Partial Discharge Tester: If “four” refers to refined testing after withstand voltage, then it is PD testing. Core function: Detect tiny, non-penetrating discharge phenomena inside the insulation of high-voltage equipment. Importance: Many insulation defects do not break down during withstand voltage tests, but under long-term operating voltage, they gradually expand due to partial discharge, eventually leading to accidents. 2026 Standard: For transformers of 110kV and above, the partial discharge level is usually required to be controlled within 100pC or even 50pC; for GIS equipment, the requirement is stricter (<5pC). Another possibility: Are you looking for a “four-in-one” integrated device? Currently on the market (2026), multifunctional integrated test carts or portable four-in-one instruments are popular, integrating the following functions into one device to improve field efficiency: DC resistance test, turns ratio and group test, dielectric loss test, insulation resistance test. (Note: Withstand voltage test, due to high power, usually still uses a separate test transformer.) This type of device is often called a “transformer comprehensive characteristics tester” or “four-in-one tester.” Summary and Suggestions: If you want to know the fourth key test besides DC resistance, dielectric loss, and withstand voltage, it is usually: Oil Chromatography Analysis (DGA) —— specifically diagnosing internal faults caused by “heat” and “electricity.” Or Partial Discharge Test (PD) —— diagnosing microscopic insulation defects. If you are referring to a specific product model or a specific “electrothermal electrical four” term (e.g., a series number from a specific manufacturer), please provide more context (such as manufacturer name, application scenario), and I can do a more precise search and answer. What specific aspect are you interested in? A. Transformer oil chromatography analysis (diagnosing overheating/discharge faults) B. Partial discharge test technology C. Multifunctional integrated test instrument (four-in-one) D. Other specific “electrothermal” related tests
