Explosion protected apparatus

Having carried out an area classification exercise, the electrical equipment to be installed in the area must then be selected for its explosion protection properties. There is a range of options to choose from, with a variety of protection techniques employed. Each of the techniques, bar one, is described in detail in a European harmonised standard and is allocated a designation letter, mainly for ease of labelling. The following text summarises each of the techniques, and identifies its letter designation and appropriate construction standard. 

The term intrinsically safe applies to the apparatus and to the circuit in which is connected. It is normally associated with instrumentation circuits and some portable apparatus. This type of equipment may arc or spark 

Increased safety equipment is more akin to ordinary non-explosion-protected designs, but special precautions have to be taken to prevent any electrical or frictional sparking or hot spots exceeding the specified temperature limits. To prevent excessive heating the equipment is liberally rated and under fault conditions the associated control gear has to interrupt the circuit before dangerous overheating occurs. 

Type N apparatus is designed so as to be non-sparking in normal operation and sueh that a fault capable of causing ignition is unlikely to occur. 

In this type of technique, the electrical apparatus or parts of it are immersed in oil in such a way that an explosive atmosphere which may be above the oil or outside the enclosure cannot be ignited. 

---

If a general reference is made to explosion protected apparatus or explosion protected systems, then this relies upon zone 1 apparatus. And this will be the guideline throughout all chapters in this book. 

Some years ago, Directive 94/9/EC (the so-called ATEX 100a Directive ), dated 1994-03-23 (following the title in French Atmospheres explosibles. .. and referring to article 100a of the Treaties of the European Community), introduced categories of explosion protected apparatus to give a guideline for their installation with respect to the risks that may be expected in different locations exposed to combustible atmospheres (Table 2.3). 

This marking system correlates the adequate zone on the explosion protected apparatus with traditional data, e.g. group, type of protection and temperature class (see Chapter 5). 

The requirements for explosion protected apparatus suitable for combustible dusts are summarized in Table 2.5. 

As an example of dust explosion protected apparatus standard IEC 61241-4, 1st edition 2001-3, types of protection pD describes a pressurization technique the interior of this apparatus is subjected to a continuous pressure from a supply of air (not containing any combustible substances) or other inert gases while electrical components within the enclosure are energized. 

Table 3.2(b) VDE 0170/0171/1969-01 Classification of explosion protected apparatus according to the MESG values of combustible gas-air or vapour-air mixtures... 

Very generally, it is considered as quite uneconomical to construct and build explosion protected apparatus in such a way that all specific values mentioned above are met jointly at their lower limits. 

It shall be emphasized that all markings described in this chapter are related to explosion protection only and shall be considered as an additional marking to that given in the standards for non-explosion protected apparatus, e.g. the manufacturer s name or his trademark, type code and rated power/ voltage/current/speed/torque, serial number etc. 

Table 5.2 Marking of explosion protected apparatus according to IEC and EN Standards...

In the following, some examples for marking of explosion protected apparatus are given (the manufacturing companies are fictitious). It is assumed that these apparatuses have been manufactured, type tested and certified under the old article 100 directives (Table 3.6), applicable until 2003-06-30. 

Table 5.3 Marking of explosion protected apparatus according to ATEX 100a Directive (94/9/EC) (this is additional to the EN marking as in Table 5.2)...

This chapter will seek to answer the question Why do standards ask for specified temperatures of windings, for limited gap and joint values and for restricted voltages and currents in electric circuits The following intends to clarify the physical background of the standards technical content, to explain the methods of explosion protection in electrical engineering, and demonstrate modem explosion protected apparatus to give an appreciation of the bandwidth of the different types of protection. 

The marking of such s -apparatus has been (Sch)s for mining equipment and (Ex)s G... for explosion protected apparatus (suitable for zone 1 applications). In many cases, protection techniques, which are covered by their own standards today, e.g. encapsulation - m - or static pressurization (as a part of pressurization - p - ), have entered the field of explosion protection as s -apparatus [17]. 

As a general rule, explosion protected apparatus (in this book restricted to electrical apparatus) shall comply with these ESRs. 

In the following, type tests which are applied exclusively to firedamp-proof or explosion protected apparatus or which represent an enormous potential in material quality verification will be considered in more detail ... 

Table 8.5 Maximum permissible PD intensities (valid in general, not restricted to explosion protected apparatus, according to [30])... 

When selecting explosion protected apparatus, there are technical (or physical) and financial aspects to be considered. 

After installation of an explosion protected apparatus and before it is brought into service, it shall be given an initial inspection. During service, regular periodic inspections or continuous supervision shall guarantee the safe operation of the apparatus or system. If necessary, maintenance or repair shall be carried out. The standards define different grades of inspection ... 

For maintenance or repair of explosion protected apparatus, three categories may be differentiated ... 

Partial discharges - ignition sources in explosion protected apparatus ... 

Part two - as the main part of the book - starts with Chapter 6. Here, detailed descriptions of the different types of explosion protection are given, combined with illustrations of today s explosion protected apparatus. Chapter 7 deals with modem contents of pressurization. According to those,... 

In the final part of the book, focal points are financial considerations (Chapter 9), inspection, maintenance and repair of explosion protected apparatus (Chapter 10), apparatus for installation in zone 0 and in zone 2 (Chapter 11) and cable protection systems, especially for underground installations in coal mines (Chapter 12). 

Certification, routine testing, and documentation are major responsibilities of manufacturers of explosion protected apparatus. Design, manufacturing, and compliance with approved design are also responsibility of the manufacturer. 

MIMS eable in very hot loeations sueh as on furnaees corrosion-resistant materials in polluted atmospheres flexible conductors where there is cable movement explosion-protected apparatus and wiring in flammable atmospheres. 

Terminations of explosion-protected apparatus should be such as not to invalidate the apparatus certificate. 

---