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A new method for assessing the condition of electric contacts and joints inside gas-insulated substations (GISs) that overcomes the limitations of conventional diagnostic methods is proposed. The contact resistance in degraded or overstressed electrical contacts varies slightly when currents of different amplitudes are passed. The diagnostic method is based on detecting this nonlinear behavior. For testing of the contacts in large GIS, excitation currents up to several thousand amperes are required. By using a capacitor-based current supply, such currents can be achieved with a portable source. The testing can be carried out without dismantling the GIS. No earlier measurements or manufacturer specifications are required. In full-scale testing under realistic operating conditions on 145-420 kV GIS using prototype instrumentation a very good sensitivity for detecting contact degradation in an early stage was obtained.