What is electrical frequency?

In terms of power cables, maintaining a consistent electrical frequency is important because connected devices are designed to operate at a specific frequency. If electricity is supplied at the incorrect frequency, the device can be damaged irreparably. This is the reason that electricity that is supplied on a national scale is done at a fixed frequency. In Great Britain the grid frequency is 50Hz. In the US it’s 60Hz. Internationally it is always 50Hz or 60Hz. 

So what impact does not having stable frequency have on connected systems? Take a motor for example: if it was designed to operate at 50Hz but were to run instead at 60Hz, it would run faster, but the torque would reduced. This reduction in torque may affect the motor's ability to drive its connected load, altering the design parameters and potentially compromising the life of the motor itself.

For data cables, the frequency relates to the transmission rate, with higher frequencies enabling quicker data transfer.  For example, category 5e cable are designed for operating  frequency of 100MHz, enabling a data transfer rate of 1Gbps (gigabit per second), whilst category 7A has a bandwidth of 1000MHz and can transfer 10Gbps.  In respect to the higher frequency, this requires the cable to have more protection against EMI (electromagnetic interference), as at higher frequency, the signal is subject to increased levels of noise and crosstalk and hence has an increased need for shielding.