The mechanical defect of gas insulated switchgear (GIS) is an important cause of equipment failure. In case of mechanical defects in GIS

abnormal vibration signals will be generated under the action of AC electrodynamic force. The detection and diagnosis on the mechanical defects of equipment can be achieved by measuring the mechanical vibration signals on the GIS enclosure. Either internal or external fastening looseness of GIS equipment is a common mechanical defect on site. However

at present

the vibration signal characteristics of internal and external fastening looseness defects are unclear

which restricts the detection and diagnosis of this type of defects on site. In view of this

such two kinds of defects as internal shield looseness and external anchor bolt looseness on real 550 kV GIS test platform are constructed

and the relevant relations among frequency domain characteristics

signal characteristic and load in case of vibration signals at different defects are compared. The result shows that neither internal nor external defects will cause the variation of 100 Hz fundamental frequency vibration signal

but only cause the slight increase of signal of other frequency points. Therefore

both internal and external looseness defects will not cause obvious increase of vibration signal amplitude of equipment; Comparatively speaking

the internal looseness defects mainly cause the increase of low-frequency signal components such as 200 Hz and 300 Hz

while external looseness defects mainly cause the increase of amplitude of high-frequency signal components above 1 500 Hz; The increase amplitude of low-frequency signal component due to internal looseness defect increases significantly with the increase of load current

while the increase amplitude of high-frequency component due to external looseness defect has little correlation with load current. The research result in this paper has provided theoretical support for the detection and analysis of internal and external looseness defects in field GIS.