Uninterruptible power supplies

Back-feed protection, in uninterruptible power supplies (UPS), prevents the risk of electric shock from any electric current feeding back from the UPS output in the event of a mains supply failure. When mains fails and connected loads are protected by uninterruptible power supplies, a back-feed protection device prevents current from being passed back to the input terminals of the UPS from the inverter output. This is extremely important for health and safety reasons because it enables a service engineer to work on the incoming supply side of the UPS without risk of receiving an electric shock.
A example of what could happen, in the event of a mains power failure, is that a bypass supply thyristor, which has short-circuited causing the output from the inverter to be passed through to the input terminals via the faulty component. It is something that must be prevented at all costs - not only to safeguard engineers but uninterruptible power supply protected loads too. Even when the input supply has been switched off, via an isolator, there is potential for it to happen, hence the need for back-feed protection. The type of back-feed device employed is determined by the size of the uninterruptible power supply.
Plug-in Uninterruptible Power Supplies
Back-feed protection for a single-phase uninterruptible power supply, up to 16A, needs to provide protection for both live and neutral input conductors using a specified air gap. The air gap is usually provided by means of a relay that opens when mains power supply fails.
For plug-in power supplies, if a fault occurs when the user disconnects it from the mains power supply (by simply unplugging it from the wall socket), the back-feed relay should prevent the exposed pins from becoming live. It should also remove any possibility of the user receiving an electric shock. UPS systems over 16A are hardwired (normally) and utilise one of two different approaches: mechanical or electronic.
Hardwired Uninterruptible Power Supply
Mechanical Back-feed Device:
Some hardwired uninterruptible power supplies are provided with the relay or contactor-based solution as used in 16A plug-in models. This, again, provides a specified safety air gap which opens when the mains power supply is disconnected or fails. Only the phase conductors are disconnected and the neutral always remains connected.
Electronic Back-feed Device:
Many hardwired uninterruptible power supplies utilise a back-feed current detection system, which continually monitors the current flow through the bypass supply. Should a fault occur within the bypass thyristors, it is detected by the UPS, which then immediately shuts down its inverter.
Back-feed protection is so simple to implement but is often overlooked. As with many potential hazards associated with electrical circuitry, such a simple problems can often, if not addressed, lead to a very complicated and costly set of circumstances. Static switches are part of online UPS design. Chances are back-feed protection will have been incorporated into the uninterruptible power supply units at the manufacturing stage but it is always best to check. This article was compiled using information available in The Power Protection Guide - the design, installation and operation of power supplies (ISBN: 9 780955 442803). By Robin Koffler and Jason Yates of Riello UPS.