What Is Low Voltage (LV)? Definition & Range

Low voltage: a voltage not exceeding a conventionally adopted limit [defined in the IEC 60050-195-2021].

Note 1 to entry: For AC, the conventionally adopted limit is 1 000 V.

Note 2 to entry: For DC, the conventionally adopted limit is 1 500 V.

British Standard BS 7671, Requirements for Electrical Installations. IET Wiring Regulations, defines supply system low voltage as: exceeding extra-low voltage but not exceeding I 000 V AC or 1500 V DC between conductors, or 600 V AC or 900 V DC between conductors and Earth. The ripple-free direct current requirement only applies to 120 V DC, not to any dc voltage above that. For example, a direct current that is exceeding 1500 V DC during voltage fluctuations it is not categorized as low-voltage.

Low voltage in international standards refers to any AC voltage up to and including 1000 V and DC up to and including 1500 V. International standards also widely use the term “low voltage”. For example, an electrical installation of a building, according to the requirements of IEC 60364, is a low-voltage electrical installation, and may consist of electrical circuits operating at voltages up to 1000 V AC and 1500 V DC.

IEC 60947-2 complex standards set requirements for low-voltage switchgear and controlgear, which can operate at AC voltages up to 1000 V and DC up to 1500 V inclusive. IEC 60439-1 standards set out requirements for complete low-voltage switchgear, which can have nominal voltages up to 1000 V AC and 1500 V DC.

IEC standards and the national standards developed on their basis classify electrical installations and equipment into low-voltage and high-voltage electrical installations and electrical equipment.

In order to provide requirements for installations and for equipment, the following bands are specified:

  • High voltage (HV). Where protection against electric shock is ensured by special measures, in particular earthing arrangements.
  • Low voltage (LV). Where protection against electric shock is ensured by basic protection and in general also fault protection.
  • Extra-low-voltage (ELV) is a part of the LV band. When ELV is applied, fault protection may not be needed, and under certain conditions basic protection is provided by limitation of voltage. These conditions include contact area, moisture, voltage, current, and others defined for particular applications.

Table 1 specifies the different voltage limits for the above mentioned bands.

The values in Table 1 are based on the following conditions:

  • a.c. systems:

– for earthed systems by the r.m.s. values of the voltages between line and earth and between lines;
– for isolated or not effectively earthed systems, by the r.m.s. value of the voltage between lines.

  • d.c. systems:

– for earthed systems by values of the voltages between line and earth and between lines;
– for isolated or not effectively earthed systems, by the value of the voltage between lines.

Voltage banda.c.d.c.Defining risk
High voltage (HV)> 1000> 1500Electrical arcing
Low voltage (LV)≤ 1000≤ 1500Electrical shock
Extra Low voltage (ELV)≤ 50≤ 120Low risk
Table 1 – Limits for voltage bands [3]

The International Electrotechnical Commission (IEC) Standard IEC 61140:2016 defines Low voltage as 0 to 1000 V AC. rms or 0 to 1500 V DC. Other standards such as IEC 60038 defines supply system low voltage as voltage in the range 50 to 1000 V AC or 120 to 1500 V DC in IEC Standard Voltages which defines power distribution system voltages around the world.

The upper limit of ELV of 120 V DC has for many years been agreed by convention. However, different environmental and contact situations as described in IEC TS 60479-1 cause different values of touch current, for a given voltage. Also the waveform of the current and the path taken through the body strongly influences the level of danger. Therefore, technical committees are requested to consider very carefully whether an ELV value less than 120 V d.c. might be necessary for their specific standard.

Standard NFPA 70E, Article 130, 2021 Edition, omits energized electrical conductors and circuit parts operating at less than 50 V from its safety requirements of work involving electrical hazards when an electrically safe work condition cannot be established.

References

  1. IEC 60050-195-2021
  2. BS 7671:2018+A2:2022
  3. IEC 61140-2016