Power Distribution Units (PDU)

Keep it Simple

Keep it Simple (KIS) is the best method of alerting on Power Distribution Units (PDU) or power strips.  PDUs can and do produce more alerts than any other device in the data center.   Therefore, take extra effort to simplify PDU alerts. 

Let’s look at an example of how many alerts a single PDU can generate if the default alerts are left unchanged.  For this example, we will assume a 3 phase PDU with 48 outlets.  Each outlet triggers an alert on Volts, Amps, and Watts for each phase of the PDU. 

  • 48 Outlets x 3 phases = 144 alerts x volts, amps, and watts = 432 alerts per PDU
  • If a customer has 100 PDUs, that is 4,320 alerts during an event.
  • 4,320 alerts are too many – the system will be ignored by most users

The method discussed below will only produce 1-3 alerts per event per PDU.

What to Monitor on a PDU

Since breakers and their trip ratings are in amps, the logical alerting method for PDUs is in amps.   Alerting on input amps and each output breaker will tell us most of what we need to know.   Measuring the input breaker high and low thresholds will tell us if the breaker is about to trip, or if the input breaker loses load.   The output branch breakers high and low thresholds will notify us if a branch breaker trips (low threshold), or if the breaker is nearing its trip rating (high threshold).

PDU Alert Best Practice Alert on Input Current (Amps) Alert on Branch Output Breaker Current (Amps) Notes
Input Power Complete Outage
A complete outage removes power from the PDU network card, so no alarms are sent. Alert on UPS output failure is a better option.
Input Power Partial Outaage
Assumes PDU network card is still powered.
Input Breaker Near Tripping
Output Power Complete Outage
Output Power Partial Outage
Output Power Breaker(s) Near Tripping

Setting Thresholds for Breakers

North America breakers are typically designed to trip at 80% of the breaker rating.  For example, a 30-amp breaker is designed to trip at 24 amps.   Internationally breakers are smaller and are typically rated at full capacity.  An International 20-amp breaker is designed to trip at 20 amps.  When setting thresholds, it is important to understand your breakers’ trip rating.  If you are unsure of your breaker rating take the more conservative approach and assume the breaker will trip at 80% of the rating.

In the example below the input plug is an L5-30, we will assume the breaker is designed to trip at 80% rating or 24 amps.   The two output breakers rated at 20 amps thresholds will be set to trip starting at 16-amps.   

We will want to know if the breaker is about to trip in advance of the breaker trip rating.  Therefore, we will set a warning threshold at 80% of the breaker trip rating.  For example, if the input trip rating is 24 amps, we will set a warning alert at 19 amps.  We will set a critical threshold at 24 amps.   These settings will send alerts when the breaker is near tripping a breaker. 

Setting a lower threshold will tell us if a breaker is no longer producing power to the load.   For example, if output breaker 1 normally runs at 5 amps of load, then suddenly drops to zero amps, breaker 1 most likely tripped.   Set lower thresholds after load is applied.  Otherwise the lower thresholds will send alarms since there is no load on the PDU.

Other Alerts

The temptation is to alert on other advanced items, but keep in mind doing so will generate too many alerts. Below are recommendations for other potential PDU alerts. 

Advanced Alerts Notes
Frequency Variation
Best managed at the UPS level.
Power Factor
Best managed at the UPS level.
Phase Balance
Best if viewed only using trend charts - alerts tend to be noisy. Use phase balance to load PDU balanced.
Amps are better because breakers are rated in amps - no conversion to watts are necessary
Alerting on volts at the output breaker level provides value when the power strip has no load. A loss of volts at the branch breaker level will notify of a branch breaker trip.

Strategies for Limiting Alerts

Alerts vs. Views

Alerts are push notifications that are sent via e-mail or text.  An alternate to alerts is view information that does not push an e-mail or text.   In some circumstances viewing information is more productive than creating an alert.   For example, help desk may prefer to monitor state changes by monitoring color changes without receiving an alarm – green is ok, red is in alarm.

Below is an example of monitoring by color changes.

PDU Load Balancing

Load balancing and temperature trending are examples of data that may be better referenced in a view vs. sending alerts.  The fewer the alerts, the more likely users will watch the system.  Load balancing will be most helpful when loading up the racks.  The better the load is balanced the more capacity the system will allow.

Raising Temperature Thresholds

Computer rooms that run hot with no immediate resolution trigger unwanted alarms.  You know the room is hot, and you plan to add cooling in the future.  Until the cooling is added, removing or raising temp thresholds will allow the system to remain in a green state.

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