How accurate is the current monitor on my APC Rack Power Distribution Unit (PDU) and Rack Automatic Transfer Switch ?

Issue
How accurate is the current monitor on my APC Rack Power Distribution Unit (PDU) and Rack Automatic Transfer Switch (ATS) ?


Product Line

• Rack Power Distribution
  • Models beginning with AP7XXX(B)
  • Models beginning with AP8XXX
• Rack Automatic Transfer Switch
  • Models beginning with AP44XX

Envrionment

• All serial numbers
• All firmware versions (unless otherwise noted)

Cause
This article outlines the accuracy for each of our Rack PDU generations and the 4th generation Rack ATS models. Sometimes this information is requested for general knowledge or to help explain why certain values are being observed.


Resolution
Refer to the information below for the AP7XXX and AP8XXX PDU models and AP44XX ATS models:
Rack PDUs (AP7XXX - first generation)
Rack PDUs (AP8XXX - second generation)
Rack ATSs (AP44XX - fourth generation)
 

---

Rack PDUs (AP7XXX - first generation) are tested at a current load of 17.5 amps at Crest Factor of 3.0 to Outlet #1 (phase L1).

Below are the accuracy specifications in regards to the load: 

Range	Accuracy
0-1A	+/- 1.0A
1-10A	+/- 0.5A
>10A	+/-5% A

Also, as of Rack PDU (rpdu) firmware >3.3.1, attached loads < 1 amp will always display as 0 amps on the PDU's LCD and network management card, regardless of the load. Lastly, readings via the Rack PDU's network management card will give measurements in decimal format. ie. 1.6 amps as opposed to 2 as seen on the Rack PDU's LCD.


 

---

Rack PDUs (AP8XXX and AP7XXXB- second generation) have an accuracy of +/- 3% of reading, +/- 1 least significant digit across the entire power and temperature range.
 Note: Accuracy is not defined below 0.5A.


rpdu2g firmware v5.1.1 and below:
Phase and bank current measurements less than 0.5 amps, and corresponding power measurements, are rounded down to zero. 

 

For AP84XX and AP86XX SKUs with individual outlet metering, outlet current measurements less than 0.5 amps and outlet power measurements less than 50 watts will be rounded down to zero.

rpdu2g firmware v5.1.2, v5.1.6:

• Phase/Bank/Outlet Current - squelched to 0A under 0.5A.
• Device/Phase/Outlet Power - squelched to 0 watts if corresponding current is squelched.
• Outlet Energy - Will NOT be accumulated if corresponding outlet power/current is squelched.
• Device Energy - Will always accumulate, since the unit does consume a very small amount of energy within the internal circuitry.
   

rpdu2g firmware v6.0.9 and above:

• Outlet Current - squelched to 0A under 0.3A. Note: Accuracy is not defined below 0.5A.
• Phase/Bank Current - squelched to 0A under 0.5A.
• Device/Phase/Outlet Power - squelched to 0 watts if corresponding current is squelched.
• Outlet Energy - Will NOT be accumulated if corresponding outlet power/current is squelched.
• Device Energy - Will always accumulate, since the unit does consume a very small amount of energy within the internal circuitry.

 

Functional Specifications - Metering
Input Metering Range	0.5 to Rated Input Current
Outlet Metering Range	0.3 to 16.0A
Allowable Crest Factor	1.75
Accuracy (Phase Current)	+/- (3% + 1 digit)
Accuracy (Phase Voltage)	+/- (3% + 1 digit)
Accuracy (Phase Power)	+/- (3% + 1 digit)
Accuracy (Phase Energy)	+/- (3% + 1 digit)
Accuracy (Outlet Current)	+/- (3% + .1Amp)
Accuracy (Outlet Voltage)	+/- (3% + .1Volt)
Accuracy (Outlet Power)	+/- (A reading * V error) + (V reading * A error)
Accuracy (Outlet Energy)	+/- (A reading * V error) + (V reading * A error)*hours
Accuracy (Bank Current)	+/- (3% + 1 digit)

Note: It is possible that a higher level measurement will show a load (such as bank or device), even when a lower level measurement is showing zero. (For instance, the phase current could show 0.8A, even though the bank currents are squelched to zero.)
 

---

Rack ATSs (AP44XX - fourth generation) have an accuracy of +/- 3% of reading, +/- 1 least significant digit.

ats4g firmware 6.4.6 and above:
Current measurements of less than 0.5 amps and corresponding VA measurements are rounded down to zero.
Note: The current ATS firmware is mistakenly squelching measurements under 1A, thus showing 0A. This will be addressed in a new firmware release.

• Load/Bank Current - squelched to 0A under 0.5A.
• Output Power - squelched to 0 VA if corresponding current is squelched.

Functional Specifications - Metering
Input metering range	0.5A to Rated Input Current
Total current metering range	0.5A to Rated Input Current
Per bank metering range (2U units)	0.5A to Bank Rated current
Allowable crest factor	1.4 - 1.75
Allowable power factor	0.9 - 1
Accuracy (Source Current)	± (3% + 1 digit)
Accuracy (Source Voltage)	± (3% + 1 digit)
Accuracy (Output Current)	± (3% + 1 digit)
Accuracy (Bank Current - 2U units)	± (3% + 1 digit)
Accuracy (Apparent Power VA)	± (3% + 1 digit)
Allowable frequency deviation	3/5/10Hz

Note: If you are connecting a small load to the ATS and it is reporting 0A it is likely the ATS is squelching the value as per the explanation provided above. Increase the load attached to the ATS output and the unit will start showing values higher than 0A.


How to verify your PDU or ATS NMC firmware version:
Navigate to the Administration tab->General->About page or the About->Network in the web UI or via local/telnet/SSH interface(s) by logging in and checking the top right of the screen or entering the about command in the CLI.