The CPT Blog

May 6, 2015 0 Comments

Can I use a 208VAC with a Make-Before-Break Bypass Switch?
A FERRUPS or UNITY/I Single Phase installation using a Make-Before-Break bypass switch will not work with a 208 VAC input.* In this configuration, the phase angle difference between the FERRUPS output and the bypass A/C circuit will cause equipment damage if switching the bypass switch from the UPS to the LINE position. It may cause the input breaker to trip, or possibly, weld the switch contacts together permanently.

Recommendation: For all installations of a Make-Before-Break bypass switch for which the input to the UPS is 208 VAC, a single-phase linear isolation transformer must be installed to step the input voltage up to 240 VAC. The linear isolation step-up transformer should have a kVA rating equal to or greater than the UPS- rated kVA, with a 208 VAC primary winding and a center tapped 120/240 VAC secondary winding. A Buck/Boost type transformer is not compatible for this application.

*208 phase-to-phase has a 120-degree phase angle difference derived from a Three-phase power source. The output of the FERRUPS is a single-phase derived output with a 180-degree phase to phase relationship. Since the neutral is bonded to ground on both systems, the phase relationship is referred to as a common point, therefore the phase difference will cause problems. This 60-degree phase difference will cause a large current impulse through the contacts of the bypass switch, leading to arcing and premature failure of the switch.

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