UPS Systems – Ground Fault & Short Circuit Handling

The G9000 is a transformerless UPS, and is designed to go to bypass to provide fault‐clearing current once a fault is detected.

In the G9000, faults are sensed many ways:

  • Faults sensed on the AC output:
    1. AC Ground Fault on the output: Phase to Ground: in this case a monitoring circuit detects when the zero sequence voltage changes. In other words, the vector addition of all three phases should be zero; when that voltage (or leakage current) rises above a certain setpoint, the unit activates the ZERO PHASE OVERCURRENT alarm, and sends the unit to This function can be disabled.
    2. Phase to Phase: here the CT’s and voltage monitoring on the output sense a rapid rise in current and simultaneous drop in voltage, and gives the INVERTER OVERCURRENT alarm, and again goes to bypass. This function cannot be disabled.
  • Faults on the DC bus/internal to the UPS:
    1. DC ground fault: there is a separate circuit in the G9000 that detects DC ground faults. This measures the difference between the unit’s virtual neutral and the input/output phases, and will detect an alarm in the case of:
      1. Leakage of one of the components in the rectifier/inverter/charger‐chopper circuits
      2. An electrolyte trail from battery to ground
      3. An output AC Phase to Ground fault in systems with ungrounded or HRG (High Resistance Grounded) source: this circuit will detect a fault in HRG systems when the voltage in the UPS’s internal virtual neutral rises to phase voltage (277V).
      4. The result will be a DC GROUND FAULT alarm and the unit will be sent to bypass. This function can be disabled.
  1. DC Short circuit: A similar circuit measures the balance between the + and – legs of the DC bus, and is there to detect a hard fault in the battery This function cannot be disabled.
  2. Note On The G9000’s Bypass Path
    The bypass path has no fuses or breakers in it and therefore has a no interrupt rating, but rather the withstand rating, required by UL from the factory. However, if an interrupt rating is specified, or if fuse protection in this path is desired, there is a provision to add fuses to the bypass path, at 65 kAIC for 80‐225 kVA models, and 100 kAIC for 300‐750 kVA models.