Accredited Tier Designer
Technical Paper Series:
Engine-Generator Ratings
August 2017
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Abstract
This technical paper focuses on the requirement and use of an engine-generator solution for on-site
power in the context of Uptime Institute’s Tier Standard: Topology. It provides additional detail and
clarification regarding the Tier consequences of engine generators and their ratings.
Tier Requirements and On-Site Energy Production
The core premise, as set forth in the Tier Standard: Topology, is that the only reliable source of power for a data center is
the on-site energy production. Typically, on-site energy production is provided by an engine-generator plant. On-site energy
production can be accomplished in many ways, such as diesel or fuel cell technology; if a unit has the requisite
performance capability as documented by the manufacturer, it can meet the applicable Tier Standard.
This paper focuses on engine generators due to the nuances of the specific ratings associated with the technology and
their impact on Tier compliance. Although the purchase of power from the local utility is an economic alternative, there are
no considerations of utility power that affect the owner’s target Tier objective. Instead, for Tier III (Concurrently
Maintainable) and Tier IV (Fault Tolerant) functionality objectives, the Tier Standard: Topology states:
“Engine generators for Tier III and IV sites shall not have a limitation on consecutive hours of operation when
loaded to ‘N’ demand. Engine generators that have a limit on consecutive hours of operation at N demand are
appropriate for Tier I or II.”
There are two key aspects of this requirement:
•
Disruptions to the utility power are not considered a failure, but an anticipated operational condition for which the
site must be prepared; and
•
A Tier III or IV engine-generator system, along with its power paths and other supporting elements, shall meet the
Concurrently Maintainable and/or Fault Tolerant performance confirmation tests while they are carrying the site on
engine-generator power.
There are two scenarios of operation that result in the engine-generator plant being required for an extended period (weeks
to months):
•
The loss of the local utility due to malfunctions within the utility system—resulting in extended outages;
•
In some designs or operational profiles, the catastrophic malfunction of the uninterruptible power supply (UPS)
system or isolation of the UPS for replacement or heavy maintenance.
The latter requires running the engine-generator plant to ensure that the most reliable and stable power is being delivered
to the IT critical environment. If the local utility is used during a time the UPS is unavailable, then any perturbation or loss of
the utility will impact the computer room operation and potentially result in an outage.
Uptime Institute field experience and Network member data shows that the availability and reliability of the infrastructure is
paramount to achieving the organization’s business objectives, i.e., it is mission imperative for the data center. Accordingly,
engine generators must have no runtime limitations at N units.
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Engine Generator Ratings
Engine generators and their ratings are governed by International Organization for
®
Standardization (ISO) Standard 8528-1. This standard covers Reciprocating
Internal Combustion (RIC) engines, alternating current (AC) generators, and
associated systems. The three principal ratings as defined in the standard are
Emergency Standby Power, Prime Power, and Continuous Power.
“Emergency Standby Power: The maximum power for which an
engine-generator is capable of delivering for up to 200 hours per year. The
allowable average power output over a 24-hour run
period is 70% of the standby rating unless otherwise agreed to by the RIC
manufacturer.”
“Prime Power: The maximum power for which an engine-generator is
capable of delivering continuously with a variable load for an unlimited
number of hours. The allowable average power output over a
24-hour run period is 70% of the prime rating unless otherwise agreed to
by the RIC manufacturer.”
“Continuous Power: The maximum power for which an enginegenerator is capable of delivering continuously for a constant load for an
unlimited number of hours.”
Standby units—allowed to
run for limited durations at
constrained capacities—do
not afford the data center
owner the capability to run
the engine-generator plant at
capacity for extended periods
to support operations during
critical events
In practice, when applying these definitions and the requirement for no runtime
limitations at N demand, standby-rated units (as defined) with limited run hours do
not comply with Tier III and IV. Standby units—allowed to run for limited durations
at constrained capacities—do not afford the data center owner the capability to run
the engine-generator plant at capacity for extended periods to support operations
during critical events, and therefore do not meet Tier III or Tier IV requirements.
Some manufacturers allow only up to 500 hours of capacity operation per year for
certain units. However, a standby-rated unit can comply with Tier III and Tier IV
requirements if there is proper manufacturer documentation that establishes the
unlimited run hour capacity of the unit at the site conditions.
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It is important to work with
the manufacturer and
obtain commitments in
writing of the specific
allowance for runtimes and
capacities.
Prime-rated units, per their definition, have more robustness than standby units.
Many manufacturers offer the same unit with both standby and prime ratings.
However, to comply with the no runtime limitations at N-load requirement, these
units must be de-rated to 70% of their prime (nameplate) rating. Note, however,
that some manufacturers will offer a derating of more or less than 70% of the prime
rating. It is important to work with the manufacturer and obtain commitments in
writing of the specific allowance for runtimes and capacities. Continuous is the
only rating that complies with the requirement without any derating.
In summary, only continuous ratings, de-rated prime ratings, or standby ratings
with no runtime limitations qualify for the Tier III or IV requirement for engine
generators.
EngineGenerator
Requirements
Rating to
Support design
load
Tier I
Tier II
Tier III
Tier IV
Any; up to
nameplate rating
to support design
load
Any; up to
nameplate rating
to support design
load
Capable of
supporting
design load for
unlimited hours
at site conditions
Capable of
supporting
design load for
unlimited hours
at site conditions
Continuous
Full nameplate capacity
Prime
Option 1: 70% of nameplate capacity
Option 2: Larger capacity than Option 1
with manufacturer letter
No additional requirement for hours of
operation limitations
Can be used for Tier III and Tier IV with
manufacturer letter; Tier Certification
capacity dependent on manufacturer
letter
Standby
De-rating for Site
Conditions
Additional derating may be required due to site conditions (e.g., ambient
temperatures, elevation)—consult manufacturer requirements
Table 1: Tier Requirements Summary­—Engine Generators
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Modifications
ATD Technical Paper Series: Engine-Generator Ratings, Version B. All
updates specific to this version are effective August, 2017.
Related Publication
Tier Standard: Topology
Accredited Tier Designer Technical Paper Series
Further information can be found at www.uptimeinstitute.com.
About Uptime Institute
Uptime Institute is an unbiased advisory organization focused on improving the performance,
efficiency, and reliability of business critical infrastructure through innovation, collaboration, and
independent certifications.
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