Back-satisfy protection, in uninterruptible strength 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 strength supplies, a back-satisfy 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 strength failure, is that a bypass supply thyristor, which has short-circuited causing the output from the inverter to be passed by to the input terminals via the faulty part. It is something that must be prevented at all costs – not only to safeguard engineers but uninterruptible strength supply protected loads too. already when the input supply has been switched off, via an isolator, there is possible for it to happen, hence the need for back-satisfy protection. The kind of back-satisfy device employed is determined by the size of the uninterruptible strength supply.
Plug-in Uninterruptible strength Supplies
Back-satisfy protection for a single-phase uninterruptible strength supply, up to 16A, needs to provide protection for both live and neutral input conductors using a stated air gap. The air gap is usually provided by method of a relay that opens when mains strength supply fails.
For plug-in strength supplies, if a fault occurs when the user disconnects it from the mains strength supply (by simply unplugging it from the wall socket), the back-satisfy 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 (typically) and utilise one of two different approaches: mechanical or electronic.
Hardwired Uninterruptible strength Supply
Mechanical Back-satisfy Device:
Some hardwired uninterruptible strength supplies are provided with the relay or contactor-based solution as used in 16A plug-in models. This, again, provides a stated safety air gap which opens when the mains strength supply is disconnected or fails. Only the phase conductors are disconnected and the neutral always remains connected.
Electronic Back-satisfy Device:
Many hardwired uninterruptible strength supplies utilise a back-satisfy current detection system, which continually monitors the current flow by the bypass supply. Should a fault occur within the bypass thyristors, it is detected by the UPS, which then closest shuts down its inverter.
Back-satisfy protection is so simple to implement but is often overlooked. As with many possible hazards associated with electrical circuitry, such a simple problems can often, if not addressed, rule to a very complicated and costly set of circumstances. Static switches are part of online UPS design. Chances are back-satisfy protection will have been incorporated into the uninterruptible strength supply units at the manufacturing stage but it is always best to check. This article was compiled using information obtainable in The strength Protection Guide – the design, installation and operation of strength supplies (ISBN: 9 780955 442803). By Robin Koffler and Jason Yates of Riello UPS.