top of page



Partial discharge activities usually occur in High Voltage Switchgear, Transformers and Cables. Partial discharge is electrical charges trying to bridge the gap or space between a point of higher potential to a point of lower potential in an electric field. For this to occur, the potential gradient must be high enough. It is for this reason that partial discharge activities occur mainly in HT installations. And it occurs regardless of whether the equipment is carrying any load or not – it is not current dependent.


Partial discharge, as the name suggests, is partial and not full discharge and it occurs intermittently. In other words, they have dormant periods. However, it is also very important to arrest partial discharge early before they become full blown discharges – ie. a flash-over!


There are 2 types of partial discharge – one that occurs within the insulating materials, eg. bushings and isolators; and one that occurs on the surface of these items. Those that occur on the surface is sometimes referred to as surface discharge.


Insulating materials may have microscopic voids inherent in them. When subjected to an electric field, electrical charges will try to bridge these voids. This is called partial discharge. When this happens, electromagnetic waves are emitted which travel towards the surfaces of metallic enclosures and induce a small voltage on them. These voltages are called Transient Earth Voltages (TEV). A measure of these TEV will give an indication of the severity of partial discharges in the HT equipment. Typically, we use an instrument call PDL1 to measure this type of partial discharge activities. The PDL1 gives a snapshot of such activities. It serves as a good start to pursue further action if necessary. We also use another instrument, PDM03 to monitor partial discharge activities. This instrument is set up on the HT equipment and it monitors the equipment 24/7. This gives a more in-depth picture of the state of health of the HT equipment.


As for surface discharge, these are typically corona discharge, tracking and arcing. These occur as a result of environmental conditions, e.g. moisture, dirt and grime. When these occur, sound waves are emitted in the frequency range of ultra sound, i.e. >20 kHz. Our human ear is insensitive to this range of sound – we can only hear from 20 Hz to 20 kHz. So we use and Ultrasound Probe 10000 to capture these sound waves and analyse them. Corona discharge, tracking and arcing each has its own signature wave pattern thus enabling us to pinpoint precisely the problem.


Our company provides this test as a high level predictive maintenance service to monitor the condition of High Voltage switchgears with expert interpretation by our experienced engineers.


We possess PDL1, Ultrasound Probe 10000, PDM 03 and the Ultra TEV Monitor (it is the PDM03 with ultrasound capabilities). We are thus able to cater to varying needs of our clients.


Information presented in this website is correct to the best of our knowledge; presents our opinion. Viewer discretion is advised.

STEPL and its employees shall not be liable for any inaccuracies or for any special, incidental, direct or indirect consequential damages, loss of profits, production or business interruption arising out of and in connection with the materials provided on this website.

bottom of page