What is SELV and PELV Circuits?

Application and arrangements of SELV and PELV circuits

1. SELV - Definition according to BS 7671

SELV - Safety (Separated) Extra-Low-Voltage
Extra-Low-Voltage system (i.e. normally not exceeding 50 V a.c. or 120 V ripple-free d.c.) which is electrically separated from earth and from other systems in such a way that a single fault cannot give rise to an electric shock.

Protection by SELV is used in high risk situations where the operation of electrical equipment presents a serious hazard to safety. One typical location is Zone 0 in a pool zone which is in the water container itself. The nominal voltage for a pool light connected in zone 0 must not rise above 12V ac or 30V ripple free dc.

In this system there is NO return path for current via earth as protective earth conductors are NOT permitted to be installed on the secondary ELV side of the transformer.

Typical applications for SELV systems include:

• Pool lighting
• Spa lighting
• Sauna lighting
• Bathroom lighting

2. PELV - Definition according to BS 7671

PELV - Protective extra low-voltageAn extra low-voltage system which is not electrically separated from earth but which otherwise satisfies all the requirements for SELV.

Protection by PELV is used where extra low voltage is required, but the risk of electric shock is much lower than what would be expected for a SELV wiring situation.

A PELV system may include the addition of circuit protection in the secondary conductors of the ELV circuit. Although a protective earth is not required on the secondary of a PELV transformer, one can be used should the connected equipment need it.

Typical applications for PELV systems include:

• Garden lightin
• General ELV lighting
• Automation busways
• ELV machine control circuits 

IEC Requirements for SELV and SELV when tested for insulation resistance

Installation Requirements

Where SELV is used, whatever the nominal voltage, protection against direct contact shall be provided by:

• barriers or enclosures affording at least the degree of protection according to SASO 980 IP2X or IPXXB, or
• insulation capable of withstanding a test voltage of 500 V a.c. for 1 min.

Sources:

• IEC
• BS 7671
• City and Guilds
• AUS/NZ 3000:2007