Question

1. What are the key differences between Separated extra-low-voltage (SELV) system and Protective extra-low-voltage (PELV)? Illustrate your answer with the help of diagrams.

Answer 1

ANSWER

Given that

Differences between SELV system and PELV system:

SELV : Separated extra-low-voltage System.

PELV : Protective extra-low-voltage System.

Separated extra-low-voltage System.

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.   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.

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.

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.

Protective extra-low-voltage:

Protective Extra-low voltage (PELV) is an electricity supply voltage in a range which carries a low risk of dangerous electrical shock.There are various standards that define extra-low voltage.

  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.

A PELV circuit only requires protective-separation from all circuits other than SELV and PELV (i.e., all circuits that might carry higher voltages), but it may have connections to other PELV systems and earth (ground).