Course EE-1 : Voltage Ranges -Part One


There are (6) ranges of voltages used in electrical world as follows:
  1. Extra low voltage range.
  2. Low voltage range.
  3. Medium voltage range.
  4. High voltage range.
  5. Extra high voltage range.
  6. Ultra high voltage range.
now, I will show these voltage ranges one by one as follows:

1- Extra low voltage (ELV) range: 

an ELV circuit as one in which the electrical potential of any conductor against earth (ground) is not more than either 25 volts RMS for alternating current, or 60 volts for direct current.

The IEC defines three types of extra-low-voltage systems:

a- Separated or safety extra-low voltage (SELV).
b- Protected extra-low voltage (PELV).
c- Functional extra-low voltage (FELV).

1-a Separated or safety extra-low voltage (SELV): see fig (1)

An electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits".

A SELV circuit must have:
  • protective-separation (i.e., double insulation, reinforced insulation or protective screening) from all circuits that might carry higher voltages
  • Simple separation from other SELV systems, from PELV systems and from earth (ground).


fig(1)

The safety of a SELV circuit is provided by:

  • the extra-low voltage
  • the low risk of accidental contact with a higher voltage;
  • the lack of a return path through earth (ground) that electric current could take in case of contact with a human body.

The design of a SELV circuit typically involves an isolating transformer, guaranteed minimum distances between conductors and electrical insulation barriers.


1-b Protected extra-low voltage (PELV):

An electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, except earth faults in other circuits.

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




fig(2)

A PELV circuit, just as with SELV, requires a design that guarantees a low risk of accidental contact with a higher voltage. As For example a transformer, this can mean that the primary and secondary windings must be separated by an extra insulation barrier, or by a conductive shield with a protective earth connection.




1-c Functional extra-low voltage (FELV):

The term functional extra-low voltage (FELV) describes any other extra-low-voltage circuit that does not fulfill the requirements for an SELV or PELV circuit. Although the FELV part of a circuit uses an extra-low voltage, it is not adequately protected from accidental contact with higher voltages in other parts of the circuit. Therefore the protection requirements for the higher voltage have to be applied to the entire circuit.


in the next lesson , we will talk about the Low Voltage Range.



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