Site Wiring Fault or Building Wiring Fault Indicator LED is on
The Site Wiring Fault LED light is only present on 120V UPS and Surge products. On APC UPS products this indicator is typically on the rear panel. The purpose of this indicator light is to warn you that there are problems with your building wiring that may result in a shock hazard.
APC recommends that you have a qualified electrician inspect your wiring for one or more of the conditions listed below.
Reasons why the Site Wiring LED light will illuminate:
1. Overloaded neutral wire.
2. Reversed polarity (hot and neutral wires are reversed).
3. Missing ground wire.
If the outlet that the APC UPS unit is plugged into is not properly grounded or properly wired, the surge protection capability of our unit may be diminished. Therefore, in order to maintain your warranty and protection under APC's Equipment Protection Policy, be sure that the APC unit is only plugged into a properly grounded outlet where the Site Wiring Fault Indicator (SWFI) light is not illuminated.
All of APC's UPSs with the exception of the Matrix-UPS and the Symmetra Power Array have a small red LED light that is called a SWFI. On some UPS models, this LED is referred to as the Building Wiring Fault Indicator. Both have the same function. On most UPS models, the light is located on the rear of the unit near where the output receptacles are located.
Taken from APC Knowledgebase