Although the Surge Arrest, Surge Station, Back-UPS, Back-UPS Pro, Smart-UPS, Smart-UPS V/S, and Matrix-UPS products will appear to work properly without a ground, APC always recommends that these units are grounded properly. Grounding is not for functionality, but rather for safety. Numerous issues are involved when there is no ground, including potential for electric shock, signal attenuation, and unpredictable operation. Therefore, APC does not support using these products in any ungrounded application, even with an earth leakage monitor or other such device in place to alarm in case of excess current or drop in resistance.
Background: Earth leakage monitors measure the resistance between phase (hot) and any current-carrying component to ensure that resistance remains high. Leakage current is typically measured by breaking the ground connection back to the mains and inserting a specific load that simulates a person interrupting the ground. Devices without a ground are highly insulated, and therefore the missing ground is not an issue during normal operation. However, were the device to become energized due to a fault or failure, it would look for a way to deflect the current to ground. Without a ground present, the current would take the path of least resistance. This would be hazardous as the return path may be through the load itself, a data line, or even a person.
In 120 Vac environments, each electrical device must reference the same ground which originates at the main service panel. Additionally, a properly grounded chassis acts as a filter against EMI/RFI interference. If this ground reference were missing, the EMI/RFI interference may result in erratic behavior of the device.
Certain types of data communication systems require a reference to ground. Additionally, some data lines use the ground as a shield against noise. RS-232, and other data cables, rely on ground to provide a return reference for the signal (although this is not the case with isolated data systems such as Ethernet). Without a ground, the signal will attenuate.