Ex i Intrinsically Safe Electrical Equipment

Ex i Intrinsically Safe Electrical Equipment

Intrinsic safety is a technique where the electrical energy available in any spark which can occur in the hazardous area will be non-incentive. Put simply, intrinsic safety is the 'safe spark' concept. The generally used for the spar ignition energies are as follows and impose a severe restriction on the design of equipment:

The above are measures of the discharge of energy stored in, for instance , an inductor when discharging into a single spark. (The sparks produced by a resistive circuit are sufficiently different in nature to product figures of 0.5-1 watt and above without risk of ignition). The IEC and European standards apply a safety factor of 2.25 to these energy values, wheras FM 3610, for example, only requires a safety factor of 1.5.

Intrinsically safe apparatus and systems are therefore designed such that a spark, even if produced accidentally (by a coming loose or a PCB track breaking, for example), will not cause ignition of the gas. It can be seen from the above ignition energies, however, that a piece of apparatus designed for IIA gases, and therefore limited to a spark of well below 320 uJ, would be inappropriate for IIB and IIC gases. Intrinsically safe equipment is always sub-grouped and should be selected according to the gas group of the hazard gas. For practical reasons, I.S. apparatus is usually given a T4 temperature class.

Intrinsically safe equipment is either 'ia' or 'ib'. The differences are as follows.
- ia: still with 2 prescribed faults - can be used in zones 0, 1 and 2 (Category 1 equipment)
- ib: still safe with 1 prescribed fault - can be used in zones 1and 2 (Category 2 equipment)
  
The term for the usual fixed installation is an 'Intrinsically Safe System' and typically consists of at least two items of equipment, one in the hazardous area and the other in the non-hazardous area, connected by cable.

Note 1: The power supply and associated apparatus may be combined into one item, which must then be certified.

Note 2: 'Barriers' and 'isolators' are two types of 'associated apparatus'

Intrinsic safety is a system concept so it is necessary to consider the safety of each item of equipment inthe loop. Equipment locatedin both safe and hazardous areas must be considered and the interaction between the pieces of equipment analysed. Using a safety 'interface' between the safe area equipment and the hazardous area removes the necessity for detailed consideration of the safe area equipment which need not be certified.

There are basically two types of interfaces, known as 'apparatus':

Zener barriers - These have a relitivly simple network of fuses, resistors and zener or ordinarydiodes to clamp the voltage and limit the current;

Galvanic Isolators -These provide galvanic isolation between the hazardousand safe areas using transformers, opto-isolators or relays.

The barrier or isolator must be able to limit the voltage and current even when the safe area power supply malfunctions. Usually, such devices are certified up to a maximum input voltage (Um,) of 250Vrms relative to earth; this is more or less the only restriction on the power supply. The barrier or isolator always defines the maximum voltage that can be supplied to the hazardous area, but some donot state the current directly: instead, a resistance is quoted and I must be calculated from Uo/R.

Most intrinsically safe systems use proprietary barries and isolators, from manufactures such as MTL, Pepperl & Fuchs, Stahl, Turck, Elcon and others. These are invariably certified and have the advantage that equipment upstream (e.g. instrument transformer, computer, etc.) need not be certified. However a manufacturer of an intrinsically safe instrument may decide to combine the power supply and signal processing circuitry with suitable interface circuitry, in which case the entire equipment requires certification (through the certifying process will concentrate on the relativly simple voltage current limiting circuitry protecting the outputs to the hazardous area terminals).

Ex e equipment is capable of being used in zone 1 or 2 with any flammable gas or vapour (subject to the temperature class being appropriate), but Ex e apparatus is not sub-grouped like Ex d or Ex i. The method of protection can only be used for equipment which does not spark or reach very high temperatures in normal operation and so there are some items which cannot be certified Ex e - switches, fuses, commutator motors, halogen bulbs, electronic components, for example - and it is common for Ex e to be combined with other protection concepts. One very common combination is a stop start enclosure, which may have a certification code as follows:

EEx ed IIC T6

This will contain one or more small component-approved flameproof switches protected by an increased safety housing.

Ex e apparatus is robust in construction and must pass an impact test on the enclosure, which must also have protection against the ingress of dust or moisture. However, these are not onerous requirments and Ex e enclosures are usually cheaper and easier to maintain compared to flameproof enclosures.

With the exception of discrete plastic glands, there is no need for certification in Ex e applications However, certified Ex e glands do exist and these can be used, as can all other types of metal gland provided an ingress protection of IP54 can be achieved at installation. When installing MICC cable, the standard glands are acceptable, however it is advisable to use certified increased safety seal to terminate the cable so as to be sure of complying with the insulation and segregation requirments for Ex e.

Ingress protection IP45

Protection of equipment against          Protection of persons against
ingress of solid foreign objects            access to hazardous parts


0     No protection                                          No protection
1     50mm diameter                                     Back of hand
2     12.5mm diameter                                  Finger
3     2.5mm diameter                                    Tool
4     1.0mm diameter                                    Wire
5     Dust-protected                                       Wire
6     Dust-tight                                                Wire

Terminals
Terminals in an Ex e enclosure must be certified. If the terminals can be removed (as with rail mounted Klippon / Weidmuller terminals, for example), then they should be individually certified. The usual coding is 'EEx e II'. Terminals which are not removable will be certified with the equipment and need not be individually marked with a certification code. Locking devices are incorporated in terminals so that they will not work loose. Clearence between terminals and creepage distances on insulating material are arranged to meet minimum requirments. 

Limitations of the Ex e concept
If a gas/air mixture is ignited inside an Ex e enclosure, unlike flamproof there is no protection left - the enclosure is not designed to withstand an explosion! The ingress protection of IP54 minimum covers the exclusion of dust and water only, so the enclosureis not gas-tight. It is therefore vital that no sources of the ignition are present in such enclosures. Items thats should definatly not be installed inside an Ex e enclosure include:
-Uncertified terminals
-Fuses and fues terminals, even if certified 'Ex N' or 'EEx n'
-Relays
-Electronic circuitry
-Switches, Circuit breakers

Junction boxes used for intrinsically safe circuits need not, strictly speaking, be certified at all, but it is common to use Ex e boxes, suitably marked on the outside with words such as 'CONTAINS INTRINSICALLY SAFE CIRCUITS ONLY'. In this case, it is generally agreed that it is permissible to use uncertified terminals and test-disconnect terminals (but not fuse terminals), since any sparks are 'safe'. Hwever, terminals should nevertheless be designed to meet the creepage and clearance distances required by the intrinsic safety standard.

T&D UK Core Product Ranges - Hazardous Areas:

Junction Boxes (Electrical Enclosures)
Local Control Stations
Control Panels
Power Switching
Lighting
Torches
Digital Cameras
Electrical Safety Equipment
Emergency Shutdown, Call Points, Breakglass
Sounders & Beacons
Trace Heating Systems
Duct Seals
Plugs and Sockets
Cable Containment (Tray, Ladder, Basket)
Cable Joints
Cable Glands
Cable Cleats
Cable Ties

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