No system connected to a computer is safe from hacking, period. The networks we rely on today for efficiency and convenience are the same networks that allow this kind of security violation to take place. It's not simply a matter of patching holes, but having a dedicated team constantly surveying the network looking for weaknesses as they are exploited. You can't always know there is a problem until someone tries to take advantage of it.
Surely there must be a way to have a functional electrical grid that does not rely on such a widespread network.