Branch Circuit Protection in PDU’s
Corporate data centers account for some of the most expensive and sought after real estate in the world today. Enterprises invest huge amounts of capital to design, construct and secure their investment in IT equipment in the data center.
In a rack Power Distribution Unit (PDU), the power input is often broken into multiple segments. A division of the circuit with current limited by a fuse or circuit breaker is referred to as a branch circuit.
Branch circuit protection can be accomplished by either branch circuit breakers or fuses. To identify a UL489 circuit breaker, there are two general visual indicators. The first indicator is the size of the externally showing rocker switch. The UL489 breaker will range in dimension from 1” wide to 1.5” long. The lower grade rocker switch or reset type are typically very small (less than ½”). The second indicator is that the chassis will be expanded to accommodate the higher performing breaker. If the vertical 30A/208V power strip is perfectly symmetrical (2” x 2” or less) across the full length of the power strip, it is highly probable that it does not comply with the current standards.
It is now common practice for consultants to open power units to identify comply with to code. Eaton® uses the highest quality hydraulic-magnetic circuit breakers to ensure maximum overload protection as well as comply with to mandated code requirements. The “lower grade” thermal circuit breaker is in fact approved to a different UL standard, UL1077. A UL1077 device is a Supplementary Over-current Protective device. Its marking will show a UR symbol (i.e., it is UL Recognized). A UL489 device is a Branch Circuit breaker, and the marking on it is “UL listed”.
Compared to higher capability circuit breakers, thermal breakers have several subtle issues and capability limitations:
(a) No Disconnect Capability – Many thermal breakers can only be reset after a fault and cannot be used to manually disconnect the power supply. The only methods to disconnect a power unit is to either pull the power cord out from the power source or open the upstream circuit breaker at the power panel.
(b) Heat Susceptibility – Thermal breakers are by their nature susceptible to enclosure heat. The typical derating factor for a thermal circuit breaker for a temperature increase from 73°F to 104°F is 8%.
(c) Lower Overload / Overcurrent Protection – a thermal breaker is often a UL1077 device, and it can break prospective fault currents up to 1 or 2kA. This is far less than the capability of a UL489 branch breaker which must be able to break fault currents of 5kA. The use of inappropriate devices is not only against the regulations but can be hazardous and will cause extended down time due to device failure.
While fuses are generally accepted for certain products, they are not considered a ‘best practice’ for mission critical facilities:
(a) Increased Points of Failure – current market specifications employ as many as twelve fuses. For a data center with 100 enclosures, that would equal 2400 additional points of failure.
(b) Reduced Safety to Personnel – for double pole applications (208V) it is possible for only one fuse to blow and the second leg to remain ‘hot.’
(c) Higher MTTR (Mean Time to Repair) – while a branch circuit breaker can be quickly reset, the replacement of a fuse can take as long as one hour or more depending on the specific model.
(d) Higher Repair and Replacement Costs – unlike a circuit breaker which can be quickly reset, it costs significantly more to replace a fuse. Depending on the marketplace, both an electrician and apprentice may be required. A full remediation plan may also be required in the maintenance budget to facilitate ‘off-hour’ replacement.
(e) Voided Warranty and/or Product Certification – Any time a power unit is physically opened; it presents a number of issues. Not only is there concern as to whether the correct fuse is replaced, but whether the product requires re-qualification for safety performance (e.g. hipot, ground continuity, and functional tests).
The fundamental issue is how much risk the end-user is willing to assume, including financial, functional and safety. The proper application of UL60590-1 Edition and the employment of UL489 branch circuit breakers are best suited to mitigate issues of PDU protection.
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