Redundant Rack Power Can Lead to Failure

How redundancy can create a false sense of security

Data Center facility design still comes down to the basics: providing uninterrupted power to hundreds or thousands of racks full of IT equipment.

Reliable redundant IT power is a basic expectation for virtually every data center application. Yet, internet colossuses, such as Amazon, Facebook, Google, and Microsoft, rely on software-based redirection for user queries in the event of a failure. Nonetheless, they still utilize redundant A-B power circuits for their critical core IT infrastructure (database servers, storage systems, routers, switches, etc.).

Utilizing IT equipment with dual power supplies is a common practice for both enterprise data centers and colocation facilities. As a result, redundant A-B power distribution circuits to the rack have become a popular provisioning option in many colos. However, power loss or mis-configuration of power distribution by IT administrators can be a major contributor to downtime.

In most cases, dual-corded IT equipment has redundant dual supplies that share the load at approximately 50% each when both supplies are active. However, if either supply fails or has lost input power, the remaining one must draw 100% of the power required. This is a normal failover scheme, but misunderstandings can lead to improper implementation.

A common issue is an overload of the power strip circuit breaker or the branch circuit breaker that feeds the receptacle to the power strip. Even if there is a metered power strip or a manual measurement performed at the distribution panel, it’s impossible to keep track of the highest maximum current being drawn on a branch circuit. The result is like playing Russian Roulette whenever a new piece of IT equipment is plugged in. Even if the added server doesn’t immediately trip a breaker, it’s possible the circuit is near (or at) capacity. Under heavy computing loads, power draw will increase, causing the circuit breaker to trip from an overload.

Most people understand the true redundancy requires that either path can sustain 100 percent of the load if the other path fails - however, the devil is in the details

Metered power distribution units (PDUs) may indirectly lead to a false sense of security. Locally metered PDUs have been available for over 20 years, but the need for improved energy efficiency has given rise to “intelligent” PDUs.

Moreover, intelligent PDUs in and of themselves cannot help if the information they convey is not fully understood. Circuit loading rules for redundant A-B power at the rack level are often misunderstood. Redundancy is only achieved when the combined total current (A+B) of each feed does not exceed 80% of the rated individual circuit value at the maximum projected power draw.

Yet it is not that simple since the loads can have wide swings over as conditions change.Wide swings in IT load current is no longer an unlikely or extreme scenario. The quest for energy efficiency in IT equipment has significantly changed the load profile and the idle-to-maximum power range for modern server.

The NEC rule regarding the 80% branch circuit breaker rating means a 20-A branch circuit should only be loaded to 16 A.

Wide swings in IT load current is no longer an unlikely or extreme scenario. The quest for energy efficiency in IT equipment has significantly changed the load profile and the idle-to-maximum power range for modern server.

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