IT supplies are used to prevent that a ground fault leads to immediate disconnection of the affected circuit like in a grounded system. Although a first ground fault results in a displacement of the potential of the entire supply, continued operation is still temporarily possible.
Special ground fault monitoring equipment reports any ground faults and hence makes it possible to quickly rectify the fault – often without disruption to the operation of the plant.
The situation is similar in supplies with high impedance grounding.
If however a second ground fault occurs in another phase, there is a short-circuit that must be immediately cleared by the short-circuit protective device. The voltage to be switched off varies depending on the locations of the short-circuits (Figure 1).
If, however, there is one ground fault on the supply side and the other on the load-side, a significantly higher breaking capacity is required because of the increased voltage load.
If both short-circuits occur on the load-side of the circuit breaker, the breaking work is shared between two contacts and the required breaking capacity corresponds to the normal 3-phase values.
If the location of one short-circuit is on the supply-side of the circuit breaker and the second short-circuit on the load-side, then one contact only of the circuit breaker has to perform the total breaking work and this at phase-to-phase voltage. In this case, the significantly lower single pole breaking capacity of the circuit breaker at the phase-to-phase voltage is critical. If the values cannot be obtained from the device documents, an inquiry should be made. If the short-circuit current at the installation site exceeds the single pole switching capacity of the circuit breaker, then a back-up fuse is required.
For three-pole short-circuits there is no difference between IT supplies and other supply types.
The ultimate short-circuit breaking capacity Icuand the service short-circuit breaking capacity Ics continue to apply. Circuit breakers under IEC 60947-2 are suitable for use in IT supplies, if they are not marked with the symbol . Testing is in accordance with Annex H.
Resource: Allen Bradley – Low Voltage Switchgear and Controlgear