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UNINTERRUPTIBLE POWER SUPPLIES
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By Alan Luscombe , Director , Uninterruptible Power Supplies
www . upspower . co . uk
Some UPSs need just simple communications capability to warn of an impending failure while more complex systems benefit from network-based solutions
The UPS ’ s primary role within a data centre or ICT room is to provide instant battery backup if the mains power fails . Sometimes , this backup resource is designed to support the load for extended periods . More usually , though , its purpose is to buy time for the load to shut down gracefully , or for a generator , if available , to start up and come online . Clearly , the UPS must communicate some type of warning signal to trigger either strategy as soon as the mains fails ; otherwise , if a blackout exceeds the battery autonomy time , a system crash is merely delayed rather than prevented .
This is one fairly obvious scenario , but there are other ways in which not only the critical load , but also other services such as building management systems and remote monitoring terminals can benefit from visibility of the UPS and incoming mains supply status , together with an ability to control the UPS if necessary . However , there ’ s no point in investing in a communications capability beyond the realistic needs of an application .
Simple communications At the simplest level , ‘ true / not true ’ information can be obtained from volt-free contacts that can communicate UPS status and alarms to other equipment on the same site as the UPS . Windows , Mac , Linux , Unix and other PC or network operating system variants support software that can detect these alarms and react by initiating an orderly system shutdown automatically . Fig . 1 shows some typical volt-free UPS status signals .
Fig . 1 : Examples of UPS volt-free status signals
A more sophisticated communications strategy can be implemented using the RS-232 connection available on many UPSs . This allows more detailed information to be transmitted , including data generated by UPS self-diagnostic activities . Examples of analogue values that can be measured and transmitted include :
• Inverter output voltage , frequency , current , kVA and kW
• Bypass voltage , frequency , current , kVA and kW
• Battery voltage , charge / discharge current and remaining battery time
• Statistics regarding mains failures and UPS operation
The RS-232 serial connection allows the UPS to be monitored in real-time as a remote computer can continuously poll it for updates . In normal practice , however , only critical alarms are continuously monitored , while operational status data is requested as needed by system administrators . The exact information profile obtained in this way from a UPS depends on the individual UPS supplier , and the software they provide to handle the serial data stream . The RS-232 protocol is also manufacturerdependent , as no European Standard for UPS RS-232 protocol exists .
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