## 2 Test Setup and the Course of the Experiment  
The first stage of the studies involved measurements of acoustic emission signals in electrically-stressed epoxy resin samples. The samples had cubicoid shapes of 25 mm \(\times\) 10 mm \(\times\) 4 mm. One sample surface of 25 mm \(\times\) 4 mm was ground and coated with varnish conducive to electric current.  
Since the process of tree channels forming in a dielectric may last very long, an electrode made of a T10 surgical needle with liquid resin poured inside the sample during sample formation was used, such that the distance between the sample bottom and the electrode end ranged from 1-3 millimeters. The procedure allowed obtaining the electric field intensity between the needle electrode and the electrode applied to the bottom of the sample, which was high enough to reduce the tree-forming duration to a few hours. The needle was connected to the neutral terminal of the transformer. The opposite base of the sample was adjacent to a plane copper electrode. The electrode was connected through a resistor of 0.5 M\(\Omega\) resistance to a high-voltage terminal of a test transformer with the ratio of 220 V/30 kV and power of 10 kVA. The voltage was measured with an electrostatic voltmeter.  
As part of the tests, acoustic signals were recorded for a dozen or so samples in which, under the influence of a high intensity of the electric field, the process of forming an electric tree began. The measurements were made for variable values of the supply voltage with a 50-Hz frequency and different distances between the electrodes.  
The elastic waves of acoustic frequencies were emitted from the analyzed sample through a wave-guide made of a steel rod of 2 mm in diameter. One of its ends was put into a hole bored in the sample, while the other was connected to an electroacoustic converter.  
As regards measurement time, the studied sample was placed in a methyl polymethacrylate vessel filled with electrical insulating oil. The voltage between the electrodes during the tests ranged from a few to several kV. The AE signals were measured by means of a Physical Acoustic Corporation (PAC) R3\(\alpha\) electroacoustic converter and a filtering and amplifying system composed of 2/4/6 type pre-amplifier from PAC, a 20 \(\div\) 1000 kHz transmission band filter, and PAC AE5A amplifier. Upon amplification, the signal was recorded in the computer memory with an data acquisition (DAQ) NI-USB6251 card, which enabled signal recording with a sampling frequency up to 1 MS/s and 16-bit resolution. The test setup schema, the view of the measuring equipment, and the view a sample holder are presented respectively in Figure 1a-c.  
Figure 1: _Cont._  
To ensure that the recorded signals concerned the electrical treeing while we were preparing the experiment, we also tested and observed under a microscope different states of the system, i.e., without any PDs, with corona, etc. We noticed that each of the PD types was connected with different waveforms of signals, and this can be found also in different researchers' results [35]. After that test, we rebuilt/reconfigured the setup and chose a voltage range to be sure that other types of PDs would not be present. Physically, electrical treeing is a combination of chemical and physical changes inside the specimen, so no one can be sure whether that particular signal is connected with the breaking polymer chain or PD inside the existing channel.  
Figure 1: The test setup schema **(a)**, view of the measuring equipment **(b)**, and view of the sample holder **(c)** for measuring acoustic signals accompanying the treeing of solid dielectrics.