Why can't I use a domestic model APC UPS on a ship
Issue:
In North America, one would expect to see approximately 120 volts when measuring from hot to neutral and from hot to ground. However, large ships use delta power. That is, there are two hot legs (center tapped 120Vac) and a ground: no neutral. Each hot is 60 volts. One can measure from hot to hot (phase to phase) and will see 120 volts. When measuring from either hot to ground, one will see 60 volts.
Product Line:
Smart-UPS
Environment:
All models with the "X93" suffix (IE: SU700X93, SUA1500X93)
Cause:
Standard North American Smart-UPS are designed for a 3 wire input comprised of a 120v Hot, Neutral, and Ground.
Resolution:
For this environment, APC offers the Smart‑UPS X93 Shipboard Series. A standard 120 VAC model should not be used on a ship with delta power. When a 120 VAC APC UPS transfer to battery, it supplies 115 V ±5 V on the hot leg. Herein lies the problem with using standard models on ships. According to the National Electric Code, the neutral leg must be passed through standard UPS units without interruption. As a result, it is possible for 180 Vrms to be passed to the load when switching to battery (120 VAC from the UPS inverter on one phase and 60 volts from the ship on the second phase). Most computers have power supplies that accept a wide range of input voltage; therefore, they would not be affected by this voltage jump. However, if the load includes any common‑mode suppression (e.g., 150 VAC MOVs), then the load may be affected.
Input to the X93 units is two hot legs at 60Vac each. The output is one hot 120Vac leg, a neutral and a ground. The primary differences between the X93 unit and the domestic are that the site wiring fault circuit is disabled, a double pole breaker replaces the single pole breaker at the input, and a triple pole switch is added that can disconnect the incoming AC as well as the bias supplies in the UPS (it won't turn on anymore). The X93 does not break both line and neutral unless the input breaker is tripped. If it just goes to battery, the output and input neutral are still continuous. The ground and neutral are not connected by the UPS so unless the neutral is bonded to ground on the ship at the supply or load, one will not necessarily measure 0 volts from neutral to ground at the output of the UPS.
Note: A customer who has decided to use a domestic model on a ship producing delta power aware that the ship must have over current protection (fuse or circuit breaker) in each hot leg. This is because a fault within the UPS (highly unlikely) from neutral (whichever hot leg corresponds to the neutral) to ground is not protected by the UPS's circuit breaker. Without this branch protection on the ship, such a fault within the UPS could be dangerous because fault current is not limited.