When testing an extinguisher against a class E fire the test is to ensure that when discharged, electricity does not flow up through the extinguisher and cause harm to the user. If an extinguisher achieves a class E rating, it means the product can put out electrical based fires and material if they were to catch on fire.
The test is performed in an electric chamber purposely built for the test for accuracy and safety reasons
Within the test 35kV (35,000 volts) will be pulsated through a large steel plate, whilst an extinguisher is discharged aiming directly at the plate from one meter away. If the current that runs back through the extinguisher is below the required amount to be considered safe, you have passed the test.
The extinguisher must pass two fire tests to be deemed for that class rating.