This is an abstract of a paper explaining the results of a numeric simulation of an attack on a simulated network grid:

ScienceDirect – Safety Science : Cascade-based attack vulnerability on the US power grid.

The researchers found that attacks on nodes withe smaller loads created more harm than attacks on nodes with higher loads. Though they said this is counter-intuitive, think about it for a minute:

If you have an uneven distribution of work, the busiest entity has less capacity to absorb additional work. Think about asking a busy Mom to answer a survey while the kids are screaming. It’s likely she will shed some of her work to someone else (Dad?) in order to answer the survey. But if there is no one there to absorb the current work, any additional work causes Mom to be over capacity, and chaos reigns.

So build networks so they have at least two entities available for critical functions, and make sure that the unloaded entity gets checked as often as the loaded entity. Now, explaining that to someone paying the bills is another matter altogether.