When there is insufficient power station capacity to supply
the demand (load) from all the customers, the electricity
system could become unstable, possibly resulting in a
national blackout. To avoid this, Eskom can either increase
supply or reduce demand.
To increase supply, Eskom runs its power stations at
maximum available capacity. In addition to the coal and
nuclear plant, hydro and gas turbine stations are used at
peak times when demand is high.
To reduce demand, Eskom will first call on customer
contracts that allow for supply to be reduced or interrupted
for specific periods and durations. If this is not enough,
Eskom is forced to cut supply to all other customers. This
could be done through either scheduled or emergency
During scheduled load shedding, parts of the network are
switched off according to a predetermined schedule, with
the impact spread equitably over the customer base. Load
shedding is predictable and allows for customers to plan
accordingly, as opposed to blackouts that are not.
In exceptional circumstances, such as many unplanned
outages at power stations, additional emergency load
shedding could be required. Control centres will then shed
load using emergency switching in order to protect the
network. This will be less predictable and all customers
may be affected at any time.