What Is Basic Insulation?

Basic insulation: an insulation that provides basic protection [defined in the IEC 60050-195-2021].

Note 1 to entry: This concept does not apply to insulation used exclusively for functional purposes.

Basic insulation: an insulation applied to live parts to provide basic protection and which does not necessarily include insulation used exclusively for functional purposes [defined in the BS 7671].

Where solid basic insulation is used, it shall prevent contact with hazardous-live-parts.

In case of high-voltage installations and equipment, a voltage may be present on the surface of solid insulation and further protective provisions shall be considered.

Where basic insulation is provided by air, access to hazardous-live-parts or entering the danger zone shall be prevented by obstacles, protective barriers or enclosures as specified in 5.2.3 [2] and 5.2.4 [2] or by placing out of arm’s reach as specified in 5.2.5 [2].

Live parts shall be completely covered with insulation which can only be removed by destruction. For equipment, the insulation shall comply with the relevant standard for such electrical equipment.

Note: The insulation is intended to prevent contact with live parts. Paint, varnish, lacquer or s imilar products are generally not considered to provide adequate insulation for basic protection in normal service.


From the definitions of the term in question in IEC 60050-195, it follows that basic insulation is applied to live hazardous-live-parts in order to protect against electric shock under normal conditions. As long as the basic insulation is intact, it prevents contact with hazardous-live-parts and also prevents them from being short-circuited to exposed-conductive-parts and causing hazardous voltages to appear on them.

Hazardous-live-parts have electrical equipment of classes 0, I and II. Class 0, I and II electrical equipment can therefore have basic insulation. The insulation of the live parts of class III electrical equipment is not basic insulation because these parts are under extra-low voltage (ELV), by means of which basic protection is provided.

The term “exposed-conductive-part” is defined in the IEC 60050-195-2021 as follows: conductive part of equipment that can be touched and that is not live under normal conditions, but that can become live when basic insulation fails.

According to this definition, class 0 and class I electrical equipment has exposed-conductive-parts because its live parts are separated from the accessible conductive parts by means of basic insulation. The accessible conductive parts of class II electrical equipment are separated from their live parts by double insulation, which is the combination of basic and supplementary insulation or its equivalent reinforced insulation.

Therefore, the touchable conductive parts of class II electrical equipment are not exposed-conductive-parts. Class III electrical equipment has no exposed-conductive-parts because its accessible conductive parts are separated from live parts by insulation which is not basic insulation.

However, the requirements of international and national standards formulated for extra-low voltage electrical circuits mention both basic insulation and exposed-conductive-parts. For example, section 414, “Protective measure: extra-low-voltage provided by SELV and PELV,” of IEC 60364-4-41 specifies requirements for extra-low-voltage electrical circuits not exceeding 50 V ac or 120 V dc.

Clause 414.1.1 [3] of the standard specifies that protection by SELV or PELV systems, among other things, requires basic isolation between the SELV or PELV system and other SELV or PELV systems and basic isolation between the SELV system and earth.

SELV and PELV circuits shall have [414.4.1, [3]]:

  • basic insulation between live parts and other SELV or PELV circuits, and
  • protective separation from live parts of circuits not being SELV or PELV, provided by double or reinforced insulation or by basic insulation and protective screening for the highest voltage present.

SELV circuits shall have basic insulation between live parts and earth.

The PELV circuits and/or exposed-conductive-parts of equipment supplied by the PELV circuits may be earthed.

Protective separation of wiring systems of SELV and PELV circuits from the live parts of other circuits, which have at least basic insulation, may be achieved by one of the following arrangements [414.4.2, [3]]:

  • SELV and PELV circuit conductors shall be enclosed in a non-metallic sheath or insulating enclosure in addition to basic insulation;
  • SELV and PELV circuit conductors shall be separated from conductors of circuits at voltages higher than Band I by an earthed metallic sheath or earthed metallic screen;
  • circuit conductors at voltages higher than Band I may be contained in a multi-conductor cable or other grouping of conductors if the SELV and PELV conductors are insulated for the highest voltage present;
  • the wiring systems of other circuits are in compliance with 412.2.4.1;
  • physical separation.

Clause 414.4.4 of the standard [3] states that exposed-conductive-parts of SELV circuits shall not be connected to ground or to protective conductors or to exposed-conductive-parts of another circuit. A note to this paragraph also mentions exposed-conductive-parts of SELV circuits.

Paragraph 414.4.5 of IEC 60364-4-41 states that the main protection is normally unnecessary under normal dry conditions for PELV circuits where the nominal voltage does not exceed 25 VAC or 60 VDC and exposed-conductive-parts and/or live parts are connected to the main earthing terminal by a protective conductor.

Where a conductive obstacle is separated from hazardous-live-parts by basic insulation only, it is considered to be an exposed-conductive-part, and measures for fault protection (see Clause 6 [2]) shall also be applied.


What is the difference between functional and basic insulation?

Functional insulation is intended to protect the user from electrical hazards, while basic insulation provides a degree of protection against mechanical and thermal hazards. Functional insulation is typically made of thicker, higher-quality materials than basic insulation. In some cases, functional insulation may also be treated with flame retardants or other chemicals to improve its performance. Basic insulation is usually less expensive than functional insulation, but it does not provide the same level of protection against electrical hazards.

When choosing between functional and basic insulation, it is important to consider the intended use of the product. If you need protection from electrical hazards, you should choose functional insulation. If you are only concerned with protecting against mechanical and thermal hazards, basic insulation may be sufficient. However, if you are unsure about which type of insulation to choose, it is always best to consult with a professional.

What is the basic insulation level?

The basic insulation level is the level of protection that an insulating material must provide in order to be considered effective. This level is typically set by government or industry standards organizations. In the United States, the most common standard for electrical insulation is the National Electrical Code (NEC). The NEC provides guidance on what levels of insulation are required for different types of electrical equipment and wiring.

The basic insulation level is important because it ensures that electrical equipment can operate safely and reliably. Insulation that does not meet the basic insulation level may not provide adequate protection against electric shock or fire. Therefore, it is important to choose an insulating material that meets or exceeds the basic insulation level.

What is difference between insulation and isolation?

There is a difference between insulation and isolation. Insulation is used to protect against heat, cold or sound. Isolation is used to protect against electricity.


  1. IEC 60050-195-2021
  2. IEC 61140-2016
  3. IEC 60364-4-41