ABB Automatic Transfer Switch OTM_C_21D
1.1 Use of symbols
Risk of Electric shock: warns about a situation where a hazardous voltage may cause phys ical injury to a person or damage to equipment.
General warning: warns about a situation where something other than electrical equip ment may cause physical injury to a person or damage to equipment.
Caution: provides important information or warns about a situation that may have a detri mental effect on equipment.
Information: provides important information about the equipment.
Table 1. Explanations of abbreviations and terms
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1.2 Explanations of abreviations and terms
OTM_C21D Automatic transfer switch, the type name
LN1-Switch I Power supply line, e.g. the primary line
LN2-Switch II Power supply line, e.g. the secondary line used in emergency cases
EMRG OFF Used to drive the automatic transfer switch transfers to the “O” position when
receiving EMRG OFF signal.
AUTO Automatic mode
Remote test A sequence to test the functionality of the automatic transfer switch
Ts Transfer delay
TBs Return delay
OV Adjustable overvoltage threshold
UV Adjustable undervoltage threshold
The OTM_C21D automatic transfer switch can be
used as a source transfer switch in a three-phase
or single-phase networks. Monitored conditions
are, no-voltage, phase-loss, overvoltage and un dervoltage detection, transfer delays, generator
start and stop, and remote test function. Source
transfer can be performed using a manually
The standard package includes:
1. automatic transfer switch, 2. handle, 3. handle storage clip 4. terminal plug, 5. mounting kit
Figure 1. OTM_C_21D automatic transfer switch
1. Handle for manual operation
2. Place for auxiliary contact blocks
3. Push button
4. Mimic panel
5. Voltage sensing connections
6. Locking clip for padlock
7. Locking latch for releasing the handle and
locking electrical control
8. Locking clip for locking manual operation
9. Dip switches
10. Rotary switches
11. Connecting terminal
12. Modbus RTU access
operated handle, locally using push buttons or
fully automatically. The automatic mode includes
several operating methods: Line 1 priority, no line
priority and manual back switching mode. OTM_
C21D can achieve communication function by
using the optional external Modbus RTU module.

The transfer sequence of OTM_C21D can be sum marized in following steps:
• An anomaly occurs on the Line 1 (LN1)
• Generator start, immediately start generator in
case of black-out, phase-loss, overvoltage or
undervoltage (If Generator mode is selected)
• The Line 2 start the normal functioning,
transfer delay Ts
• Change-over switch (Switch I) to the position 0
• Change-over switch (Switch II) to the position II
And the return sequence can be summarized in
the following steps:
• The Line 1 will start the normal functioning
• Return delay TBs
• Change-over switch (Switch II) to the position 0
• Change-over switch (Switch I) to the position I
• Generator stop delay Gs
• Generator stop
The transfer sequence of OTM_C21D can be sum marized in following steps:
• An anomaly occurs on the Line 1 (LN1)
• Transfer delay Ts, in case of black-out, phase loss, overvoltage and undervoltage conditions
• Change-over switch (Switch I) to the position 0
• Change-over switch (Switch II) to the position II
And the return sequence can be summarized in
the following steps:
• The Line 1 will start the normal functioning
• Change-over switch stays in position II
• An anomaly occurs on the Line 2 (LN2)
• Return delay TBs
• Change-over switch (Switch II) to the
position 0
• Change-over switch (Switch I) to the position I
The transfer sequence of OTM_C21D can be sum marized in following steps:
• An anomaly occurs on the Line 1 (LN1)
• Generator start, immediately start generator
in case of black-out, phase-loss, overvoltage or
undervoltage (If Generator mode is selected)
• The Line 2 start the normal functioning,
transfer delay Ts
• Change-over switch (Switch I) to the position 0
• Change-over switch (Switch II) to the
position II
Figure 4. Automatic Switching Sequences in OTM_C_21D, Manual return mode
And the return sequence can be summarized in
the following steps:
• The Line 1 will start the normal functioning
• Change-over switch stays in position II
• An anomaly occurs on the Line 2 (LN2)
• Change-over switch stays in position II
• Change-over switch can be transferred
manually back to position I

To operate the switch manually:
1.Attach the handle to the switch panel. You can
attach the handle in any position.
2.When the handle is attached, the automatic
transfer switch will automatically be in Manual
mode and won’t operate automatically in case
of line failure. The AUTO LED on the mimic panel
is OFF.
Figure 5. Operating the switch manually
When the handle is inserted into the switch, the switch will enter “manual mode” with the
automatic operation disabled.
Do not adjust wires when the transfer switch is being energized.
Before the power-on operation of the transfer switch, please operate the switch manually
to confirm it is in normal function.
With the power supply function in “normal” and without the handle inserted and EMRG OFF
signals, the initially energized switch will enter automatic mode and transfer to the main
line. Keep the handle inserted if you do not want the switch to be in automatic mode upon
initial energization.
OTM_C21D must be in automatic mode and the
“AUTO” LED is on in order that the switch can
perform automatic transfer cycles according to
the pre-set operating mode.
To operate the switch electrically:
• If the handle inserted
1. Press handle locking clip and remove the handle
from the switch.
2. Press “AUTO” button and the “AUTO” LED will
be ON, indicating automatic mode.
Figure 6. Selecting the automatic transfer OTM_C_21D, switch to Auto Mode
2 1
3
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3.2 Automatic operation
• If handle is not inserted
1. If “AUTO” LED blinks, press “AUTO” button and
the “AUTO” LED will be ON, indicating auto matic mode.
2. Automatic operation includes three operating
modes: Line 1 priority (factory default setting),
No line priority, and manual return mode.