What happens to your servers if the electronics inside a switched PDU fail?
But there is another question that deserves just as much attention:
What happens to the connected IT equipment if the control electronics inside the PDU itself fail?
A failure in the intelligence of a PDU should not automatically become a power failure for the connected servers. However, what happens during a fault depends on how the PDU's switching architecture has been designed.
For data centers where uptime is critical, it is therefore important to look beyond what a switched PDU can do and understand how it actually switches power.
Not every switched PDU switches in the same way
A switched PDU uses relays to control power to individual outlets. How these relays operate is important for both energy efficiency and reliability.
Schleifenbauer uses electromechanical bistable relays in its Switched and Managed PDUs.
A bistable relay has two stable mechanical states. Energy is only required when the relay changes from one state to the other. Once it has switched, it does not require continuous electrical power to maintain that position.
This differs fundamentally from a relay design that requires continuous energization to maintain a particular switching state.
Why does a bistable relay matter if electronics fail?
Intelligent PDUs use electronics for metering, communication and switching. These capabilities provide data center operators with valuable control, but it is also important to consider the dependencies introduced by this intelligence.
A bistable relay does not require continuous electrical actuation to maintain its mechanical switching position. Energy is only required when the relay actually changes state.
This design principle provides several advantages:
- lower internal energy consumption;
- reduced heat generation;
- no continuous relay energization to maintain the selected position;
- a clear distinction between controlling a switching event and maintaining the physical relay position.
The last point is particularly relevant in a data center environment.
PDU intelligence should add functionality and control to the power infrastructure. But the architecture behind that intelligence deserves just as much consideration as the features listed on the specification sheet.
Switching should add control, not unnecessary risk
Remote outlet switching has clear advantages.
Servers and other IT equipment can be power-cycled remotely without sending an engineer to the rack. Equipment can be started in a controlled sequence and power operations can be automated.
But as soon as a PDU can switch an outlet, the switching relay becomes part of the critical power path.
That is why Schleifenbauer has considered not only how the relay maintains its position, but also what happens to the relay when an outlet is actually switched.
This is where Near-Zero Voltage Switching becomes important.
Protecting bistable relays is just as important
Modern IT equipment typically contains switch-mode power supplies with capacitors. When equipment is switched on, these capacitors can draw a very high current for a very short period.
This is known as inrush current.
And the peak can be surprisingly high.
During testing for Schleifenbauer's technical white paper, a 65 W laptop power supply was switched on near the peak of the mains sine wave. Despite its relatively low power consumption, a peak current of 150 A was measured. Its normal RMS current was only approximately 0.33 A. Whitepaper Schleifenbauer-NEAR-…
For a switched PDU, this matters because that current passes through the relay contacts.
High switching currents can damage relay contacts. Tests described in the Schleifenbauer white paper found that, in some cases, relay contacts became stuck together after switching a 65 W adapter only at times. Whitepaper Schleifenbauer-NEAR-…
Reliable switching is therefore not only about what happens if the control electronics become unavailable. It is also about protecting the switching components throughout their operational life.
Near-Zero Voltage Switching reduces stress on the relay
This is why Schleifenbauer uses Near-Zero Voltage Switching (N-ZVS).
The mains voltage follows a sine wave. If electronic equipment is switched on near the peak of that wave, the resulting inrush current can be extremely high.
Around the zero crossing, however, the voltage is at its lowest.
Schleifenbauer's N-ZVS technology determines the appropriate switching moment and switches the connected equipment close to this zero crossing.
In the same test in which an inrush current of 150 A was measured previously, switching near the zero crossing reduced the measured peak to approximately 35 A. Whitepaper Schleifenbauer-NEAR-…
Reducing these high switching currents puts less stress on the relay contacts and contributes to a more reliable PDU. It can also reduce mains distortion and improve EMC behavior and Power Quality. Whitepaper Schleifenbauer-NEAR-…
Two design choices with the same objective: reliable power distribution
Bistable relays and Near-Zero Voltage Switching address two different aspects of PDU switching.
Bistable relays only require energy when changing state. They do not require continuous energy to maintain their position.
Near-Zero Voltage Switching reduces the electrical stress placed on those relays when an outlet is actually switched.
Together, these technologies demonstrate an important principle for intelligent rack PDU design:
Additional intelligence and control should not come at the expense of reliable power distribution.
That becomes increasingly important as rack power densities rise and the potential impact of an interruption becomes greater.
Look beyond the features on the specification sheet
When comparing switched and managed PDUs, attention is often focused on features such as outlet-level metering, remote switching, SNMP, APIs, environmental sensors and monitoring software.
Those capabilities matter.
But for critical data center infrastructure, it is equally important to understand the underlying switching architecture.
Questions worth asking include:
What type of relays does the PDU use?
Do the relays require continuous energy to maintain their state?
How does the PDU reduce high inrush currents during switching?
What happens to power distribution if part of the PDU's control electronics stops functioning?
These questions may not always appear at the top of a PDU specification sheet, but they relate directly to the most fundamental purpose of every rack PDU:
delivering reliable power to the connected IT equipment.
Schleifenbauer Switched and Managed PDUs
Schleifenbauer designs and manufactures its rack PDUs in the Netherlands. Its Switched and Managed PDUs combine remote outlet switching with bistable relays and Near-Zero Voltage Switching.
Programmable outlet switching delays can also be used to sequence equipment start-up, helping to avoid multiple devices being switched on simultaneously.
Depending on the selected configuration, Schleifenbauer PDUs can also be combined with EnerTree DCEM for centralized monitoring, management, configuration and energy analysis.
EnerTree is included without recurring software license fees, with open APIs and support for integration with other data center management systems.
But underneath all that intelligence, the principle remains the same:
An intelligent PDU should provide more control over your power infrastructure without introducing unnecessary dependencies into the power path.
Frequently asked questions about switched PDUs and bistable relays
What is a bistable relay in a PDU?
A bistable relay has two stable switching positions. It requires energy to change from one state to another, but does not require continuous energization to maintain its position. Schleifenbauer uses electromechanical bistable relays for outlet switching in its switched PDU technology. Whitepaper Schleifenbauer-NEAR-…
Why does Schleifenbauer use bistable relays?
Bistable relays only consume energy when switching. Continuous energization is not required to maintain the selected relay position. This reduces energy consumption and heat generation associated with switching and means that maintaining the relay's physical position is not dependent on continuous coil energization.
What happens to a bistable relay if its control signal disappears?
A bistable relay is mechanically designed to remain in its last stable position without continuous energization. The behavior of the complete PDU during a specific electronic or hardware failure also depends on the overall PDU architecture and the nature of the failure.
What is Near-Zero Voltage Switching?
Near-Zero Voltage Switching, or N-ZVS, is a switching technique in which equipment is switched on close to the zero crossing of the AC mains voltage. This can significantly reduce inrush current. In a Schleifenbauer test, a measured peak current was reduced from 150 A to approximately 35 A. Whitepaper Schleifenbauer-NEAR-…
Why is inrush current important for a switched PDU?
High inrush currents place additional stress on relay contacts and other electrical components. They can accelerate wear, damage relay contacts and contribute to mains distortion. Reducing these currents therefore contributes to switching reliability and improved Power Quality. Whitepaper Schleifenbauer-NEAR-…
What is the difference between a Schleifenbauer Switched and Managed PDU?
a Switched PDU provides input-level energy metering combined with remote outlet switching. A Managed PDU adds individual outlet-level metering to input metering and remote outlet switching. Both can be used where individual outlets need to be controlled remotely.
How reliable is the switching architecture behind your rack PDUs?
Discover Schleifenbauer Switched and Managed PDUs, or talk to us about a build-to-order PDU configuration for your data center.
The post What happens to your servers if the electronics inside a switched PDU fail? appeared first on Schleifenbauer – Bespoke PDUs.
Source: https://www.schleifenbauer.eu/en/switched-pdu-failure-bistable-relays/