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# DETAIL

Voltage is a physical quantity to measure the energy difference of unit charge in electrostatic field due to different potential. Its magnitude is equal to the work done by the unit positive charge moving from point A to point B under the action of electric field force. The direction of voltage is defined as the direction from high potential to low potential. The international unit system of voltage is volt (V for short), and the commonly used units are millivolt (mV), microvolt (uV), kilovolt (KV), etc. This concept is similar to the "water pressure" caused by the water level.

## Classification of Voltage By Size

The voltage can be divided into high voltage, low voltage and safe voltage. The difference between high and low voltage is based on the voltage value of the electrical equipment to the ground. High voltage refers to the voltage above or equal to 1000 volts to the ground. Low voltage refers to the voltage with ground voltage less than 1000 volts. Safety voltage refers to the voltage that the human body contacts for a long time without the risk of electric shock. According to the national standard "GB3805-83", the safety voltagerefers to a series of voltage supplied by a specific power source for preventing electric shock accidents. In China, there are five levels of power frequency safety voltage, namely 42V, 36V, 24V, 12V and 6V.

## Classification of Voltage By Function

### 1.Impedance Voltage

(1) Definition of Noun

A winding in a two-winding transformer is short-circuited with a voltage of a rated frequency applied to the other winding, and the applied voltage value when the rated current flows. For a multi-winding transformer, except for the pair of windings tested, the remaining windings are open and the applied voltage value is passed through the rated current corresponding to the winding of the pair of windings having a smaller rated capacity. The impedance voltage of each pair of windings is the value at the corresponding reference temperature and is expressed as a percentage of the rated voltage value of the applied voltage winding.

(2)Impedance VVoltage Calculation Method

When the secondary winding of the transformer is short-circuited (steady state), the voltage applied by the primary winding to circulate the rated current is called the impedance voltage Uz. Usually Uz is expressed as a percentage of the rated voltage, ie uz = (Uz / U1n) × 100%. The turn potential: u = 4.44 * f * B * At(V). B refers to magnetic density in the core, At refers to core effective intercept area. It can be converted into a common formula for transformer design calculation:

When f = 50Hz, u = B * At / 450 * 105(V)

When f = 60Hz, u = B * At / 375 * 105(V)

If the phase voltage and the number of turns are known, the turn potential is equal to the phase voltage divided by the number of turns.

### 2.Medical Voltage

The voltage of electrocardiogram refers to the distance between two horizontal lines on electrocardiogram paper. It is often used to measure the amplitude of electrocardiogram. Units are usually expressed in mm or mV. The value of voltage is related to the regulation of * constant quasi-voltage. If a standard voltage of 1 mV is input to shift the baseline by 10 mm, the distance between the two thin horizontal lines is 1 mm and the voltage is 0.1 mV. When measuring the amplitude of an electrocardiogram, the voltage calibration side makes sense.

Voltage can be said to be ubiquitous in the electrical power, but sometimes it is harmful if the voltage is too high. For example, if lightning strikes on buildings without lightning protection measures, such buildings are poorly grounded, and lightning currents cannot disappear in time, resulting in local high voltages that endanger internal people or electrical equipment. Buildings that use lightning rods or lightning arresters, because of their good grounding, can direct lightning from the top of the building to itself and can safely drain into the ground through the grounding electric cable.

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