Clear guidelines exist to calculate harmonic emission limits for a new customer installation in the planning stage. No clear methodology exists to determine harmonic emission of an operating customer installation based on field measurements. This paper provides a comprehensive framework to determine the harmonic contribution of a customer installation partly based on methodologies recently discussed in CIGRE/CIRED JWG 4.42. The respective indices are implemented into an existing PQ measurement instrument and a comprehensive set of field measurements was performed in Switzerland. The contribution indices are compared with the respective emission limits. The framework consists of two stages. While the first stage analyses the current harmonics, the second stage analyses the voltage harmonic contribution. Practical challenges regarding the application of the method are identified and open issues for future research are discussed.