker Enclosures,” or fuses listed for branch circuit protection, such as those listed under UL 248-5, “Low-Voltage Fuses – Part 5: Class G Fuses.”

Using the Right Circuit Breaker Means Avoiding Short-Circuit Disasters

In addition to UL Standard 489, Underwriters Laboratories also publishes UL Standard 1077, which is the “Standard for Supplementary Protectors for Use in Electrical Equipment.” Devices certified to this standard are considered “Supplementary Protectors” and are called “Recognized” components, not “Listed” devices (as UL 489 breakers are). UL Listed Circuit Breakers meet more stringent requirements for branch circuit protection than Supplementary Protectors with UL Recognition. UL 489 requires the breaker to be functional after being subjected to a short-circuit test. UL 1077 and the IEC standard EN 60934 allow for breakers to clear a short-circuit condition but become safely destroyed in the process.

Circuit breakers are used in a variety of ways:

They are mounted in panel boards (also referred to as building PDUs) and rackmount PDUs to protect branch-circuit wiring.
They are also built into equipment to protect components and systems. Interrupting a short circuit—current flow limited only by the resistance of wiring—is a severe test of a circuit breaker.
If the interrupting capacity of the breaker is not adequate, the device could literally explode.

While that last bullet sounds a little dramatic, it is a friendly reminder that the amount of energy being harnessed in the electrical systems of most data centers, even at the rack or individual circuit level, is significant. Keep in mind UL 489 breakers can interrupt short circuits of 5,000 amps or more and UL 1077 breakers can interrupt fault currents of 1,000 amps.

Overload Protection and Rack Mount PDUs

If you would like more information on the topic of data center overload protection, check out our White Paper “Data Center Power Overload Protection: Circuit Breakers and Branch Circuit Protection for Data Centers”.