Video: OT-90

Video transcript

The OT-90, and the breakdown voltage test of insulating oil to IEC 60156: the cell, the sample, the ramps, and how to trust the number at the end. Moisture and particles lower the breakdown voltage of an oil long before the transformer shows it. The OT-90 turns a sample into that figure, in the laboratory or next to the equipment, because it runs for a full day on its own battery. The test voltage runs from 10 to 90 kilovolts on the electrodes, rising at two kilovolts per second, measured to 3 per cent.

The mean and the spread of the breakdowns are calculated by the instrument. Open the lid and the test compartment is all there is. The cell sits on two high voltage terminals. The lid is interlocked: open it and no voltage can be applied. The caps cover the plugs of the generator, which is filled with oil. The panel is simple. The power switch, a green light for power and a red one for high voltage, the display, and 4 keys: start, stop, the results, and a pause.

With the instrument come the cell and its two electrodes, the stirring rod, a reference gauge for the gap, a remote control, the earth and mains cables, and a spare fuse. The Bluetooth version adds the PC software. Safety first. Two qualified people, authorised for more than a 1000 volts. The power switch at O. The case earthed before anything else. Then the mains. And never run the tester with a faulty interlock. At the back, two connections, in this order.

The earth wire from the clamp to protective earth. Then the mains cable into its socket, unless you run on the battery. The standard fixes the geometry. Two mushroom-profile electrodes with a 25 millimetre radius face each other in the cell, which holds at least 400 cubic centimetres. The gap is checked with the reference gauge: one face must pass freely, the other must not. A good result starts with the sample.

Clean the cell with a solvent and rinse it with the liquid to be tested. Turn the sample container over gently, several times, without making bubbles, and let it reach room temperature. Then fill the cell and put in the stirring rod. The IEC programme is built in. Six breakdowns, automatic stirring, a settling time after the cell is filled and another after each breakdown, and the voltage rising at two kilovolts per second until the oil breaks down.

Switch on. Until the cell is in place and the lid is closed, the display says so, and nothing happens. Then READY. Press start: the settling time counts down, the warning appears, the red light comes on, and the voltage climbs. This is the whole sequence. The sample settles. The voltage rises at two kilovolts per second until the oil breaks down, and is cut at that instant. The oil is stirred and left to settle, and the next ramp begins, 6 times.

The mean of the 6 is the result. One figure alone proves little, so the OT-90 checks its own result. It calculates the mean, its standard error, and the coefficient of variation. Above 20 per cent, it asks for a fresh sample from the same container and 6 more breakdowns. If the scatter is still too high, the oil fails. The results key pages through everything: date, time, number of tests and breakdowns,

the mean, the statistics and each breakdown value. The thermal printer issues the ticket as soon as the test ends, and over Bluetooth the PC stores the results and prints the reports. When a laboratory procedure differs from the standard, the custom programme sets each parameter: up to 20 breakdowns, stirring time and speed, and both settling times up to 20 minutes. The IEC programme stays one menu away.

Look after the electrodes and they look after the result. Clean and polish them, rinse with petroleum spirit and acetone, then run 24 breakdowns in clean oil before the first real test. Store the cell covered and full of dry liquid, and have the tester verified every year. For transformer makers and service companies, for oil circuit breakers, instrument transformers and capacitors, and to check the oil after treatment. Insulating oil testing, from Amperis.