DeLonghi TRD2000T oil filled heater trips mains switchboard RCD?

This 2000W oil column heater has a small and large heater element. Power settings I, II & III presumably use either the small element (I), the large element (II) or both elements together (III). The 30mA threshold RCD (I think these may be known as GFI in the U.S.) on the switchboard is being tripped when you switch from setting II to III. Have removed the control panel and all wiring from the from the chassis of the heater. Have not been able to find any earth leakage by doing a continuity test between the electrical contacts of the element and chassis. I'm in the southern hemisphere and it's perhaps significant that this heater has not being used since last winter (around 6 months ago) although there is no excessive build-up of dust inside. It went for around 10 minutes initially before the first RCD trip. Other components on this heater include thermostat, timer, LED's (x2), non-upright safety cut-out switch plus another as yet unidentified. No earth leakage detected at 3-pin plug before taking apart. Heater also trips RCD in another building which suggests the RCD is not at fault. Perhaps the earth leakage only occurs under greater load than what my meter is testing at. Any thoughts on what to test for next in this situation? Thanks.