Sizing Considerations for
Closed-Transition Transfer Operation
Author : Benjamin O. Medich P.E.
Ballinger, Philadelphia
Closed Transition Equipment
to critical services,
where momentary
interruptions during
transfer scheme
testing cannot be
tolerated. Hospitals,
laboratories and data
centers are among the
occupancies that will
often consider the use
of closed transition in
their automatic transfer
switches, and for their
main and/or substation
switchgear.
ASCO 7000 Series Closed
Transition Transfer Switch
Closed-Transition equipment
is equipment (switchgear and
automatic transfer switches)
that uses a make-before-break
operating sequence in order to
maintain uninterrupted power
to essential loads throughout a
transfer between live sources.
The importance of this is tied to
the requirement to operate the
transfer equipment periodically
to comply with mandatory
testing requirements, with
the desire to avoid causing a
disruption to electrical loads.
In general, the use of closedtransition equipment is limited
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Where closed-transition
switchgear is used, it
causes the short-term
paralleling of two, separate
utility services, and these
services may originate from
different sources within the
utility grid. In these cases,
coordination with the servicing
utility and their engineering
requirements is crucial to the
success of the project. The
serving utility will normally
require that the transition be
automatically supervised by
synchronism-check relays,
and that breaker interlocking
controls are provided to limit
the time duration of the parallel
operation (100 ms, or 6 cycles,
is a typical requirement, but it
does vary from utility to utility).
There are some utilities that
do not allow their services to
be paralleled for any length of
time at all, allowing only opentransition switchgear.
More common is the use of
closed-transition automatic
transfer switches. These
switches also have a makebefore-break sequence,
that allows the emergency
(typically, generator) source
to be paralleled with utility
power for a short period
of time (again, 100 ms is
a typical requirement),
during transfers to and from
generator power, where a
stable utility source is present.
The use of closed-transition
switches is especially popular
in hospitals, where testing
of generators and transfer
switches can be performed,
without interruption to hospital
activities, if closed-transition
equipment is used.
Effects on Short-Circuit Current Available
Many engineers are hesitant to
use closed-transition equipment.
This is largely due to the understanding that closed-transition
adds size and cost to the project,
due to the need for increased
fault duty ratings. This engineering opinion is based on their
interpretation of the NEC, and
specifically Sections 110.9 and
705.16 of the NEC, which state:
• “110.9 Interrupting Rating.
Equipment intended to interrupt current at fault levels shall have an interrupting rating not less than the
nominal circuit voltage and
the current that is available
at the line terminals of the
equipment.” [NEC 2011]
• “705.16 Interrupting and
Short Circuit Current Rating. Consideration shall be
given to the contribution of
fault currents from all interconnected power sources
for the interrupting and
short-circuit current ratings
of equipment on interactive systems.” [NEC 2011]
Taken on their face, these references seem to imply that the
equipment specified, whether
closed-transition switchgear
or closed transition transfer
switches, and all of the equipment located downstream of
these closed-transition devices,
should be rated to interrupt the
full available fault current of all
utility and/or generator sources
that may be connected during
a closed transition switching
procedure. However, digging
deeper starts to reveal a picture
that is not as clear-cut.
In reviewing the NEC Handbook,
sections 110.9 and 110.10 are a
matched pair of requirements.
Under the code commentary
for 110.10, it is noted that,
“Literature on how to calculate
short-circuit currents at each
point in any distribution system
generally can be obtained by
contacting the manufacturers of
overcurrent protective devices
or by referring to IEEE 141-1993
(R1999), IEEE Recommended
Practice for Electrical Power
Distribution for Industrial Plants
(Red Book).” Furthermore, the
code commentary for 705.1
notes that, “Article 705 sets
forth basic safety requirements
for the installation of generators and other types of power
production sources that are
interconnected and operate in
parallel as distributed generation.”
The NEC Code Making Panel has
reviewed proposed changes to
the wording of section 110.9 in
2002 and again in 2005 to specifically allow the short-circuit
rating to be exceeded in cases of
a (momentary) closed transition, and both proposals were
defeated. However, it is instructive to review the statement of
the Code Making Panel in their
rejection of the 2002 proposal,
“Complex systems design criteria
such as closed transition are inappropriate for specific inclusion in
the NEC. Existing sections, such
as 90-4 may be an appropriate
avenue to deal with such issues.”
Therefore, the NEC has specifically taken no position on the
proper way to conduct a shortcircuit study, but merely is stating that the results of the study
need to properly inform the
selection of equipment with regards to short-circuit withstand
and interrupting currents.
Although the NEC commentary
invokes IEEE 141 as an appropriate standard for the performance of a short-circuit study,
that standard is silent on the
issue of how to handle closedtransition between of sources, as
is IEEE 242-2001 “IEEE Recom3
Effects on Short-Circuit
Current Available (continued)
mended Practice for Protection and Coordination of Industrial and Commercial
Power Systems” and IEEE 399-1997 “IEEE
Recommended Practice for Industrial
and Commercial Power Systems Analysis”. In fact, the only standard that does,
indeed directly address the issue is IEEE
666-1991 (R2007) “IEEE Design Guide
for Electric Power Service Systems for
Generating Stations”. In this standard,
Part 4.6.1 states that, “The major concern when paralleling both sources is
fault current, which will be larger than
that calculated for a single source. However, it is acceptable practice to design
for the single-source condition if the
duration of parallel operation is short.”
Although this paragraph is specifically
for manual transfer schemes, Part 4.6.2
regarding automatic transfers does
incorporate Part 4.6.1.
The IEEE 666 standard, while not directly
applicable to facilities other than Generating Stations, is the only standard that
does directly address the issue of closed
transition versus fault current, and as
such, is the most appropriate standard
to apply in the design of closed-transition systems. Based upon this, it is a
reasonable practice to ignore the contribution of parallel sources in properly
supervised closed-transition schemes
when the design considerations on the
next page are included.
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Design Considerations
In design of a closed-transition system, it is
imperative that the system be designed with
interlocks that prevent the inadvertent and indefinite paralleling of sources. There are several
design rules that should be used in the design of
closed-transition systems:
1. Closed-transition switchgear should be
designed such that manual transfers are
manually initiated and automatically interlocked. This will prevent the utility sources
from being paralleled for an excessive
amount of time. This transition should take
place in the range of 100 ms, depending
upon specific utility requirements.
2. Closed-transition switchgear should be
designed with a non-defeatable safety
circuit timing relay, which will cause source disconnection
within a pre-determined time if the sources are manually
paralleled, or the closed-transition interlocking scheme fails to
perform. (Example – A timing relay that opens the tie breaker
if both utility mains are closed for 10 seconds would serve this
function).
A. Transfer schemes for closed-transition switchgear shall be
designed such that a transfer cannot be initiated into a
down stream fault condition.
3. Where closed-transition automatic transfer switches are speci-
fied, a shunt-trip circuit breaker on the emergency feeder could
be specified to force the emergency feeder to open in the case
of a failed closed transition.
4. All of the functions noted in 1-3 above should be alarmed and
annunciated.
Closed-transition systems, due to their nature and the required supervision capabilities are somewhat more expensive than open-transition systems. However, the cost of these systems, where properly
designed, need not be prohibitive.
Summary
Based on our review of applicable codes and
standards, there is no need to consider both
sources of fault current when sizing equipment
for closed-transition transfer schemes. However,
when making this determination, it is important
that the proper electrical interlocking or other
supervision techniques are used to ensure that
the system cannot be inadvertently be placed
into a maintained parallel state, which would
then require that the equipment be sized for the
combined parallel sources. This will assure a safe,
cost-effective installation, in compliance with the
requirements of NEC, and consistent with the
guidance provided by IEEE.
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