Potential induced degradation (PID) is one of the factors that could contribute to the long-term degradation of PV modules. It is, therefore, essential to carry out the PID test for different open-air conditions as well as both voltage polarities for modules that maybe designated by the manufacturer to operate in either polarity. This paper evaluates the influence of PID test temperature, humidity-based and carbon-based surface conductivity of glass, and system voltage polarity on the degree of PID effects on fresh and pre-stressed (thermal-cycling or damp-heat) mono- and poly-crystalline silicon modules. Irrespective of PID test temperature (85oC or 60oC) and pre-history (fresh or pre-stressed in damp-heat or thermal-cycling), this work indicates that the positive-voltage has little or no PID effect on all the tested modules when humidity or carbon is used for the glass surface conductivity. In contrast, irrespective of PID test temperature (85oC or 60oC) and pre-history (fresh or pre-stressed in damp-heat or thermal-cycling), all the tested modules appear to be susceptible to the negative-voltage when conductive carbon (carbon layer) is used for the glass surface conductivity. However, when humidity is used for the surface conductivity instead of carbon, only the fresh and damp-heat stressed modules, excluding thermal-cycling stressed modules, appear to be susceptible to the negative voltage irrespective of PID test temperature (85oC or 60oC). It is also concluded that the humidity based approach may be a better replication of the field degradation (if any) pattern as compared to the metallic/carbon conductive layer approach.