What is a Buchholz relay and what fault does it detect?

• A. A transformer oil-immersed relay that detects internal faults via oil/gas movement indicating insulation problems.
• B. A relay that detects external short circuits on the primary side.
• C. A sensor for cooling system performance.
• D. A device that monitors transformer winding temperature.

Answer: (A)

The Buchholz relay is a protective device used in oil-filled transformers that detects internal faults by sensing movement of oil and the generation of gas inside the transformer. When insulation within the transformer begins to break down or a fault such as arcing occurs, gases are produced and oil flow patterns change. This gas accumulation and oil movement travel through the transformer’s oil sump to the relay, causing a float or gas-sensing mechanism to trigger a trip or alarm. That early detection helps isolate the transformer before severe damage occurs. This is why the description that matches is an oil-immersed relay that detects internal faults via oil and gas movement indicating insulation problems. It’s not about external short circuits, cooling system performance, or winding temperature monitoring, which are handled by other protection or monitoring devices.