Low voltage

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.
The International Electrotechnical Commission (IEC) 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.
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.
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.
In electrical power , 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 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.