The Transmille 1000, 3000A, 4000 and 9000A units ship with the High Voltage output limited to 20 minutes for safety reasons.
If required, users can disable this functionality so that the output can be shortened (for example to 5 minutes) or disabled completely so that the output will not time out.
To perform this, first make sure that the Transmille Calibrator Setup Utility is downloaded and installed on your computer (Support Link)
Connect the calibrator via USB and start the Calibrator Setup Utility, and click on 'Settings'
On the settings window that appears, navigate to 'High Voltage Timeout'
Enter either the desired time in seconds, or to disable enter 0 and then click 'Store'
The changes are permanent and will stay with the unit across restarts/firmware updates unless otherwise specified.