The term high voltage characterizes electrical circuits, in which the voltage used is the cause of particular safety concerns and insulation requirements. High voltage is used in electrical power distribution, in cathode ray tubes, to generate X-rays and particle beams, to demonstrate arcing, for ignition, in photomultiplier tubes, and high power amplifier vacuum tubes and other industrial and scientific applications.

Definition

The definition of high voltage depends on the context of the discussion. Two factors considered in the classification of a "high voltage" are the possibility of causing a spark in air, and the danger of electric shock by contact or proximity.

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 circuits as those with more than 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 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.

The general public may consider household mains circuits (100–250 V AC), which carry the highest voltages they normally encounter, to be high voltage. For example, an installer of heating, ventilation and air conditioning (HVAC) equipment may be licensed to install 24 Volt control circuits, but may not be permitted to connect the 240 volt power circuits of the equipment.

Voltages over approximately 50 volts can usually cause dangerous amounts of current to flow through a human being touching two points of a circuit, so safety standards generally are more restrictive where the chance of contact with such high voltage circuits exists.

In digital electronics, a high voltage is the one that represents a logic 1; this may be only several hundred millivolts for some logic families.

The definition of extra high voltage (EHV) depends on the context of the discussion.

In electric power transmission engineering this refers to equipment designed for more than 235,000 volts between conductors.

A power supply that provides an e.m.f. greater than 275,000 volts is known as an "EHV Power Supply". It is often used in experiments in physics.