Three easily underestimated issues with data center PDUs ( Part 2)

Question 2: The “cumulative effect” of branch voltage drop

Phenomenon: The PDU input voltage measurement is qualified, but the voltage at the remote output port is lower than the device’s allowable range. As a result, the server power supply operates in an undervoltage state, shortening its lifespan and even causing frequent restarts.

Mechanism: Voltage drop = Current × Resistance. From the PDU input terminal to the farthest output port, the current needs to pass through multiple conductors, including the busbar, circuit breaker, internal wires, and output socket. Each segment has resistance, and each segment will generate a voltage drop.

At low current, the total voltage drop may be only a few tenths of a volt, barely perceptible to measuring instruments . However, under full load, the voltage drop can accumulate to several volts— an unacceptable deviation for server power supplies requiring ±5% voltage accuracy .

A more subtle problem is that the voltage drop in different branches may vary. Branches closer to the input line experience smaller voltage drops, while those farther from the input line experience larger voltage drops. This problem would go unnoticed if only the total input voltage is measured, without measuring the output voltage of each branch.

The purpose of PDU verification loads is to connect rack-mounted dummy loads to different output branches of the PDU, measure the terminal voltage of each branch under full load conditions, and calculate whether the voltage drop is within the allowable range (typically required to be ≤3% of the input voltage). Fude Electronics’ rack-mounted loads support multi-module parallel expansion, and each unit is compact and can be flexibly deployed in different rack locations. In a smart computing center project, Fude Electronics deployed 426 rack-mounted load banks at once, achieving rack-by-rack verification of the entire power supply system .

cumulative effect ” of multiple branches carrying loads simultaneously

Phenomenon: During branch-by-branch testing, the current and voltage of each branch are within acceptable ranges. However, when all branches are simultaneously under full load, the total current exceeds the design value, causing the circuit breaker to trip or the busbar temperature to rise excessively.

Mechanism: The design capacity of the PDU is calculated based on the condition that “all branches are simultaneously fully loaded”. However, in branch-by-branch testing, only one branch is operating at full load at any given moment, while the other branches are unloaded or lightly loaded. This testing method cannot verify two key indicators:

Total current carrying capacity : Does the total current after the sum of the currents of all branches exceed the design capacity of the PDU input terminal or busbar?

Heat superposition effect: The heat from each branch circuit breaker and terminal block is superimposed inside the PDU, which may cause the overall temperature rise to be much higher than the temperature rise during single-branch testing.

The purpose of the PDU verification load is to simultaneously connect rack-mounted dummy loads to all output branches of the PDU, synchronously load them to their rated power , and run them continuously for a sufficient period of time to observe the total current, busbar temperature rise, and the status of circuit breakers in each branch. Fude Electronics’ rack-mount loads support multi-unit parallel expansion, “like building blocks, flexibly handling test scenarios of different scales from single racks to full systems.” Their 12kW air-cooled rack-mount load series has already delivered over 1200 units to overseas markets.