Compaq ProLiant BL e-Class System Overview and Planning

Compaq ProLiant BL e-Class System Overview and Planning 22
The Domino Effect
If a power supply, PDU, or AC source fails or is unplugged, the power load is automatically
re-distributed. The remaining AC source and PDUs must support 100% of enclosure power
requirements. If the resulting load exceeds the capacity of the remaining PDU, that PDU’s circuit
breaker may trip, resulting in a potential “domino effect”.
Similarly, if the resulting load exceeds the capacity of the remaining AC source, that AC source
may also fail.
Example
Figure 11 shows four fully loaded enclosures using two low-line voltage (100-120VAC) PDUs
(P/N 207590-D71). The configuration is not PDU redundant, but it appears to be power-supply
redundant. In reality, this configuration is susceptible to the domino effect if even a single power
supply fails.
Figure 11. Domino effect caused by low-line current overload
Normal Operation
At low-line, each fully loaded enclosure draws 10 A that are shared approximately 50/50%
between the enclosure’s two power supplies. The output circuit breaker on each PDU supports
5A, which is within the breakers’ 15 A limit. The PDU supports 4 * 5 = 20 A total which is also
within the breakers’ input limit of 24 A.
Failure scenario 1
If a single power supply fails or is unplugged (for example, power supply 2 in
enclosure 4), PDU 1 trips, followed quickly by PDU 2. This chain reaction occurs after
power supply 1 fails because PDU 1 must now support the full 10-A load of enclosure 4.
This is within PDU 1’s output circuit breaker limit of 15 A. However, PDU1’s total
output current becomes 5 + 5 + 5 + 10 = 25 A, which exceeds PDU 1’s input rating of 24
A. By exceeding the PDU’s input rating, the PDU’s input circuit breaker may potentially
trip. If the AC source is designed for 24 A, it is also at risk for tripping. If PDU 1 trips or
its AC source fails, the entire 40-A load of 4 enclosures falls on PDU 2 and its AC
source, which are likely to fail, resulting in power loss to all enclosures.