The presence of free metallic particles and some other conditions which might lead to breakdown in 420 kV gas insulated substations (GIS) can be detected by measuring their partial discharge activity. The discharge pulses excite the GIS chambers into many modes of resonance at ultra high frequency, which may be picked up by couplers built on to hatch cover plates. Signals from the couplers can be displayed in both the time and frequency domains. Because the oscillations spread throughout the GIS, only a few couplers are needed to cover a complete substation. The method has excellent sensitivity; for example, particles smaller than those likely to cause breakdown can be detected from many tens of metres. Moreover, the nature of the discharging source can be determined, and its position located with good accuracy by timing the wavefronts arriving at couplers on either side of it. Tests using various discharge sources have been made both in the laboratory and on site, and the results are reported. This technique is intended in the first instance for use during GIS commissioning tests. However, simpler equipment operating on the same principle has been developed to monitor continuously the partial discharge level of the GIS in service.