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A daisy chain is a wiring series that connects several systems or devices together in sequence. While customers often consider joining two or more plug-and-play UPSs, under no circumstances should the input of one UPS be attached directly or indirectly to the output of another. This method dramatically increases the chance of UPS failure.
Daisy-chaining introduces another point-of-failure into the system ── with potentially disastrous results. One threat is that the current draw can cause the upstream UPS to go into an overload condition or distort the output voltage waveform. When this occurs, it can prompt the UPS to detect a power fluctuation, revert to battery power, and continually cycle between the battery and normal operation. Plugging one UPS into another can also spark inverter compatibility issues, where the unit closest to the load doesn’t recognize the other UPS’s power as sufficient. This compels both UPSs go to battery, defeating the intent. Daisy-chaining can also force UPS controllers to work against each other, leading to harmonic distortion on power lines.
There are much better approaches to achieving the outcomes that customers most commonly seek from daisy-chaining UPSs, such as bolstering redundancy, gaining additional runtime and attaining surge protection.
One of the most frequently cited reasons for daisy-chaining UPSs is the desire to enhance reliability and eliminate the possibility of a single point-of-failure. However, the process of daisy-chaining actually increases the risk of UPS failure by combining failure rates for the two units. If either unit fails, the remaining UPS provides no value.
For customers desiring redundancy, an architecture utilizing dual power supplies is recommended. In this type of solution, two UPSs are isolated from one another so a failure in one does not affect the other. In configurations where dual power supplies are not available, consider using an automatic transfer switch (ATS) PDU, which reallocates power between sources without interruption if the primary source fails. Learn more about redundancy and system-level design in Eaton’s network closet white paper.
Learn more about redundancy and system-level design in Eaton’s network closet white paper.
Daisy-chaining is not a viable option for customers hoping to bolster runtime in an existing UPS. While large three-phase UPS systems have the ability to rely on huge battery banks to deliver hours of backup in the event of a power failure, most smaller UPSs are limited by the runtime capabilities of their internal batteries. However, some UPS models such as the Eaton 5PX G2 offer the option of adding extended battery modules (EBMs), which provide a pluggable battery extension to achieve longer runtime. Eaton’s interactive battery graphs make it easy to pinpoint the optimal UPS solution and runtime for your specific needs and environment.
It is also important to note on this topic how UPSs work with generators as an ideal complement for applications that must remain online 24/7 without interruption.
While virtually all UPSs provide an adequate level of surge suppression for most applications, additional protection may be needed in certain harsh environments, such as sites plagued by frequent thunderstorms or where industrial equipment impacts the integrity of the electrical system. In either case, daisy-chaining UPSs will not mitigate incoming surges. In fact, cascading multiple units will often instigate a more dangerous outcome; since a UPS is designed to be self-sacrificing, if a surge is too powerful for it to handle, the UPS will go offline and the system will go down regardless.
For sites with known surge problems, it is best to consult with an electrician to develop a multi-stage approach. Surge protection at the UPS is best when paired with a second surge device at the panelboard. Those concerned about the impacts on lighting can also explore the option of lightning arrestor solutions .
