Residual Current

A residual current, commonly called a differential current, is the difference between the current in the forward conductor and the return conductor of a powered device. If a residual current occurs on a device, it usually means that there is an undesirable current path that causes the current supplied through the forward conductor to not flow through the return conductor. Causes of fault currents can be, for example, insulation faults or defective electronic components through which an unwanted current can flow. Fault currents pose a significant safety risk for devices that operate at voltages that can be life-threatening when touched. However, there are also a number of electronic components that cause a small harmless differential current due to their design. For example, many devices of protection class 1 (devices that are operated at 230V and are connected to the mains voltage in 3-wire configuration with phase (L), neutral (N) and protective earth (PE)) contain interference suppression capacitors that dissipate a small leakage current via the protective earth conductor.

In accordance with legal regulations, fault currents in many electrical installations are monitored by residual current circuit breakers (RCDs) and, if a limit value is exceeded, lead to the rapid disconnection of the corresponding power line in order to avert possible personal injury and damage to property.

By monitoring residual currents of type B/B+, in principle all possible types of occurring residual currents, e.g. in devices with power supplies that rectify the input AC voltage into a DC voltage, can be detected.

In particular, residual current monitoring is of interest to operators of data centers in the context of the legally required electrical repetition testing: When operating electrical systems with high availability, a costly and personnel-intensive shutdown of the system can thus be dispensed with under the right technical and organizational conditions.

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