Definition
| IEC voltage range | AC | DC | defining risk |
|---|---|---|---|
| High voltage (supply system) | > 1000 Vrms | > 1500 V | electrical arcing |
| Low voltage (supply system) | 50–1000 Vrms | 120–1500 V | electrical shock |
| Extra-low voltage (supply system) | < 50 Vrms | < 120 V | low risk |
The numerical definition of high voltage depends on context. Two factors considered in classifying a voltage as "high voltage" are the possibility of causing a spark in air, and the danger of electric shock by contact or proximity. The definitions may refer to the voltage between two conductors of a system, or between any conductor and ground.
In electric power transmission engineering, high voltage is usually considered any voltage over approximately 35,000 volts. This is a classification based on the design of apparatus and insulation.
The International Electrotechnical Commission and its national counterparts (IET, IEEE, VDE, etc.) define high voltage as above 1000 V for alternating current, and at least 1500 V for direct current—and distinguish it from low voltage (50–1000 V AC or 120–1500 V DC) and extra-low voltage (<50 V AC or <120 V DC) circuits. This is in the context of building wiring and the safety of electrical apparatus.
In the United States 2005 National Electrical Code (NEC), high voltage is any voltage over 600 V (article 490.2). British Standard BS 7671:2008 defines high voltage as any voltage difference between conductors that is higher than 1000 V AC or 1500 V ripple-free DC, or any voltage difference between a conductor and Earth that is higher than 600 V AC or 900 V ripple-free DC.
Electricians may only be licensed for particular voltage classes, in some jurisdictions. For example, an electrical license for a specialized sub-trade such as installation of HVAC systems, fire alarm systems, closed circuit television systems may be authorized to install systems energized up to only 30 volts between conductors, and may not be permitted to work on mains-voltage circuits. The general public may consider household mains circuits (100–250 V AC), which carry the highest voltages they normally encounter, to be high voltage.
Voltages over approximately 50 volts can usually cause dangerous amounts of current to flow through a human being who touches two points of a circuit—so safety standards, in general, are more restrictive around such circuits. The definition of extra high voltage (EHV) again depends on context. In electric power transmission engineering, EHV refers to equipment that carries more than 345,000 volts between conductors. In electronics systems, a power supply that provides greater than 275,000 volts is called an EHV Power Supply, and is often used in experiments in physics.
The accelerating voltage for a television cathode ray tube may be described as extra-high voltage or extra-high tension (EHT), compared to other voltage supplies within the equipment. This type of supply ranges from >5 kV to about 50 kV.
In digital electronics, a logical high voltage is the one that represents a logic 1. It is typically represented by a voltage higher than the corresponding range for logic 0, but the difference may be less than a volt for some logic families. Older systems such as TTL used 5 volts, newer computers typically use 3.3 volts (LV-TTL) or even 1.8 volts.
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