Tech Talk: Riello UPS Units & Bypass Switches

How does a Riello UPS operate when switching to an internal or external maintenance bypass?‘ Technical Services Manager Jason Yates explains all…

All Riello UPS from 10kVA upwards are supplied with an internal maintenance bypass switch, in addition to a specific pair of bypass input signal contacts, which can be used to electrically interlock the UPS to the external bypass switch.

When an operator switches a Riello UPS to internal bypass, the switch they are operating is fitted with an additional auxiliary switch (typically closed), which when operated immediately notifies the UPS’s control system. In turn, this action activates the UPS’s internal static bypass and shuts down the inverter. By doing this, the UPS effectively controls the transition to prevent any potential damage from occurring to the system.

In reverse, when the operator opens the maintenance bypass switch, the signal from the auxiliary contact notifies the UPS’s control system of this action. The system then carefully controls the transition from the static bypass to the inverter, by first restarting the inverter and synchronising it to the bypass supply. Then the inverter output voltage is raised or lowered in order to match the bypass supply voltage.

Finally, when safe to do so, the UPS will automatically transfer the load from the static bypass supply onto the inverter output. This changeover typically occurs at the zero-crossing point of the waveform to minimise any possible disruption to the load. Once the load has been securely transferred back onto the inverter output, the inverter will then return the output voltage level to that as specifically configured within the UPS.

External UPS Bypass Switch

If the system is installed with an external maintenance bypass switch, it is very important to ensure that the external bypass switch is fitted with an auxiliary contact (one set per UPS). This contact can be used to notify the UPS when the switch is closed, in order for the UPS to take the appropriate action, effectively interlocking it.

When switching to external bypass it is important that the UPS is switched to internal bypass first. Then when returning the external bypass to the normal state (open), it is preferred that the operator first opens the external bypass switch and then opens the UPS’s internal bypass.

By performing these operations this way, it ensures that the UPS has greater control of the bypassing actions, providing a higher level of protection for the connected load during the transition.

The auxiliary contacts provided within the internal or external bypass system are specifically designed to provide protection to the UPS and the connected load during manual bypass operations. Typically, manual bypass operations are undertaken during planned maintenance or repair of the UPS system.

Although these interlocks provide the required protection, it is imperative that any switching actions performed on or around a UPS follow a strict switching process and are performed by trained and authorised personnel. This recommendation is far more important when switching multiple parallel UPS, where the switching operations can become more complicated.

Learn more about the technical aspects of uninterruptible power supplies with our ‘Tech Talk‘ series.