The Importance Of UPS Maintenance Bypasses In Critical Power Installations

11/05/2014

By Eco Power Supplies

ups maintenance bypasses,critical power systems,uninterruptible power supplies

When you installed an uninterruptible power supply between your load and mains power supply, did you give much thought to the UPS bypass switch arrangements? When planning critical power installations of several hundred kVA or larger MVA-sized installations it is mandatory common sense and sound engineering to install some form of external maintenance bypass on each UPS module or as a wrap-around bypass switchgear for the entire UPS system.

The role of the UPS bypass switch is to allow the uninterruptible power supply to be completely powered down and therefore isolated from its connected mains power supply. The battery circuit breaker can also be opened to ensure that the UPS maintenance engineer can safely work on the UPS without the risk of exposure to either AC or DC power sources.

An external maintenance bypass is different to an internal maintenance bypass. With external bypass switch may be breaker or rotary switch based. The bypass may also be fitted with Castell interlocking, remote alarm signal contacts and even a socket for an engineer to power his laptop from the local mains supply.

Most three phase (3/3 or 3/1) and some single phase (1/1) UPS systems will have an internal maintenance bypass switch. The switch will be breaker-based and will allow parts of the UPS to be isolated for maintenance work. The bypass arrangement will not however allow the UPS to be removed from the circuit without interruption to the connected distribution board and loads.

For critical power installations and external bypass is always recommended. Whilst most external bypasses are hardwired, small single-phase bypasses may also be socket-based for plug and play connection. The bypass may provide either sockets to match the local power (square-pin in the UK) or IEC based. The IEC sockets may also have retention clips to prevent accidental disconnection. The UPS input and output 3kVA may also be plug and play.

Additional considerations include whether to adopt a wall-mounted or rack mount UPS bypass arrangement. It is more common for rack mount UPS systems to be installed with a rack mount bypass switch and this will generally be 1-2U in size. Overall this type of arrangement is ideal for a rack server based installation. Additional resilience may be added through the installation of a rack mount static transfer switch (STS) or automatic transfer switch (ATS). These may also provide additional metering to the UPS and also output distribution sockets.

For whichever size or format of uninterruptible power supply is installed, a UPS bypass switch is recommended. For critical power systems it should be mandatory to ensure the loads (servers or industrial processes) can be kept running whether the UPS is in circuit or not.

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