Machinery Safety 101

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 PELV? Elec­tric­al design­ers run into all kinds of spe­cial­ized ter­min­o­logy as part of their work. IEC is notori­ous for cre­at­ing spe­cial­ized terms that are not famil­i­ar 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. For that, we have to 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 have to 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 is what does the dam­age, but the touch voltage is what 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 there are two upper voltage lim­its that 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 itself. 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 serves to reduce the inter-wind­ing capa­cit­ance and the leak­age cur­rent that will flow through that capa­cit­ance under nor­mal con­di­tions. The design of the trans­former 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 is what provides the shock pro­tec­tion, assum­ing that the PELV source is cap­able of more than 30 mA.

Bond­ing the sec­ond­ary to earth is dif­fer­ent 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, but 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 basic 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 you can use 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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