There is no universally accepted definition, although some industry standards do specify various minimum voltages, above which is considered high voltage. These definitions are generally based on safety considerations or the voltage where arcing will occur. It would be convenient if high voltage was universally accepted to start at a nice round number, for example, 1kV. Instead, we've seen voltages as low as 5V referred to as high voltage. In contrast, according to the Bonneville Power Administration, to be considered high voltage, it needs to be 100kV and above! We'd like to define this relative term from a more practical viewpoint instead:
High voltage starts at the point where designers have to consider additional technical issues, and where there are significantly fewer component suppliers to choose from.
Technical Issues: Certainly, for most engineers, high voltage is different voltage. It's no longer engineering-as-usual. Other variables enter into the design and manufacturing processes. You need to take into consideration creepage distance, insulation thickness, corona and geometrical arrangement. It's no longer a good idea to use your finger to see which component is running hot. Components exhibit unexpected behavior, like resistors that change value as a function of applied voltage.
Component Supplier Issues: Designers need to be more careful in vendor selection. That's because there aren't tons of suppliers, and the technology isn't as well known. Thus, there are larger differences in quality and reliability among high voltage manufacturers. For example, if you need a 5V power supply, there are hundreds of companies that will make them. But if you need, say, 100V or 10,000V, there are only a handful. And of that handful, there is only a couple that makes high quality, reliable supplies.
Any voltage greater than 40 volts is generally considered potentially (!) dangerous.
Voltage of 208V and higher.
High voltage is defined by the DOE Electrical Safety Guidelines as over 600 volts.
Generally considered to be a wire or cable with an operating voltage of over 600 volts.
Any electric potential capable of producing breakdown in air at STP, or around 600volts.
A voltage higher than that used for power distribution. The lower limit is usually taken as either 5000V (Bell) or 8700V (National Electrical Safety Code).
10 kV, because that's about where you have to start worrying about corona and it gets harder to find off the shelf components. [Editor’s note: this is a good practical definition, however, other issues besides corona occurs at lower voltages.]
Generally, a wire or cable with an operating voltage of over 25, 000 volts.
An electrical system or cable designed to operate between 46 kV and 230kV.
Descriptive of transmission lines and electrical equipment with voltage levels from 100 kV through 287 kV.
Least useful definition?: Adjective:. High-voltage - operating on or powered by a high voltage; "a high-voltage generator"
Medium Voltage: This is a term used by the electrical utility and power distribution industry. One of the definitions is from 1kV to 35kVac. Other definitions of the medium voltage range start as low as 600V, and go as high as 72.5kV. This certainly overlaps with most generally accepted definitions of high voltage.