S&C Instruction Sheet 515-600 11
Operating Description
●
Factory-set at 85 .0 Volts .
■
Factory-set at 2 .00 seconds .
▲
Factory-set at 105 Volts .
◆
Factory-set at 3 .00 minutes .
With adequate voltage available from both utility sources, the preferred-source interrupter
switch should be closed and the alternate-source interrupter switch should be open with
its associated circuit available as a standby. The manual/automatic operation selector
switch should be set to
Automatic
mode and—if the
Supervisory Control
option is
enabled—the supervisory manual/automatic dry contact is closed and the left source
voltage, right source voltage, and automatic-transfer READY indicating lamps should
be lit. See the “Conditions required to light automatic-transfer READY indicating lamp”
section on page 32.
At installations using single-phase source voltage sensing, the Micro-AT control
continuously monitors the signal-input voltage level on phase 2 of each of the two sources
and compares these inputs to the predetermined refer ence level to determine the status of
each source. The control will initiate a loss-of-source transfer when each of the following
conditions exist:
• The signal-input voltage from phase 2 of the source serving the load is reduced below
the predetermined loss-of-source voltage setting
●
,for a period of time considered
sufficient to confirm the condition is not transient—the predetermined loss-of-source
time setting
■
• The signal-input voltage from phase 2 of the standby source exceeds the predetermined
return-of-source voltage setting
▲
• The
Overcurrent Lockout
feature, if programmed, is not “set up” to prevent an auto-
matic transfer opera tion that would close a source interrupter switch into a fault (See
the “Overcurrent Lockout Condition” section on page 12.)
At installations using three-phase source voltage sens ing, the Micro-AT control con-
tinuously monitors the signal-input voltage level on each phase of the two sources and
compares these inputs to the predetermined reference level to determine the status of
each source. The control will initiate a loss-of-source transfer when each of the follow-
ing conditions exist:
• The signal-input voltage from one or more phases of the source serving the load is
reduced below the pre determined loss-of-source voltage setting
●
for a period of time
considered sufficient to confirm the condition is not transient—the predetermined
loss-of-source time setting
■
• The signal-input voltages from all three phases of the standby source exceed the pre-
determined return-of-source voltage setting
▲
• The
Overcurrent Lockout
feature, if programmed, is not “set up” to prevent an auto-
matic transfer oper ation that would close a source interrupter switch into a fault (See
the “Overcurrent Lockout Condition” section on page 12.)
In addition, at installations using three-phase source voltage sensing, if the
Unbalance
Detection
feature is pro grammed, the control will initiate a transfer as a result of an
open-phase condition when the system unbalance exceeds the predetermined unbalance-
detection voltage. See the “Transfer on Unbalance Condition” section on page 12.
If a loss-of-source transfer occurs, the automatic-trans fer READY indicating lamp will
extinguish—indicating the normal condition no longer exists.
Upon return of preferred-source voltage for a period of time sufficient to establish
that the return is not temporary—the predetermined return-of-source time setting
◆
,
automatic return-of-source transfer to the normal condition will occur if the control
has been programmed for
Automatic Return
mode. The return-of-source transfer will
either be a “closed transition” or an “open transition,” depending on the control settings.
With a “closed transition” return, the preferred-source interrupter switch will close
before the alternate-source interrupter switch is opened, so that there is no interruption
of service to the load. With an “open transition” return, which prevents an automatic
operation that would parallel the power sources, the alternate-source interrupter switch
will open prior to closing of the pre ferred-source interrupter switch.
Normal Condition
Transfer on
Loss-of-Source
Voltage
Transfer on Return-of-
Source Voltage
