UFC 3 -520-01
June 10, 2002
Maintaining Ground Fault Protection.
13-5.7.1 Refer to NEC Article 250.20 (2002 Edition) for grounding requirements.
13-5.7.2 Ground fault protection requires special consideration with an ATS. The NEC
provides the basic requirements for GFP. NEC Article 230.95 (2002 Edition) requires
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. FPN No. 3 in NEC Article 230.95 recognizes the
inherent difficulty of the above requirement when a transfer switch is used. It states that
where GFP is provided for the service entrance disconnect and interconnection is made
with another supply system by a transfer device, means or devices might be needed to
ensure proper ground fault sensing by the ground fault equipment.
13-5.7.3 The selected d esign approach depends on whether the generator is a
separately derived system. If the generator system is a separately derived system, a
4-pole ATS should be used in which the neutral is also switched.
Additional System Design Considerations.
13-5.8.1 If allowed by the facility layout, locate the transfer switch near the load. This
increases system reliability by minimizing the length of the run common to both power
sources from the transfer switch to the load.
13-5.8.2 Design feeder routing wi th physical separation between the normal power
feeders and the emergency feeders. This minimizes the possibility that both power
sources will be simultaneously interrupted by a localized problem within the facility.
13-5.8.3 Where possible, use a greate r number of small transfer switches rather than a
lesser number of large transfer switches. By this approach, failure of a single transfer
switch should not affect the entire facility. NFPA 99 endorses this approach by
separating the electrical system into subsystems.
13-5.8.4 Include a fully rated break and load maintenance bypass switch in parallel
with a closed transition ATS. It is important that the ATS be designed for maintenance
and repair without requiring shutdown of the associated system.
13-5.8.5 Refer to NFPA 99 for any transfer switch applications involving medical
Information Sources. The following references provide additional
information regarding automatic transfer switches.
EGSA On-Site Power Generation: A Reference Book--provides a detailed overview
of design and application issues.