If it is only dripping out of the tube that the pressure relief valve is on when the water heater is on (gas or 230v) then that is normal. The hot water cylinder is pressurized when the heater is on due water expansion. Nothing to worry about.
These heaters have over pressure/temperature valves which sometimes stick, they usually have a small lever on them - try activating this lever a dozen times or so (more water will spill) and see if that helps.
If it doesn't then call a plumber.
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"I beseech you in the bowels of Christ think it possible you may be mistaken"
Oliver Cromwell, 3rd August 1650 - in a letter to the General Assembly of the Kirk of Scotland
