Considerations for a Highly Available Intelligent Rack Power Distribution Unit

Overview

A rack PDU sits at the back of the rack in hot-aisle air. The paper says ambient there is commonly 50 degC or more, so PDUs rated at least 55 degC should be specified. Idle power and relay type also matter: bistable relays can save about 7,600 EUR per year in a 100-rack data center, though that is a vendor estimate.

For overcurrent protection the paper favours resettable hydraulic-magnetic breakers over fuses. A rating detail deserves attention: a 20 A breaker rated at 80% allows only 16 A continuous, whereas a 100%-rated one allows 20 A. Because server power-supply inrush can exceed 50 A for tens of microseconds, sequential outlet power-up is recommended.

Threshold setting is critical in racks with 2N feeds: each branch alarm threshold should sit below 50% of the PDU rating, because when one feed drops its whole load lands on the other. Locking outlets, IEC 60320 C13/C19 connectors (up to 250 V) and metering accuracy come under functionality, while maintainability and adaptability cover outlet type and monitoring. The 2017 paper is written from one manufacturer's viewpoint, so the savings figure and product features tie to that family. The temperature, derating and threshold rules are nonetheless usable independent of vendor, since they rest on physical and electrical limits.

Key points

1. Rear-of-rack PDU ambient commonly >=50 degC; specify PDUs rated >=55 degC
2. Branch breakers rated at 80% -> 20 A breaker allows only 16 A continuous; 100%-rated allows 20 A
3. Server PSU inrush can exceed 50 A for tens of microseconds; use sequential outlet power-up
4. In 2N rack feeds, set branch thresholds below 50% of PDU rating
5. Common outlets IEC 60320 C13/C19, up to 250 V
6. Bistable relays can save ~7,600 EUR/yr in a 100-rack data center (vendor estimate)
7. Resettable hydraulic-magnetic breakers preferred over fuses

Using it in design

• In A/B (2N) racks keep each feed's design load at or below rating x 0.8 (80%-rated breaker) x 0.5; for single phase usable kW = V x I x 0.8 x 0.5.
• Add catalog fields maxAmbientC (default filter >=55), ocpType, branch rating percent and outlet types.
• Raise a lint warning when a PDU with an ambient rating under 55 degC is placed in a hot aisle.
• Put sequential outlet power-up on the inrush-current checklist.

Who should read it

Electrical designers selecting in-rack power distribution, and operations teams.

Related standards

• IEC 60320
• UL 489
• IEC 62368-1
• ASHRAE TC9.9 power equipment white paper

Publication details

Authors

Vertiv

Publisher

Vertiv

Year

2017

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation