8 S&C Instruction Sheet 515-600
Overview
The Micro-AT Source-Transfer Control In Weatherproof Enclosure ensures a high
degree of critical-load continuity for primary-selective overhead distribution systems
by minimizing interruptions resulting from the loss of one source. Excluding the inten-
tional time delay to coordinate with upstream protective devices and/or transition dwell
time
●
, transfer is achieved in 1.5 seconds maximum (when Type AS-1A Switch Opera-
tors are used) or 2.4 sec onds maximum (when Type AS-10 Switch Operators are used).
Under normal operating conditions, the preferred-source interrupter switch is closed
and the alternate-source interrupter switch is open. See Figure 3.
The Micro-AT control monitors the condition of both power sources and initiates
automatic loss-of-source transfer switching when preferred-source voltage has been
lost (or reduced below a predetermined level) for a period of time sufficient to confirm
that the loss is not transient. The preferred-source interrupter switch is automatically
opened and the alternate-source interrupter switch is then automatically closed, restor-
ing service to the load.
Following a loss of the preferred source that has resulted in a transfer away from
the preferred-source and then to the alternate-source, the preferred-source interrupter
switch is open and the alternate-source interrupter switch is closed. While in this condi-
tion, if the preferred source voltage meets the criteria for being considered normal but
has not yet been normal for a sufficient period of time to satisfy the return-of-source
time setting and the alternate-source is lost (or reduced below a predetermined level)
for a period of time sufficient to confirm that the loss is not transient, the Micro-AT
control will initiate a loss-of-source transfer. The alternate-source interrupter switch
is automatically opened and the preferred-source interrupter switch is then automati-
cally closed, restoring service to the load.
Both types of loss-of-source transfers in the common-bus primary selective system
are always in “open transition.” The primary reason for the loss-of-source transfer always
being in “open transition” is to best ensure that a fault on the distribution system is not
fed by both sources simultaneously. In addition to the open transition loss-of-source
transfer providing protection for closing both sources into a fault on the distribution
system, the use of the transition dwell time can be made to further minimize the risk
of creating a system fault condition by closing a source interrupter switch to connect
a large motor load with the motor load out of synchronization during completion of a
loss-of-source transfer.
Load
Preferred source
Alternate source
S&C Micro-AT Source-Transfer
Control in a weatherproof enclosure
S&C Type AS-1A or Type
AS-10 Switch Operator
Figure 3. System diagram, normal configuration.
Voltage
transformer(s)
STC
SO
SO
●
An adjustable time delay to allow motor residual voltage—the voltage appearing at the terminals of a
connected motor when the source is inter rupted—to drop sufficiently before service is restored .
Alduti-Rupter Switch
