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Determining the Proper Use of Low and High Voltage in Electrical Applications

June 25, 2019 by Sam Holland

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What is High Voltage (HV)?

Electrical standards associations from across the globe define high voltage differently. For example, the International Electrotechnical Commission (IEC), IET, and IEEE defines high voltage (HV) as anything above 1,000 VAC or 1,500 VDC; whereas the U.S. 2011 National Electric Code has no fixed value for HV (but while making a distinction between voltages greater or less than 600V). Engineers working in various countries must take local and national standards into consideration while designing applications.


An electrical power transmission and distribution network diagram. Image courtesy of National Park Services.


Electrical power is often transmitted over long distances at a high voltage and low current (amperage). The main purpose of this is to increase efficiency by minimising losses due to high resistance and I2R heating in the conductors. Consequently, power can be transmitted over great distances using thinner conductors. On the other hand, power transmission at high currents would require significantly larger conductors, which would make the process quite expensive. Some important applications for high voltages are in power distribution networks, the scientific study of particles, in cathode ray tubes, and amplifier vacuum tubes.


What is Low Voltage (LV)?

Like high voltage, low voltage (LV) is also a relative classification. The IEC considers low voltage to be anything between 50 and 1,000 VAC or 120 and 1,500 VDC. In power distribution systems, a low voltage may refer to the mains voltage used in domestic and light industrial/commercial applications, which can be anywhere from 100 up to 240 VAC. Electrical fixtures, LED lights, fans, and personal computers can be regarded as low voltage devices. Important industrial applications for LV include axial and radial fans, centrifugal and semi-submersible pumps, chillers, and milling machines.


Sag and clearance for overhead transmission line. Image courtesy of Research Gate.


Working with High and Low Voltages

Determining the proper use of voltage levels allows engineers to design efficient and safe devices and installations.

Three essential considerations for working at higher voltages relate to:

  • Physical dimensions
  • Passive components
  • Connectors and cabling


Physical Dimensions

Conductor spacing is a crucial consideration when designing HV systems. The basic 'rule of thumb' is to allow for 7.5 kV to 10 kV per inch in air. Two essential variables, which the engineer in question must determine, are clearance and creepage.

Clearance is the shortest distance between a conductor and the ground, which is measured through an insulating medium. Creepage is the shortest distance between two conductors that is measured along the surface of the insulating material. The minimising of creepage is vital to prevent hazards like electric arcs and flashovers. To determine the ideal spacing for the conductors, we multiply the root-mean-square (RMS) voltage by 3. Engineers can use IEC guidelines and specifications in their designs.


Passive electrical components. Image courtesy of Pixabay.


Passive Components

Engineers designing HV systems are often more concerned with the tolerances and maximum voltage ratings of the passive components that they work with (e.g., resistors, capacitors, and switchgear) than their LV-designing counterparts, who rarely have their components subjected to higher voltages. In HV design, choosing passives with higher maximum voltage ratings will prevent premature, or even catastrophic, failures. To be on the safe side, engineers must choose components with a safety factor that is a few times greater than the expected maximum voltage.


Connectors and Cabling

Connectors and cabling provide connectivity between various components of an electrical installation. To design an optimal layout for electrical wiring, engineers must again put creepage and clearance into consideration. Also, as is the case when choosing passive components for HV installations, designers must choose cables that have sufficient maximum voltage ratings, to avoid issues such as overheating and insulation breakdown. 


Voltage regulating devices being worked on by hand. Image courtesy of Pixabay.


Vital Safety Considerations for HV and LV

Maintaining a high degree of safety in HV and/or LV applications is critical to prevent hazardous workspaces, damage to electrical equipment, and loss of life. Both high and low voltages are dangerous when standard safety precautions are not adhered to strictly. For example, exposure to 220 volts of alternating current can induce atrial fibrillation, and HV electric shocks may cause physiological damage to one's vital internal organs.

The most common ways that an energy source can be hazardous is via contact or electric arcing. The former happens when a person’s body touches a naked conductor; and the latter occurs when a person at a short distance from an electrical installation, particularly when a high voltage electrical discharge occurs through the surrounding air. Live contact can cause electrocution while electric arcs can cause severe burns.


Electrical system isolation through a 'lockout, tagout' procedure. Image courtesy of Bigstock.


To minimise incidences of contact while working with low and high voltages, engineers must utilise adequately-rated components, keep installations and workspaces moisture-free, use suitable personal protective equipment, and isolate power sources when they're not in use, (namely using industry safety procedures, such as 'lockout, tagout').

Engineers can prevent electric arcing by using overcurrent protection devices, such as circuit breakers and fast-acting fuses that quickly open a circuit under abnormal conditions. It’s also vital to de-energise equipment and 'prove dead' at all HV installations to ensure that no stray electric currents will be discharged into the surrounding air.

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