UPS the basics

Dual input / Dual supply availability

With most smaller, 3kVA and under UPS systems, power is provided via a 16A IEC input, 10A IEC input or a moulded cord with a plug suitable for a wall socket. In the case of the UK, a 3 pin plug. Without the inclusion of other devices, such as automatic transfer switches, they are limited to 1 input. For example, on a 1kVA tower UPS it may be powered by a 13A wall socket, utilising a 13A UK plug and a 10A female IEC connector. 1 input only powering the UPS and, should this supply line develop a fault, the UPS will supply the downstream load with power until either the fault on the mains is corrected and restored, or its batteries reach their cut-off voltage and the output of the UPS ‘drops the load’. The failsafe line within UPS systems is the static bypass line. With single input UPS systems, the same source that is feeding the UPS is also feeding the bypass line.

From around 6kVA onwards, there are often options to include dual input kits, although it may be a standard feature depending on the size and/or the manufacturer. Dual input UPS systems allow for 2 inputs to feed the UPS. The first input source will feed the rectifier and the secondary power source will feed the bypass. Should the UPS develop a problem which causes the input breaker to trip, the fault may be isolated to that power pathway, meaning mains can still support the critical equipment via the bypass line. Without dual input being available, a fault on the rectifier line causing the input breaker to trip would mean mains is no longer available via the rectifier line or the bypass line. Unless intervention occurs, batteries will deplete until cut-off voltage.

Clients wishing to utilise dual input UPS systems should take caution. Power feeding both bypass and rectifier lines must be taken from the same source/transformer to ensure that the UPS always retains a neutral reference point. If this isn’t the case floating neutrals can occur, which can result in both safety and operations problems.

Should a client wish to utilise two separate pathways there are a few ways to handle this scenario. Breaking the connection when primary power is lost before transferring to secondary can avoid this problem such as with an ATS panel. Alternatively, an input isolation transformer can be installed to create a permanent reference point for the rectifier and bypass line.