Hi spider_pig,
No that doesn’t sound right. Most heating devices today include “thermal protection” circuitry. This is simply a backup method of assuring that heating products do no over-heat and cause wires and elements from burning which could result in fires. It sounds as if your heater is defective.
Did you
acclimate the heater before powering it on? Aquarium heaters need to sit for 30 minutes in the aquarium before being powered on to allow the thermostat to adjust to the water temperature. If not, try unplugging the heater for 30 minutes and re-trying.
Good luck!