Low voltage

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In electrical engineering, low voltage is a relative term, the definition varying by context. Different definitions are used in electric power transmission and distribution, and electrical safety codes define "low voltage" circuits that are exempt from the protection required at higher voltages. These definitions vary by country and specific codes or regulations.

IEC Definition[edit]

Voltage range AC RMS voltage (V) DC voltage (V) Defining risk
High voltage (HV) > 1000[1]* > 1500[1]* Electrical arcing
Low voltage (LV) ≤ 1000[1]* ≤ 1500[1]* Electrical shock
Extra Low voltage (ELV) ≤ 50[1]* ≤ 120[1]* Low risk
* May depend on the applicable standard used.

The International Electrotechnical Commission Standard IEC 61140:2016 defines Low voltage as 0 to 1000 V a.c. rms or 0 to 1500 V d.c.[1] 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[2] which defines power distribution system voltages around the world. Since the definition may vary between standards, always refer to the definition provided in the document being used.

In electrical power systems low voltage most commonly refers to the mains voltages as used by domestic and light industrial and commercial consumers. "Low voltage" in this context still presents a risk of electric shock, but only a minor risk of electric arcs through the air.

United Kingdom[edit]

  • British Standard BS 7671, Requirements for Electrical Installations. IET Wiring Regulations,[3] defines supply system low voltage as:

exceeding 50 V ac or 120 V ripple-free dc. but not exceeding 1000 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.

United States[edit]

In electrical power distribution, the US National Electrical Code (NEC), NFPA 70, article 725 (2005), defines low distribution system voltage (LDSV) as 0 to 49 V..

The NFPA standard 79 article defines distribution protected extra-low voltage (PELV) as nominal voltage of 30 Vrms or 60 V dc ripple-free for dry locations, and 6 Vrms or 15 V dc in all other cases.

UL standard 508A, article 43 (table 43.1) defines 0 to 20 V peak / 5 A or 20.1 to 42.4 V peak / 100 VA as low-voltage limited energy (LVLE) circuits.

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Further reading[edit]