UFC 3 -520-01
June 10, 2002
CHAPTER 5
POWER DISTRIBUTION AND UTILIZATION --SWITCHGEAR, LOAD CENTERS,
AND BREAKERS
5-1
SERVICE ENTRANCE.
5-1.1
Locate service equipment at the service entrance p oint.
5-1.2
Ensure equipment is capable of safely performing all interrupting functions
based on the available system capacity and characteristics. Protective devices must be
able to clear the available short circuit current without damaging the unaffected portions
of the system. If the available short circuit current exceeds the ratings of standard
electrical equipment, evaluate the following options:
5-1.2.1 Current Limiting Fuses. This is usually the most cost-effective method of
reducing downstream fault currents. Base downstream equipment ratings on the
maximum let-through current of the current limiting fuses. Ensure that the fuse will
function in its current-limiting range of operation for the available short circuit current.
5-1.2.2 Current Limiting Circuit Breakers. Similar to current limiting fuses,
breakers can perform a current-limiting function. Periodic breaker maintenance is
crucial to maintaining this current-limiting capability. Also, discuss this option with the
manufacturer to ensure that the current-limiting capability is understood completely.
Series-combination ratings for MCCBs are not allowed.
5-1.2.3 Current-Limiting Reactors. Detailed design analysis is necessary to ensure
voltage drop is not excessive and to verify t hat the installation is adequately braced for
short circuit conditions. This option is not recommended without a detailed analysis of
all other options.
5-1.2.4
5-1.2.5 High Impedance Transformers. Although high impedance transformers can
reduce the fault current, they also suffer from poor voltage regulation and higher energy
costs. This option is not recommended without a detailed analysis of all other options.
5-1.3
Provide a single disconne cting means for each facility. Avoid multiple
disconnects unless major economies can be realized in large capacity services or if
multiple service voltage requirements exist.
5-1.4
NEC Article 230.95 (2002 Edition) requires ground fault protection (GFP) to
be provided for solidly grounded, wye electrical services of more than 150 volts to
ground, but not exceeding 600 volt phase-to -phase for each service disconnecting
means rated 1,000 amperes or higher. If GFP is provided at the service entrance, it
should be provided at downstream branch panels also. Although this will add to the
overall system cost, it can be difficult to coordinate the service entrance GFP by itself
5-1
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