Why can't I use a domestic model APC UPS on a ship
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.
All models with the "X93" suffix (IE: SU700X93, SUA1500X93)
Standard North American Smart-UPS are designed for a 3 wire input comprised of a 120v Hot, Neutral, and Ground.
For this environment, APC has the Smart-UPS X93 Shipboard Series. A standard
120Vac model should not be used on a ship with delta power. When a 120Vac
APC UPS transfers to battery, it supplies 115V +/-5V on the hot leg. Herein
lies the problem with standard models on ships. According to the National
Electric Code, the neutral leg must be passed through the standard UPS
units without interruption. As a result, it is possible that 180Vrms may be
passed to the load when switching to battery (120Vac 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 the voltage jump. However, if the load has any common mode
suppression (i.e. 150Vac 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.