Application and arrangements of SELV and PELV circuits

In an electrical circuit, SELV (Safety Extra-Low Voltage) and PELV (Protective Extra-Low Voltage) refer to circuits that operate at a voltage level that is considered safe for humans to touch. The exact voltage level will vary depending on the specific standard or regulation being used, but generally SELV and PELV circuits operate at a voltage level of 30V or less. 

These circuits are typically used in applications where there is a risk of electrical shock, such as in medical equipment or in areas where water is present. The main difference between SELV and PELV is that SELV circuits are completely separated from other circuits, while PELV circuits are protected by a safety barrier or isolation device.

According to British Standard 7671.

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.

The main feature of SELV circuits is that they are completely separated from other circuits, including those operating at higher voltage levels. This separation is achieved through the use of galvanic isolation, which ensures that there is no electrical connection between the SELV circuit and other circuits. This complete separation provides a high level of safety, as there is no risk of electrical shock even if the circuit is touched or a component fails.

If there is a single failure in the SELV system, like an earth fault in another circuit, the voltage should not rise above the extra low voltage. Since the SELV systems are not connected to the ground, even a small malfunction will not result in an electric shock for anyone who comes into contact with it.

The design makes use of an insulation transformer and establishes prescribed minimum clearances between conductors and insulating materials. SELV systems must be physically isolated from any high-voltage circuits by measures such as double insulation or protective screening. When the SELV wires will be in close proximity to other circuits, they must be doubly insulated or run through plastic tubes to prevent electrical interference.

In addition, there must be no connection between the wire and any higher-voltage systems or cables, any other SELV or PELV circuits, or the earth ground. For lower voltages up to 25 Volts, it is acceptable to have un-insulated current carrying elements such terminal screws. However, some sort of insulation should be used to prevent batteries from short circuiting.

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

Protective Extra-Low Voltage (PELV) is a type of electrical circuit that is used to power low-voltage equipment like lighting, audio-visual systems, and control systems. PELV circuits are designed to give off a low level of electrical energy that is safe for people to use and won't cause electric shock or death if exposed conductors are touched. A PELV circuit's voltage level is usually less than 42.4 volts (AC) or 60 volts (DC).

PELV circuits are used when there is a chance of coming into contact with live parts, like in wet or damp places or in places where the public can access the circuit. They are often used in homes, businesses, and factories, as well as in parks and gardens and other places outside.

Most PELV systems have a transformer to lower the voltage coming from the main supply and safety devices like RCDs and fuses to prevent overcurrents and short circuits.
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.

PELV (Protective Extra-Low Voltage) is similar to SELV, but these circuits are not completely separated from other circuits. Instead, PELV circuits are protected by a safety barrier or isolation device, which ensures that the voltage level is safe for humans to touch. The safety barrier or isolation device is designed to limit the amount of current that can flow in the event of a failure, further reducing the risk of electrical shock.

A PELV system may include the addition of circuit protection in the secondary conductors of the ELV circuit. Although 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 lighting
  • 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.

Both SELV and PELV are used in applications where there is a risk of electrical shock, such as in medical equipment or in areas where water is present. These circuits are also commonly used in low voltage lighting systems, audio and video equipment, and other electronic devices.

It's also worth noting that, in some cases, SELV and PELV can be used together. For example, a SELV circuit could be used to power a PELV circuit, which would provide a high level of safety for both circuits.

What is the difference between SELV and PELV? 

The level of protection is the main difference between the two. SELV circuits offer the most protection because they are "double insulated." This means that there is no chance of getting an electric shock even if both the primary insulation and the secondary insulation fail. PELV circuits offer less protection because they are only "single insulated." This means that if the primary insulation fails, there is a chance of getting an electric shock.

The voltage level is another difference between SELV and PELV. The voltage level of SELV is less than 42.4V AC or 60V DC, and people are not supposed to touch it. PELV is meant to be touched by a person and has a voltage level of less than 42.4V AC or 60V DC.

Different things are also meant to be used. SELV circuits are usually used in places where there is a high risk of electric shock, like in medical equipment, lab equipment, and electronic devices where the user is not expected to touch the live parts. Most of the time, PELV circuits are used in places where there is less chance of getting an electric shock, like in lighting and audio-visual systems where the user is expected to touch the live parts.

Both SELV and PELV systems usually have a transformer to lower the voltage coming from the main supply and safety devices like RCDs and fuses to prevent overcurrents and short circuits.

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


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