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 50 volts RMS for alternating current, or 120 volts for direct current.

Extra-low voltage is used where the risks are great: swimming pools, wandering-lead hand lamps, and other portable appliances for outdoor use, etc.
 
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".

Safety by extra low voltage SELV is used in situations where the operation of electrical equipment presents a serious hazard (swimming pools, amusement parks, etc.). This measure depends on supplying power at extra-low voltage from the secondary windings of isolating transformers especially designed according to national or to international (IEC 60742) standard. The impulse withstand level of insulation between the primary and secondary windings is very high, and/or an earthed metal screen is sometimes incorporated between the windings. The secondary voltage never exceeds 50 V rms.

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).
  • No live conductor at SELV must be connected to earth
  • Exposed-conductive-parts of SELV supplied equipment must not be connected to earth, to other exposed conductive parts, or to extraneous-conductive-parts
  • All live parts of SELV circuits and of other circuits of higher voltage must be separated by a distance at least equal to that between the primary and secondary windings of a safety isolating transformer.
  • SELV circuits must use conduits exclusively provided for them, unless cables which are insulated for the highest voltage of the other circuits are used for the SELV circuits
  • Socket outlets for the SELV system must not have an earth-pin contact.
  • The SELV circuit plugs and sockets must be special, so that inadvertent connection to a different voltage level is not possible.


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.

This system is for general use where low voltage is required, or preferred for safety reasons, other than in the high-risk locations noted above in SELV. The conception is similar to that of the SELV system, but the secondary circuit of the isolating transformer may be earthed at one point.

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.

Basic protection is generally necessary, except when the equipment is in the zone of equipotential bonding, and the nominal voltage does not exceed 25 V rms, and the equipment is used in normally dry locations only, and large-area contact with the human body is not expected. In all other cases, 12 V rms is the maximum permitted voltage, where no basic protection is provided.


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.

A functional extra-low voltage (FELV) system can be used just for functional purposes, for instance for machine control systems.

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.

Where, for functional reasons, a voltage of 50 V or less is used, but not all the requirements relating to SELV or PELV are fulfilled, appropriate measures described in IEC 60364-4-41 must be taken to ensure both basic and fault protection, according to the location and use of these circuits.

Protection against direct contact (basic protection) must be provided by insulation, barriers and enclosures - this includes a FELV transformer used for generation of voltage in a FELV system.

Note: Such conditions may, for example, be encountered when the circuit contains equipment (such as transformers, relays, remote-control switches, contactors) insufficiently insulated with respect to circuits at higher voltages.


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


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