Breakdowns in the devices within electrical network on electrical lines that are connected cause harmonic.

What could cause harmonical breakdowns?

Internal and external harmonics we can say.
External Harmonics: Harmonics that affect our system over the electrical network

Example: Deployments of the harmonic from the other lines
Internal harmonics: Harmonics from the devices used in the plant
Phase controlled rectifiers and invertor circuits
Motors

Power control tools (Thyristor and Triax)
Charge ammeter systems
Flurescent lamp and ballasts
Pcs
Drivers
Owens
etc.

Can it be measured?

Harmonical breakdowns can be measured by harmonical analyzers (energy analyzer, network quality analyzer, etc.) and recorded.

Following is the device that can be used.