Electrical networks are at the heart of modern economies -- which makes them a prime target for disruption by Global Guerrillas. Electrical systems are complex networks exhibit power laws (in failure plots and network organization) and self organizing criticality (SOC). The load on a electrical network is tightly correlated with the propensity of the network to suffer system-wide cascades of failure. Ian Dobson et. al. have modeled this in their paper "Complex Systems Analysis of Series of Blackouts: Cascading Failure, Criticality, and Self-Organization." What is interesting to our analysis is how quickly an electrical network moves from a survivable state to a critical state when the load on the network is increased. See the chart (right) from their model that depicts the inflection point at where the transient loads shunted from a failed node increase the potential of a large network failure.
This implies that electrical networks are at their most vulnerable during summer days (80% plus utilization), when the load is well within the range of cascade responses (electricity use is strongly seasonal). Systems like Iraq are fully utilized (electricity is rationed nationally) and therefore likely to be in a critical range 24/7. Of course, the potential of mismanagement (during a power crisis) and complicating events increases the potential of cascades of failure at much lower levels of utilization.
NOTE: Given this, it's likely that the Moscow blackout was made possible due to an unseasonable heat wave.