Restströme

Restströme, or residual currents, are small currents that continue to flow in an electrical installation or equipment even when it is considered de-energized or when insulation appears intact. They arise from leakage paths, parasitic capacitances, and the behavior of protection and measurement devices, and can travel through insulation, protective earth, or signal lines within a system.

Common sources of restströme include moisture or dirt on insulation, surface leakage along cables, capacitive coupling between live conductors and earth or other conductors, leakage through EMI suppression capacitors, and galvanic currents between dissimilar metals in contact with electrolytes. In complex systems, stray currents may also originate from ground loops or from equipment with internal switching.

Restströme are usually small, ranging from microamps to milliamps depending on design, humidity, and contamination. In safety practice, residual currents that trip protective devices are typically specified at 30 milliamps or lower for general use, with more sensitive types (for example 10 mA) used in specialized environments. In precision instrumentation, leakage must often be kept at very low levels to avoid interference with measurements.

Effects of restströme can include nuisance tripping of protective devices, unwanted interference with control or measurement systems, and, in some installations, electrolytic or galvanic effects such as corrosion in piping systems. They can also contribute to electromagnetic interference and degraded signal integrity in sensitive equipment.

Mitigation and management involve good insulation and sealing, proper bonding and grounding, shielding and careful routing of conductors to minimize capacitive coupling, and the use of protective devices such as residual-current devices (RCDs) that detect current imbalances and disconnect the circuit. Regular insulation resistance testing and adherence to relevant standards (for example IEC 60364) help reduce restströme and enhance safety and reliability.