Preventive replacement: By preventive replacement of the electromechanical relays the chance on a defective relay is reduced and therefore also the chance on failure of a relay. As the preventive replacement is carried out as a project, the cost for replacement will be lower than in case of single corrective replacements after a failure. Specifically for relays in the HV/MV substations the inspection frequency can be lowered from once per year to 1x/3 year, resulting in lower maintenance costs. In the calculation of investment cost versus risk reduction it is assumed that the new (digital) relay has to be replaced again after 20 years.