What is Separated Extra Low Voltage (SELV)
Separated Extra Low Voltage (SELV) system is an extra low voltage electrical circuit that is electrically separated from other circuits that carry higher voltages, isolated from the earth and from the protective earth conductors of other circuits.
In the SELV system, the voltage should not exceed the extra low voltage under the normal conditions or under single-fault conditions such as those from earth faults in other circuits. The SELV systems are separated from the earth such that a single fault cannot cause an electrical shock to anyone in contact with the system.
The design involves using an insulation transformer, guaranteed minimum distances between conductors and insulation barriers. The SELV systems must have protective-separation such as reinforced insulation, double insulation or protective screening from all circuits carrying higher voltages. The SELV cables must be double insulated or pass through plastic conduits especially when they are likely to come into contact with other circuits.
In addition, the wiring should be completely separate from systems or cables with higher voltages as well as from other SELV circuits, PELV circuits, and from the earth ground. It is permissible to have un-insulated current carrying parts such as terminal screws for lower voltages of up to 25 Volts. However, it is recommended to use a kind of insulation to avoid short circuiting such as those form batteries.
What makes SELV systems safe
The safety of the SELV systems stems from:
- Low voltages which should never exceed 120 Volts DC and 50 Volts AC, these levels are too low to provide current that would give a lethal electric shock
- Very low risk accidental contact with the high voltages
- Absence of ground or return path that the current would take
SELF system Requirements
For a power system to be classified as a SELV, it must comply with the following:
- The installation should be physically separated from the conductors for the other systems
- The extra-low voltage source should not come into contact with the low voltage system. Commonly used sources are output from an isolating transformer, battery, a motor generator set, or a power supply unit which does not have any low voltage at its output terminals.
- Use of sockets and plugs that are not interchangeable with those from other systems; this prevents accidentally plugging in a SELV equipment into a low voltage system
- No connection between the live parts or exposed conductive parts of the SELV equipment and exposed conductive parts, the protective system, or earth of the low voltage systems. This prevents the dangerously high potential that may arise due to fault conditions from getting into the SELV system
- Do not use luminaire support couplers with grounding option
- The sockets and plugs should not have the earth connection, to avoid mixing the SELV and FELV devices.
Once an SELV system such as isolation transformer is earthed, it ceases to be an SELV and instead becomes FELV (functional extra-low voltage) as illustrated below.
Fig 1: SELV transformer operation (without earth) – Image Credit
Fig 2: FELV system resulting from earth to SELV system – Image Credit
Typical SELV systems
The SELV systems are used in applications where operating some electrical appliances poses a serious safety hazard. Typical locations include amusement parks, swimming pools as well as some domestic electrical appliances such as radios.
Typical examples of SELV are:
- Access control systems
- Lighting control panels
- CCTV and security systems
- DATA and computer cabling systems
The Separated Extra Low Voltage (SELV) systems can be thought of as having a protective partition from the other higher voltage systems. Combining the low voltage, basic insulation, and protective separation makes SELV systems one of the safest electrical systems.