Denser cabinets with high-power devices are driving today’s power- distribution needs. As this high-density trend continues, 208 V power and three-phase 208 V power is being brought down to the cabinet level. Most organizations consider this equipment mission critical, with downtime measured in thousands of dollars for each minute a system is unavailable. With so much at risk, a better understanding of overcurrent protection and the differences between fuses and circuit breakers protection is key to maintaining uninterrupted service when implementing cabinet-level power distribution units (PDUs).

Branch circuit protective devices typically fall into two categories, molded-case circuit breakers listed to UL489 or fuses listed to UL248. By definition, a circuit breaker is a means of automatically opening a circuit at a specified level of overcurrent, on either an overload or a short- circuit condition. Fuses are also designed to open the circuit at specified levels of overcurrent. Fuses are typically the less costly up-front solution, and are replaced after each occurrence. Breakers are typically the more costly upfront solution and, if listed to UL489, are large, bulky and do not fit into a 1U enclosure.

Proper selective coordination eliminates unnecessary power outages and reduces costly downtime. Selective coordination is the act of isolating a faulted circuit from the remainder of the electrical system, while maintaining uninterrupted power to the unaffected circuits. The faulted circuit is isolated by the selective operation of only that overcurrent protection device (OCPD) closest to the overcurrent condition.

Fuses open the circuit when they “see” a specific level of current passing through the fuse. Lower amperage-rated fuses require less energy to open the circuit than higher amperage-rated devices. This allows selective coordination of the fuses.

Circuit breakers require a coordination study to ensure selective coordination. Overlap of circuit breaker trip curves between the upstream and downstream devices often results in simultaneous operation of both devices. A circuit breaker system will clear the fault condition and open the circuit, but it will also remove power to all of the remaining loads being served by the PDU.

According to the NEC 110.10, overcurrent protection devices shall be selected to permit the OCPD to clear a fault without damage to the electrical components of the circuit. By reducing the amount of energy that passes through to the protected device, the damage that reduces repair and downtime is reduced. In order to successfully protect sensitive equipment, the upstream overcurrent protective device needs to be able to operate in a short amount of time, and consistently limit the amount of fault current/energy that passes through to the downstream devices.

Fuse operation is based on a simple thermal principle: the internal fuse element will rapidly melt/vaporize at a specific level of energy. This amount of energy is well below the total amount of energy potential available during a faulted condition. The resultant clearing time and the subsequent let-through current is significantly reduced, which results in less energy that a downstream component is required to withstand. Per UL248 listing, fuses are required to meet maximum allowable energy let-through values under fault conditions, which allows for protection of components.

Most thermal-magnetic molded-case circuit breakers are not listed and marked as current limiting. They do not interrupt short-circuit currents in less than a half cycle, and typically require a full cycle to clear a fault condition. This means that the full peak current and energy of the first cycle of the fault will be let through. Per UL489 listing, standard thermal-magnetic molded-case circuit breakers are not tested to limit the maximum amount of energy let through to downstream components.

Proper maintenance of overcurrent protection devices, as specified by the manufacturer, is critical to effectively and consistently operate within manufacturing specifications in the event of an overcurrent condition. Fuses do not require maintenance. Molded-case circuit breakers require periodic inspection and manual operation as part of their prescribed maintenance procedures.

Failure to manually exercise the mechanism can cause the breaker to open slower than specified or not operate. Most manufacturers recommend that if a molded-case circuit breaker has not been operated, opened or closed within six months time, it should be removed from service, and the mechanical operation and the tripping mechanism manually exercised.

Fuses ship from the factory calibrated to a specific set of operating parameters, ensuring that the fuse will operate as specified without maintenance and upkeep concerns.

All modern current-limiting fuses listed to UL248 employ a simple method of current limitation and are able to easily achieve interruption ratings of 100,000 amps or higher. Standard UL489 circuit breakers typically are tested to safely interrupt much lower levels of fault current, and are not inherently current limiting.

When operating under a short-circuit condition, as the contacts of a circuit breaker separate an arc is created between the contacts. The circuit breaker utilizes arc chutes to divide and dissipate the resulting arc. As a mechanical device, the breaker cannot internalize the resultant expansion of ionized gases. These must be vented safely from the breaker into the surrounding equipment.

Fuses are required to contain any violence or fire that occurs during the internal arc that is created when the fuse opens under a short-circuit condition. This includes maximum amounts of physical deformity, of which little is allowed.

The fuses utilized in many PDUs have a specific physical footprint and rejection-style fuse holder that prevents the wrong fuse from being installed. This prevents unqualified personnel from replacing an open fuse with an incorrect fuse.

There are several misconceptions concerning the suitability for using resettable devices for reliable overcurrent protection. Per OSHA 1910.334(b)(2), after a circuit has been de-energized by the operation of a circuit-protective device, the circuit may not be reenergized until it has been determined that the circuit can be safely energized.

Circuit breakers that have interrupted a fault approaching their listed ratings shall be inspected and tested to the manufacturer’s instructions, according to NFPA70E 225.3. After a circuit breaker safely interrupts one short-circuit fault, the breaker needs to be evaluated to determine if it can safely be put back into service, and it may need to be tested in order to determine if it will safely interrupt a short circuit in the required amount of time.

Kevin S. Arnold and Cooper Bussmann are with Server Technology, Reno, Nev.

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