Do UPS main and bypass inputs have to be 'same source'?

We often encounter a core issue in project design: must the main circuit (rectifier) input and bypass (static bypass) input of UPS come from the same power source (i.e. "same source")? This may seem like a simple wiring issue, but it actually involves multiple aspects such as the working principle, topology, system reliability, and grounding safety of UPS.

What is' same source '?

In the UPS system, "same source" refers to the main (rectifier) input and the bypass (static switch) input coming from the same low-voltage distribution point, that is, the same transformer and the same section of busbar, so their neutral line (N) is unique and shared. On the contrary, 'different sources' refer to two inputs that may come from different transformers or power grids, and the neutral line system is independent of each other.

Not all UPS systems require same source

Whether to require same origin depends primarily on the topology design within the UPS, especially the way rectifiers and inverters generate neutral lines.

UPS without neutral wire for rectifier input:

Topological features: For example, the design uses a full bridge inverter (with a Δ/Y output transformer generating the neutral line) or a three-phase four bridge half bridge inverter (with the neutral line drawn from the midpoint of the bridge arm). Its rectifier side operation does not rely on the input neutral line.

Same origin requirement: Same origin is not mandatory. Because the neutral wire is generated within the UPS, even if the main and bypass inputs are from different sources (with different neutral wires), it will not affect the normal operation and switching of the UPS.

The rectifier input requires a neutral line UPS:

Topology feature: Commonly seen in some IGBT rectifier UPS systems, it adopts a half bridge inverter design. The neutral wire is led out from the midpoint of two sets of batteries and directly connected to the neutral wire of the rectifier and bypass input.

Same origin requirement: must be of the same origin, that is, the neutral line must be unique. If the two input neutral lines are different, it may cause problems such as zero line current loop and potential difference, which may damage the equipment. If the same source cannot be achieved on site, an isolation transformer (such as a zero phase shifted D/Z transformer) must be installed at the bypass input end to reset the neutral line.

Important exception: Parallel systems and industry standards

When UPS is running in parallel: This is a special and strict situation. Even though the main circuits of each UPS may not be from the same source, the bypass inputs of all parallel units must be from the same source. This is because if there is a voltage or phase difference in the bypass power supply of each UPS, a huge circulating current may burn out the bypass switch when the system switches to bypass operation.

Power industry standard requirements: In scenarios with extremely high reliability requirements, such as the main building of a power plant, the industry standard DL/T 5491-2014 clearly stipulates that the AC main power supply and AC bypass power supply of UPS should be connected by different plant bus sections. This is a non mandatory requirement for "different sources", aimed at achieving redundancy at the power level and avoiding a single busbar failure that could cause the entire UPS to lose power.

Same sources vs different sources: advantages and disadvantages and application scenarios

✅ When can "different sources" be used?

Not all situations require same origin, and there are also flexible solutions in terms of technology:

UPS with rectifier without neutral wire: For UPS with internal topology such as full bridge or three-phase four bridge arm, the rectifier input does not rely on the neutral wire, which is generated by the midpoint of the output transformer or inverter bridge arm. Whether the main and bypass of this type of UPS are of the same origin has no impact on its operation.

Implementing 'logical same source' through ATS: The best practice is to use an automatic transfer switch (ATS) to interconnect two different sources of mains power and output one power supply to the UPS. This not only satisfies the redundancy backup of external power sources, but also allows the UPS to operate in the same source mode, with the load always under protection.

Installation of isolation transformer: If different source inputs must be used on site and the UPS rectifier requires a neutral wire, a zero phase shift isolation transformer (D/Z type) can be installed at the bypass input end to solve the problem of different neutral wire sources