For the development of resistive fault current limiters understanding of the transition intothe normal state is a crucial point. Thereby, there is a ongoing discussion of whether thisquench formation is due to a thermal or a non-thermal process. Fast propagation velocityof the quench and a transition clearly below the critical temperature are hints at anon-thermal effect.We have investigated quench formation optically. Thereby, we found that the quench formation velocity isabove 10 m s−1 along a YBCO stripe. After a certain quench length, which depends on the applied voltage,the propagation mechanism changes and the propagation velocity is reduced to0.5 m s−1. We proved the optical results by measuring the quench propagation velocity via thevoltage drop at taps separated by a defined distance.To explain quench formation we build up a thermal model within a numerical simulation.Thereby, flux flow motion leads to a thermally induced transition. Dissipated energy istransferred into the substrate, where it diffuses further on. This leads to a fast enlargementof the normal zone. The calculated propagation values are in good agreement with theexperimental results.