Transformer oil is typically tested for its closed-cup flash point.
The reasons are as follows:
Simulate Actual Operating Conditions: Transformer oil resides within a relatively sealed environment (the tank) inside the transformer. The closed-cup flash point test method (such as ASTM D93 - Pensky-Martens closed-cup method) more closely simulates the conditions where vapor accumulates above the oil in a confined space and could potentially be ignited.
Safety Assessment: The closed-cup flash point is more sensitive for detecting small amounts of light volatile contaminants (like gasoline, solvents) that may have mixed into the oil. These contaminants can more easily accumulate to a flammable concentration in a confined space, significantly lowering the flash point, which is a critical safety hazard for transformer operation. The closed-cup method can detect this risk more sensitively.
Standard Specifications: Major international and national standards for transformer oil specifications (such as IEC 60296, ASTM D3487, GB 2536) all explicitly stipulate that the flash point test for transformer oil must use the closed-cup method. This is the industry-recognized standard test method.
Result Comparability: Using a unified standard method (closed-cup) for testing ensures that flash point data from different batches, different manufacturers, and oil in service is comparable.
Summary:
Open-Cup Flash Point: The test apparatus is open, making it more suitable for simulating oils used in open or semi-open environments (e.g., engine crankcases, open oil storage tanks), like lubricating oils and fuel oils.
Closed-Cup Flash Point: The test apparatus is sealed (with only a small opening to introduce the ignition source), making it more suitable for simulating oils used in sealed systems, like transformer oil and aviation hydraulic fluids.
Therefore, considering the characteristics and application environment of transformer oil, the closed-cup flash point is the more relevant, standard, and safer testing parameter.