Gas insulated substation is widely applied in power system because of many advantages

including high reliability

easy maintenance

relatively less space etc. However

very fast transient（VFT） process can be produced

during the operating of disconnector switch

earth switch and circuit breaker. VFT is mainly characterized as very fast transient overvoltage on GIS bus-bar bushing

and as the raise of transient earth voltage on GIS enclosure

and as transient electromagnetic field （TEM） around the GIS. In this paper

the mechanisms about TEM generation and TEM production

caused by VFT

are analyzed. Meanwhile

the influences of VFT on the cable and the transformer connected with GIS are studied.