Does your Geyser drain out?
If your geyser drains when you have a water outage, there is a high likelihood that your element will burn out. When there is no water around the element the thermostat does no pick up that the element is really hot, and it continues to heat up until the inner resistance wire burns out.
To stop this from happening you need a fully functioning set of vacuum breakers on the cold inlet and hot outlet pipes at the geyser. This will stop the geyser from back siphoning out. If you have these vacuum breakers already installed and they geyser still drains, you have a faulty vacuum breaker that is sticking in the closed position. Replacing the offending vacuum breaker will solve your problems