An uninterruptible power supply is an in-line power protection device that sites in the critical power path from your mains building incomer to your power distribution points. As such it is imperative that the UPS system can be bypassed without disruption to any connected loads.
Uninterruptible power supplies come in three different design topologies that are classified as standby, line interactive and on-line double-conversion. On-line UPS are the most popular for critical power protection and incorporate a built-in automatic bypass.
A sensing circuit monitors the output waveform of the UPS and compares this to a sinewave. If the output waveform is seen to distort due to an overload condition or a failure within the UPS itself, the load is automatically transferred to the bypass supply (normally connected to the mains power supply).
The UPS automatic bypass may be relay or static switch based. In a relay-based transfer switch there may be a short break in supply as the load is transferred from the output of the UPS to the mains power supply. In a static switch based bypass the transfer is instantaneous and ‘no-break. The most common type of UPS maintenance bypass is the static switch variety. The rela-based bypass is a cheaper alternative and can be found in smaller on-line UPS systems (sub 1.5kVA).
Internal Maintenance Bypasses
Three phase and some smaller UPS systems will also incorporate a maintenance bypass switch. The switch will be breaker based and must be manually pushed into position to activate. In maintenance bypass, the UPS can be safely serviced and inspected, though the system itself still has live components and the load is powered through the UPS and connected mains power supply. When the UPS service is completed, the maintenance bypass switch can be used to put the UPS back into full on-line protection.
External Maintenance Bypass
An external bypass can also be installed. This is a separate box (normally wall or rack mounted) and connected to both the UPS input and output supplies. The bypass is manually activated and when the output is switched to bypass, the load is supplied directly from the raw mains power supply. In bypass mode, the UPS can be safely powered down without interruption to the connected loads and removed for UPS service work or swapped out. When the operation is completed (and the UPS reconnected) the bypass can be turned or switched to put the load back onto UPS output.
External maintenance bypass switches are commonly used with UPS systems in critical power protection installation as they prevent downtime and the need to power down connected loads (servers and comms systems).
Wrap-around Maintenance Bypasses
Another name for an external bypass arrangement is a wrap-around bypass. This type of arrangement is more commonly associated with parallel UPS installations. Each individual uninterruptible power supply will have its own internal maintenance bypass. The wrap-around bypass allows the entire UPS arrangement to be bypassed without disruption to the load. The wrap-around may be a separate wall mounted device or form part of local switchgear arrangements.
These are a form of lock and switch device that can be incorporated into larger external maintenance bypasses, wrap around bypasses and switchgear. The locking arrangement prevents complex switching procedures being performed out of sequence and ensures that the right procedure is followed to maintain power resilience.
At EcoPowerSupplies we manufacture our own UPS maintenance bypass switches up to 400A in 1/1, 3/1 and 3/3 arrangements. Options include cabinet colour, Castel interlocks, auxilliary connects for remote status monitoring and local sockets for powering engineer devices.