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It discusses an example of a "fully loaded" Sun Fire[TM] 15K server, and explains the theories behind the answer to the question 'how many supplies are enough?'.

The principles herein can be applied to all servers with N+1 Power Supply Redundancy, though it is specifically targeted at those with dual power grid AC infeed.

NOTE: The E10000 only supports dual grid power with the optional Dual Grid Power installation.

If the customer is aware of a PLANNED partial power outage, it is highly recommended to either power down affected systems -OR- bring them to the OBP prompt to faciliatate rapid return to operation once the planned partial power outage work has been completed. However, should the customer absolutely need to keep their system operational during a planned partial power outage, the information contained in this document will help to determine if such action is possible. Ultimately, the decision to keep the system powered up will rest with the customer after all factors are considered.

• 6 AC power inputs from one Power Distribution Unit (PDU), which we will call PDU0 for this example.
  One input is connected to 'AC0' of each of the 6 AC Power Supplies.
• Another 6 power inputs from a different PDU, PDU1 for this example.
  One input is connected to 'AC1' of each of the 6 AC Power Supplies.

Ideally, each of the PDUs should be supplied from different Uninteruptable Power Supply (UPS) units, and different utility power grids.

See the "Sun Fire 15K/12K Systems Site Planning Guide" (806-3510-14) chapter 3 for details. 

The Sun Fire 15K power system (using A141 power supplies) has been architected to be able to survive a complete power loss to one of its input power sources. (ie: Input 0 on all 6 power supplies, or one complete PDU).

In the event of an external power loss, it is possible to have the system remain operational even with the loss of up to half of the external power cords or circuits (a whole PDU). A site may need to power off one of their power distribution units for maintenance and the question arises whether the system will stay running during the planned (or unplanned) power source outage.

This may place a greater than expected load on the remaining functional PDU, if it has not been sized appropriately. Each PDU should be sized to be able to supply the required output to power the entire 15K frame.

There is no direct way to determine what the current demand is on each of the external power cords. However, one can make some simple calculations to figure what an internal DC power supply demand approximately translates to in external AC circuit terms.

Data obtained via showenvironment or similar interrogation commands will display some useful information regarding the power input to the system. Be aware the current values shown by the showenvironment command are the 48VDC ones.  Several items must be kept in mind when observing these indications.
Among them :

1. Power supply current demands are what is used at the DC voltage of the supply. For example - a 20.32A demand for PS0 is obtained at 48.80VDC.  There may be a second amperage demand for PS0 if a second AC input cord is connected. If the second DC current indicated is also 20.32A, then total current draw for that power supply is 40.64A measured at 48.80VDC.

2. To determine what the facilities electrician will measure on the AC power cords connected to PS0, we must do some simple calculations.
For AC-0 input to PS0 :
DC voltage is 48.80 VDC
DC current demand (current0) is 20.32A
DC current demand (current1) is 20.32A
Power supplied via current0@48.80VDC is : 48.80VDC x 20.32A = 991.62W
Power supplied via current1@48.80VDC is : 48.80VDC x 20.32A = 991.62W

AC equivalent is found by dividing the DC wattage by the AC line voltage.  For purposes of our example, we will assume a steady 208VAC.

Therefore:

Approximate AC current input on AC-0 is : 991.62W / 208VAC or 4.77A.
Approximate AC current input on Ac-1 is : 991.62W / 208VAC or 4.77A

3. The above calculated AC input currents are approximate, as we have not compensated for factors such as power supply inefficiency. However, the calculated AC current input should be sufficient for planning purposes.

4. As an aside, if you note an obviously abnormal or disproportionate variation between the two current readings of any power supply, it would be wise to
further investigate if a power supply problem is starting.

5. Once we have determined what can be approximately expected on the external AC power cords, one can perform an assessment of the external power sources for sufficient capacity to handle the equipment under any operating condition.

Let's look at a real world example to determine what the impact would be if one of the external AC power sources were to fail. Basically, we need to determine if the system will stay up.

For purposes of this example, it is assumed the external power source is a well-regulated UPS providing steady 208 V AC power to the equipment circuits.

For purposes of our example, the Sun Fire 15K (or similar) derives power from two power distribution units. Power from PDU-0 are supplied to the AC-0 input of each DC power supply and power from PDU-1 are supplied to the AC-1 input of each DC power supply.

Situation is as follows: The customer needs to power down PDU-1 for some emergency maintenance. They want to know if the SF15K needs to be powered off during their maintenance or if they can remain operational. While it is highly recommended to power off systems during a planned partial power outage, this is not always possible due to operational demands on the system. If the customer absolutely needs to keep their system running (if possible), then details contained in this document will help them to make an informed decision to do so.

F15K Frame#3

POWER           VALUE     UNIT    STATUS
---------       -----     ----    ------
PS0
Current0       20.32     A       N/A
Current1       20.32     A       N/A
48VDC          48.80     V       N/A
Power          1983.23   W       N/A       

Translates to external power cord readings of : 4.77A @ 208V AC (AC0 input)
4.77A @ 208V AC (AC1 input)

PS1
Current0       20.32     A       N/A
Current1       21.91     A       N/A
48VDC          49.60     V       N/A
Power          2094.61   W       N/A       

Translates to external power cord readings of : 4.85A @ 208V AC (AC0 input)
5.22A @ 208V AC (AC1 input)

PS2
Current0       21.51     A       N/A
Current1       20.32     A       N/A
48VDC          49.40     V       N/A
Power          2066.40   W       N/A       

Translates to external power cord readings of : 5.11A @ 208V AC (AC0 input)
4.83A @ 208V AC (AC1 input)

PS3
Current0       20.71     A       N/A
Current1       21.51     A       N/A
48VDC          48.80     V       N/A
Power          2060.34   W       N/A       

Translates to external power cord readings of : 4.86A @ 208V AC (AC0 input)
5.05A @ 208V AC (AC1 input)

PS4
Current0       21.91     A       N/A
Current1       20.32     A       N/A
48VDC          49.40     V       N/A
Power          2086.16   W       N/A       

Translates to external power cord readings of : 5.20A @ 208V AC (AC0 input)
4.83A @ 208V AC (AC1 input)

PS5
Current0       22.71     A       N/A
Current1       22.31     A       N/A
48VDC          48.80     V       N/A
Power          2196.98   W       N/A       

Translates to external power cord readings of : 5.33A @ 208V AC (AC0 input)
5.23A @ 208V AC (AC1 input)

Total Power     12487.72  W       N/A

Under fault conditions (ie: loss of one of two PDU feeding the frame) the current normally supplied from one PDU or the other will be shifted onto the surviving power source.

As an example, assume the entire frame is powered from two power distribution units. Further assume the second PDU experiences a fault and drops power.

All the AC-1 input current demands would shift to the surviving AC-0 power source.

Examining this further :

PS0 AC-0 current demand shifts from 4.77A@208V AC to 9.54A @208V AC
AC-1 current demand shifts from 4.77A@208V AC to zero (loss of power)

PS1 AC-0 current demand shifts from 4.85A@208V AC to 10.07A @208V AC
AC-1 current demand shifts from 5.22A@208V AC to zero (loss of power)

PS2 AC-0 current demand shifts from 5.11A@208V AC to 9.94A @208V AC
AC-1 current demand shifts from 4.83A@208V AC to zero (loss of power)

PS3 AC-0 current demand shifts from 4.86A@208V AC to 9.91A @208V AC
AC-1 current demand shifts from 5.05A@208V AC to zero (loss of power)

PS4 AC-0 current demand shifts from 5.20A@208V AC to 10.03A @208V AC
AC-1 current demand shifts from 4.83A@208V AC to zero (loss of power)

PS5 AC-0 current demand shifts from 5.33A@208V AC to 10.56A @208V AC
AC-1 current demand shifts from 5.23A@208V AC to zero (loss of power)

The surviving power source (PDU#1) has a sudden increase of 29.93A @208V AC
or a load increase of 6225.44Watts (half of the 12487.72 total load)

Possible conclusions :

• The loss of an entire Power distribution unit may permit the system to stay powered up without any loss of operation.
  Reason(s): The surviving power distribution unit has sufficient capacity to accept the sudden shift of power demand - including sudden power demand shifts of other equipment previously powered by the faulty power distribution unit.
  Caveat(s): The surviving power distribution unit may now be heavily loaded and raise concerns with powering large pieces of equipment on. Powering a piece of equipment on can create a temporary large demand that can exceed the capacity limits of a power distribution unit.

• The loss of an entire Power Distribution Unit may not allow the system to stay powered up.
  Reason(s): The surviving power distribution unit may have already been loaded to near capacity - now with a second power distrbution unit failing, the added load is in excess of tolerable limits.
  OR
  The UPS utilized for the surviving power source may not be able to handle the abrupt load change to maintain acceptable power to critical equipment.

• Recommendations:
  • Do not overload power distribution units within the data center.
    Per National Electric Code, a power source can be loaded up to 80% for continuous running purposes. Computer equipment needs and power failover redundancy can be cause to drastically reduce the overall loading of any given power distribution unit.
  • Consider what equipment may shift load demand to a given power distribution unit. Total the current or wattage demand that may be abruptly imposed to determine if the target power distribution unit and/or associated UPS is able to accomodate the added load.
  • Else - plan on a power-related equipment outage.

• Assuming there is adequate reserve capacity on the surviving power distribution unit and the abrupt load shift of other equipment does not exceed the capacity of the surviving power distribution unit, all will be OK.

• Once repairs have been made to the faulty power distribution unit, it can be returned to service. Current demand will again be shared between the AC-0 and AC-1 inputs as originally noted. The system can continue running during the entire switchover without loss of operation.

• If the system has been powered down for the planned power work or if the system has been brought down to an OBP level, then either re-power the system or boot the system back to operational status.

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