The Quick Overview (Based on Common International Standards)
Low Voltage (LV): Up to 1000 V (1 kV). This is the voltage you encounter in daily life.
Medium Voltage (MV): 1000 V to 35 kV. Used for distributing power from substations to neighborhoods and large buildings.
High Voltage (HV): 35 kV to 230 kV. Used for sub-transmission, connecting large substations.
Extra High Voltage (EHV): 230 kV to 800 kV. Used for long-distance, high-power transmission.
Ultra High Voltage (UHV): > 800 kV. The cutting edge of transmission technology for vast distances.
Detailed Breakdown
1. Low Voltage (LV)
This is the "useable" voltage for most consumers and equipment.
Typical Range: 50 V to 1000 V AC (or 120 V to 1500 V DC).
Common Examples:
Residential/Commercial:
120 V, 230 V: Standard wall outlets and lighting in homes and offices.
240 V, 480 V: For larger appliances (electric stoves, dryers) and some commercial equipment.
Industrial:
400 V / 480 V (3-Phase): Powers motors, pumps, and machinery in factories.
Extra-Low Voltage (ELV): A sub-category for very low, generally safe voltages (typically < 50 V AC). Examples include USB ports (5V DC), doorbells (12-24V AC), and telephone lines (48V DC).
Primary Use: Final distribution to end-user equipment and appliances. Safety standards are strict, but the insulation and clearance requirements are less demanding than for higher voltages.
2. Medium Voltage (MV)
MV is the workhorse of distribution, acting as the middleman between high-voltage transmission and local low-voltage consumption.
Typical Range: 1 kV to 35 kV (Some standards extend this to 69 kV).
Common Examples:
5 kV, 15 kV, 25 kV, 35 kV
Railways: The overhead lines for electric trains (e.g., 25 kV AC is a global standard).
Primary Use:
Distributing power from a primary substation (which steps down HV to MV) to multiple smaller distribution transformers in neighborhoods, industrial parks, or large facilities (like universities or factories).
These local distribution transformers then step the voltage down to LV (e.g., 120/240V) for final use.
Key Characteristics: Requires specialized switchgear, transformers, and cables. Work on MV systems requires significant training and personal protective equipment (PPE).
3. High Voltage (HV) & Extra-High Voltage (EHV)
HV and EHV form the backbone of the national power grid, focusing on moving vast amounts of power over long distances efficiently.
Typical Range (HV): 35 kV to 230 kV
Typical Range (EHV): 230 kV to 800 kV
Common Examples:
69 kV, 115 kV, 138 kV, 230 kV, 345 kV, 500 kV, 765 kV
Primary Use:
Transmission: Carrying bulk power from large power plants (nuclear, hydro, coal) to main substations at the edge of cities.
Interconnectors: Linking different regional grids for stability and redundancy.
Key Characteristics:
Efficiency: According to the formula P = V x I, for a given power (P), a higher voltage (V) means a lower current (I). Lower current reduces energy losses due to resistance in the wires (P_loss = I²R). This is the primary reason for using extremely high voltages.
Infrastructure: Large steel lattice towers (or "pylons"), significant air gaps for insulation, and massive substations with circuit breakers and transformers. Work is highly specialized and hazardous.
Summary Table
| Category | Typical AC Voltage Range | Primary Application | Common Examples |
|---|---|---|---|
| Low Voltage (LV) | Up to 1000 V | End-user consumption | Homes (120V/230V), Offices, Factories (480V) |
| Medium Voltage (MV) | 1 kV to 35 kV | Regional Distribution | Neighborhood feeders, Large Factories, Rail Traction (25kV) |
| High Voltage (HV) | 35 kV to 230 kV | Sub-Transmission | Connecting substations within a region |
| Extra High Voltage (EHV) | 230 kV to 800 kV | Long-Distance Transmission | Bulk power from dams, nuclear plants (500kV, 765kV) |
| Ultra High Voltage (UHV) | > 800 kV | Extreme Long-Distance | Intercontinental links (China uses 1100kV) |
Why Do These Classifications Matter?
Safety: Voltage level dictates the required safety procedures, PPE, and regulations for working on electrical systems. An arc flash at 480V is dangerous, but one at 230kV is catastrophic.
Equipment Design: The design of cables, switches, transformers, and insulators is entirely dependent on the voltage class. A medium-voltage cable has much thicker insulation than a low-voltage one.
Regulation and Standards: Electrical codes (like NEC, IEC) have different rules for installation, protection, and maintenance based on these voltage classifications.
Efficiency: As explained, higher voltages are crucial for efficient long-distance power transmission, which is the foundation of a reliable electrical grid.