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Approved Scenarios and Scenarios Not Approved for Dual DC Power Supply Configuration
You can connect a single DC power source to either the A input or the B input. If there are dual power sources, connect one
source to the A input and one source to the B input. Both sources must be the same polarity (with respect to ground) and voltage
(within 0.25 volts). Do not connect –DC grounded and +DC grounded dual sources to Cisco 2811, Cisco 2821, and
Cisco 2851 series integrated services routers.
Dual sources with opposite-polarity grounding damage equipment.
Caution
In Figure 21, either the positive source terminal or the negative source terminal is tied to ground.
Figure 21
Connecting to One Source Only—Source A or Source B
A-
+
A+
B+
B-
In Figure 22, source A and source B share common negative terminal connections.
Figure 22
Connecting Source A and Source B with Common Negative Terminals
Va
+
+
Vb
In Figure 23, source A and source B share common positive terminal connections. This is allowed only if Va equals Vb (within
0.25 V).
When connecting source A and source B with common positive terminals, if source A and source B voltages are
Caution
unequal by more than 0.25 V, the higher-voltage source can discharge into the lower-voltage source through the A-
and B- input terminals. Excessive discharging currents through these terminals can cause one or both of the dual
input DC power supply's internal A- or B- fuses to open, resulting in lack of redundancy or system failure. When
source A and source B are within 0.25 V, discharge current is minimal.
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+
A-
A+
B+
B-
A-
A+
B+
B-