Although both schemes can be considered as N equipment, the UPS module design is different. Unlike small UPS systems, systems that exceed single-phase capacity (approximately 20kW) are equipped with internal static bypass switches to safely switch the load to mains power in the event of internal issues with the UPS module. The conversion points from UPS to static bypass are carefully selected by the manufacturer to provide the best maintenance for critical loads and to ensure that the UPS module itself is not compromised. Here is an example of one of these maintenance methods: In three-phase UPS applications, modules generally have a rated overload to achieve the target. One way to express this goal is that the module will carry 125% of the rated load for 10 minutes. Therefore, once the load reaches 125% of the rated value, the module will start a timing program, and its internal clock will start counting down 10 minutes. After 10 minutes, if the load has not returned to normal levels, the module will safely switch the load to the static bypass. There are many situations for enabling bypass, which will be discussed in detail in the standard specifications of the UPS module. One way to expand N equipment is to provide a “maintenance” or “external” bypass for the system. If an external bypass is chosen, the entire UPS system (modules and static bypass) can be safely enclosed when maintenance is required. The maintenance bypass and UPS share the same distribution panel and are directly connected to the UPS output terminal. Of course, under normal circumstances, this circuit is in a disconnected state and only closes when the UPS module switches to a static bypass. In the design process, certain measures must be taken to avoid maintaining the bypass circuit connection when the UPS is not converted to a static bypass. If installed correctly, maintaining the bypass can ensure the safe operation of the UPS module without worrying about load shutdown, making it an extremely important component in the system. Most “N” system equipment, especially equipment below 100kW, is used in construction environments that do not have special requirements for the entire power system equipment. The power system of buildings usually uses “N” equipment, so “N” APC UPS equipment is just enough to meet this situation. Figure 1 shows the commonly used single module APC UPS system configuration.
Defects of the “N” system:
There are multiple single fault points, which means that the reliability of the system is determined by its thinnest link, with limited availability. If the UPS module fails, the load will be switched to bypass power supply and then placed under maintenance free power supply.
During maintenance of APC UPS, batteries, or downstream equipment, the load is under maintenance free power supply.
Lack of redundancy constrains the ability to maintain loads in the event of UPS malfunctions.