Insulation basic

BIL is the basic insulation level of equipment. When the system TOV or voltage surges exceed this level, the equipment may yield. In the latest international and national standards it is defined as follows ... 

Surge arresters Electrical characteristics of a ZnO surge arrester Basic insulation level (BIL) ... 

The important and widely studied copper-oxide-derived high-temperature superconductors, known as cuprate superconductors, are basically insulators. Doping converts these into metallic materials, many of which are superconductors over rather more restricted composition ranges. Several of these materials have already been discussed La2Cu04 and Sr2Cu02F2 (Section 4.3.3), La2 A.SrxCu04 (Section 8.5.1), and Nd2, Ce,Cu04 (Section 8.5.2). 

But consider the case of a high-voltage mosfet (switch) mounted on the (earthed) metal enclosure (for better heatsinking). Clearly, we can t provide any level of protection through physical separation. So in this case, we need to place one layer of approved insulator between the mosfet and the enclosure. Note that in this position, the insulator serves as basic insulation. ... 

What if we have an exposed conductor that is not connected to earth (such as for equipment with a two-wire ac cord), or if earthing is itself not an acceptable level of protection for that particular type of equipment as per safety regulations Then, besides the layer of basic insulation, we need another insulating layer (with identical dielectric withstand capability). This is called supplementary insulation. Together these two layers (basic + supplementary) are said to constitute double insulation. We could also use a single layer of insulation, with dielectric withstand properties equivalent to double insulation (i.e. 3000 V ac or 4242 V dc). That would then be called reinforced insulation. So for example, if the equipment is by design, meant only for a two-wire ac cord, we would need two layers of approved insulators (or... 

This can be confirmed by simply pressing the tips of an ohmmeter at two points on the surface of any bare ferrite lying around in the lab. Further, if we are trying to rely on the enamel coating of a typical copper magnet wire to protect from shorts, we should know that the coating is considered to be just operational/functional insulation, and is not considered to be even basic insulation. 

The European safety standards stress an important principle called the singlefault concept that is at the core of safety philosophy. This principle states that even under a single fault (a wire coming loose, component failure, short/open circuit, insulation failure, etc.) at least one level of protection (basic insulation) must be... 

Insulation separates live parts of the product from the user and prevents live parts from coming into contact with each other. For protection against direct and indirect contact with live parts insulation is safer than grounding or fusing. The protective measures are shown in the relative product standards and apply to all products and machines. The types of insulation applied between circuits of different potentials and metal parts depend on the equipment class, possible faults, and the parts in question. The voltage and part or circuit accessibility are also important for proper selection of the insulation type, such as double or reinforced insulation between live parts and user-accessible parts. Basic insulation is required between live parts and grounded metal parts. 

Basic insulation. Insulation to provide basic protection (one level) against electrical shock. 

Supplementary insulation. An independent insulation applied in addition to basic to ensure protection against electric shock in the event of a failure of the basic insulation (equals two levels). 

The majority of fixed apparatus is of Class I construction, as defined in BS 2754 1999 Construction of electrical equipment for protection against electric shock. This means that it usually has a metal case enclosing the live parts and that the five parts are insulated from each other and from the carcass by basic insulation only. The case, which is earthed by connection to the protective conductor, protects the internal wiring and components from damage and prevents direct contact. 

Portable apparatus may be of Class I construction, in which case the metalwork is earthed by a protective conductor which is a separate core in the flexible supply cable, or it may be of Class II construction, having no protective conductor terminal as its metalwork is not earthed. The essential safety feature of Class II apparatus is that the basic insulation is supplemented by additional insulation to provide a further safety barrier and danger arises only if both insulating layers fail. There are two types all insulated where the supplementary insulation is a plastic case, and double insulated where there is a metal case but live parts are separated from it by two layers of insulation. It is not advisable to use enclosed ventilated Class II apparatus in wet environments, because moisture may penetrate and provide a conductive film between the touchable surfaces and internal live conductors. 

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