Earth Loop Impedance Testing UPS - Master Dialog
Due to the design of all UPS systems, any Earth Loop Impedance Tests carried out downstream of the system will result in high impedance readings (transformer less systems especially). The UPS system cannot be considered the same as the mains supply due to the current limitations of the UPS output. All UPS systems are designed in accordance with the relevant standards. The main standards for UPS being EN 50091 and EN 62040 amongst many.
A UPS system does not have the same quantity of energy available as the mains supply. This is due to the fact that the output stage of a UPS is completely separated from the incoming mains supply. This method of operation is known as the On-Line or double conversion system. This basically means that the output power to the critical load is continuously supplied by an electronically generated sinewave (using IGBT switching). The switching is controlled by a microprocessor which ensures a stable and regulated output voltage regardless of the input supply or output load conditions. This independency is referred to as VFI (Voltage and Frequency Independent) as defined in EN 62040-3.
When carrying out Earth Loop Impedance Tests, the values obtained will reflect the impedance of the output stage of the UPS known as the inverter. The inverter output is manufactured using high frequency switching and filtering circuits, and at no time will the inverter output replicate the impedance of the supply transformer or generator alternator winding. The inverter output power is limited to the size of the components from which it is manufactured; these components must be protected to ensure that the UPS can continue to operate without any consequential damage following an output overload or short circuit.
The input neutral and earth remain unchanged through the UPS, therefore the input neutral is connected to the output neutral and the input earth is connected to the output earth, and the UPS must be connected to the neutral and earth of the supply source.
When carrying out Earth Loop Impedance Testing to determine the fault current and therefore the breaker or fuse discrimination, the two operational states of the UPS must be taken into consideration; these being normal operation when the mains or generator supply is present and battery operation when no mains or generator supply is present.
During normal operation when the mains supply is present, if a fault occurs downstream of the UPS the load and fault will be transferred onto the bypass line which provides a direct connection from the UPS input to the UPS output. The bypass operation protects both the inverter and enables the necessary fault current to be generated by the mains supply, So to test this scenario the earth loop impedance testing must be carried out with the UPS maintenance bypass circuit breaker closed. This operation replicates an overload or fault scenario when the UPS has the mains supply available. During this test the values obtained should be normal.
During battery operation when no mains supply is present and therefore no bypass supply is available, the UPS cannot transfer to the bypass if a fault occurs downstream. Therefore the UPS must attempt to clear the fault using the power supplied by the batteries via the inverter. However, as previously stated the inverter does not have an unlimited quantity of energy available, and to protect the inverter and prevent a failure within the UPS, the system is limited to provide a maximum short circuit current of up to 200%In (current limited) for 100mS. After this time the UPS output will be automatically shutdown. Shutting down the inverter protects the inverter and disconnects the supply to the fault, therefore preventing any cable overheating or further problems. If the automatic inverter shutdown fails then the inverter output or battery supply fuses will rupture prior to the thermal limitations of the inverter being exceeded. This inevitably, will cause the inverter switching components to fail. Once the shutdown has occurred the unit will remain off until reset.
The UPS systems are specifically designed to operate this way, during manufacture the inverter overload protection shut down is tested in the factory on all units to ensure perfect and safe operation.
Jason Yates
Technical Manager
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