What exactly is PELV?

Photo of the interior of an electrical control cabinet showing the components and wireways on the backplane.
Electrical control enclosure interior

What exactly is Protected Extra-Low Voltage (PELV)? Electrical designers run into lots of specialized terminology as part of their work. IEC is notorious for creating specialized terms unfamiliar to North American designers. PELV is a definition of a limited range of system voltage and current and is related to some other terms you may run into. Let’s dig in!

Defining PELV

PELV stands for protective extra-low voltage. IEC defines common voltages in IEC 60038 [1]. This standard defines common system voltages but does not directly address some of the specialized ranges like PELV. To get this information, we must go to IEC 60364-4-41, Clause 414 [2].

PELV is defined in the IEC electropedia as:

PELV system
electric system in which the voltage cannot exceed the value of extra-low voltage:
  • under normal conditions and
  • under single fault conditions, except earth faults in other electric circuits
Note ? PELV is the abbreviation for protective extra-low voltage.
IEV reference 826-12-32

Defining ELV

The definition raises another question: What is extra-low voltage (ELV)? To get the definition for ELV, we must go to yet another standard, IEC 61140 [3].

extra-low voltage
ELV
voltage not exceeding the maximum value of the prospective touch voltage which is permitted to be maintained indefinitely under specified conditions of external influences
IEC 61140:2016, 3.26

Touch Voltage

The touch voltage limits vary somewhat between standards and jurisdictions, from 50 Vac to as low as 25 Vac in dry conditions and as low as 6 Vac in damp or wet locations. The touch voltage is the voltage that will exist during a single fault on the exposed conductive parts of a product. The design goal is to limit this voltage as much as possible so that the shock current is limited to as low a value as possible. The shock current does the damage, but the touch voltage creates the shock current. Shock currents less than 30 mA are generally considered low risk for most non-medical products.

Voltage Bands

To finally get a voltage range, we have to read a bit further down in [3] to find the limits for the voltage bands.

Voltage bands table from IEC.
[3, Table 1]

So we now know that two upper voltage limits apply to PELV, depending on whether we are designing an a.c. or a d.c. system. The PELV definition gives two specific conditions under which the ELV limits must be maintained, normal conditions and single fault conditions within the PELV system. Let’s look at a circuit diagram that will help explain this.

Diagram showing four different ELV sources in schematic form, including PELV and SELV.
[4]

In the schematic above, there are differences in the construction of the transformers used to supply the xELV sources, as well as differences in the overcurrent protection used and the use of secondary earthing. Since we are talking about PELV in the article, let’s focus on that construction.

The transformer

The transformer is an isolating transformer with an earthed electrostatic shield between the primary and secondary windings. This reduces the inter-winding capacitance and the leakage current that will flow through that capacitance under normal conditions. The transformer design will limit fault currents under single fault conditions in the PELV circuit.

The secondary circuit is also bonded to earth, which helps to stabilize the secondary voltage with respect to earth. The earthing connection also ensures that electrical faults between the un-earthed secondary conductor and any conductive parts that are also bonded to earth, such as a power supply chassis, for example, will result in the overcurrent protective device operating. The overcurrent device provides shock protection, assuming that the PELV source is capable of more than 30 mA.

Bonding the secondary to earth differs from SELV systems where the secondary is not bonded to earth.

PELV systems must be run separately from all other systems except SELV. Basic insulation is required between SELV and PELV systems.

There are two different interpretations of the “S” in SELV, with some sources using “separated” and some using “safety.” Some sources [6] indicate that the absence of a secondary earth connection is where the term “separated,” as in “separated extra-low voltage,” comes from. Still, I have not found an authoritative source that corroborates this interpretation. Since [2] uses “safety,” I will stick with that interpretation for this post. Just know that both are considered correct and describe technically identical requirements.

Functional Extra-Low Voltage (FELV)

If you have a circuit like the 24 V I/O circuits commonly used with PLCs, the source may not meet all of the requirements for SELV or PELV. In this case, it likely falls into what the IEC calls functional extra-low voltage (FELV). FELV circuits have the same upper voltage limit as shown in Table 1 but may not have the double insulation or screening in the transformer. FELV systems are required to meet the basic requirements for protection against direct contact, i.e., correct insulation for the system voltages, and single fault protection against indirect contact, i.e., system bonded to earth and overcurrent protection in the un-earthed conductors.

Why use PELV?

When designing the switchgear and controlgear assemblies for machines, one method for shock protection offered by IEC 60204-1 [5] is the use of PELV. You can use this method if it makes sense in the application or one of the other shock protection methods provided for in that circuit.

Additional resources

If you would like to read some more on this topic, you might want to check out these articles:

  • Extra-low voltage – Wikipedia
  • SELV and PELV what is it?? Learn the difference between the two types of transformer. – YouTube [6]

References

[1] IEC standard voltages, IEC 60038. International Electrotechnical Commission (IEC), Geneva. 2009.

[2] Low voltage electrical installations – Part 4-41: Protection for safety – Protection against electric shock, IEC 60364-4-41. International Electrotechnical Commission (IEC), Geneva. 2005+AMD1:2017.

[3] Protection against electric shock – Common aspects for installation and equipment, IEC 61140. International Electrotechnical Commission (IEC), Geneva. 2016.

[4] Dmitry G., “Low Voltage Installations for Low Voltage Devices.” Wikimedia.org. 2011. Available: https://commons.wikimedia.org/w/index.php?curid=13665417. [Accessed: 2021-04-14].

[5] Safety of machinery – Electrical equipment of machines – Part 1: General requirements, IEC 60204-1. International Electrotechnical Commission (IEC), Geneva. 2016.

[6] Learn Electrics, SELV and PELV what is it? ? Learn the difference between the two types of transformer. 2021. Available: https://youtu.be/2_0fZpcDYGw. [Accessed: 2021-04-14].

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