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Types of Protection

     

N-Type protection

Code: Ex n

Suitable for zone 2 use. (Not suitable for zones 1 or 0) Equipment Category 3G.

Relies of good engineering and robust construction to ensure that, in normal operation, there are no ignition-capable parts. (ie no sparking parts and not hot surfaces.)

Used for lighting, junction boxes, rotating machines and instrumentation.


Increased Safety

Code: Ex e

Suitable for zones 1 and 2. (Not suitable for zone 0) Equipment Category 2G.

Relies of good engineering and robust construction to ensure that, in normal operation and under specified fault conditions, there are no ignition-capable parts. (ie no sparking parts and not hot surfaces.)

Used for lighting, junction boxes, rotating machines. One of the three most popular types of protection.


Flameproof

Code: Ex d

Suitable for zones 1 and 2. (Not suitable for zone 0) Equipment Category 2G.

Relies on the equipment enclosure being strong enough to withstand an internal explosion in such a way that there is no ignition of an external explosive atmosphere. Joints in the enclosure (for example enclosure openings) will have machined flange surfaces which are closely tolleranced such that flame transmission across the flange is prevented.

Used for lighting, junction boxes, rotating machines. One of the three most popular types of protection.


Purge - Pressurize

Codes: Ex px, Ex py, Ex pz

Ex px: suitable for zones 1 and 2 (not zone 0). Equipment Category 2G

Ex py: suitable for zones 1 and 2 (not zone 0). Equipment Category 2G

Ex pz: suitable for zone 2 (not zones 1 and 0). Equipment Category 3G

Basic principle is that the enclosure containing the electrical equipment is first purged with clean (non-hazardous) air (to ensure there is no internal hazardous atmosphere) and then subjected to a small overpressure to ensure that any external hazardous atmosphere cannot enter the enclosure. Since there will normally be some leakage from the enclosiure, the overpressure air supply must be continuously connected.

The installer may have to ensure that the appropriate alarm indication or automatic shut-off is provided and will operate if the pressure falls below the permitted minimum. Consult the equipment documentation for details.

Mostly used for control panels etc.

Purging is also used as a pre-start up operation for some hazardous area rotating machines.


Oil immersion

Code Ex o

Suitable for zones 1 and 2. (Not suitable for zone 0) Equipment Category 2G.

The idea is to submerge the electrical equipment under oil - thus preventing any ignition-capable parts from coming into contact with a surrounding explosive atmosphere.

This type of protection is not common. It may be used for such equipment as transformers and for some control electronics (which cannot be designed to comply with, for example, intrinsic safety). There are various practical difficulties such as the change in oil volume with temperature - thus preventing the use of a sealed enclosure and requiring a breathing device which will not allow damp air (which could degrade the oil) to enter etc.


Quartz filling

Code Ex q

Suitable for zones 1 and 2. (Not suitable for zone 0) Equipment Category 2G.

The Ex q enclosure if filled with sand so that flame transmission (from any ignition capable part within the enclosure) is prevented.

Usually  used for low current applications including small power supplies for weighing apparatus etc. The technique is not particularly commonly used.


Encapsulation

Code Ex m, Ex ma, Ex mb, Ex mc

Ex m: suitable for zones 1 and 2 (not zone 0). Equipment Category 2G

Ex ma: suitable for zones 0 and 1 and 2. Equipment Category 1G

Ex mb: suitable for zones 1 and 2 (not zone 0). Equipment Category 2G

Ex mc: suitable for zone 2 (not zones 1 and 0). Equipment Category 3G

The electronics is encapsulated (in, for example and epoxy resin) which prevents and internal ignition-capable parts igniting the surrounding explosive atmosphere. The T-Class will refer to the maximum temperature the external surface will exhibit.

Mostly used for small and low cost items because repair is not possible. (It's a solid block of resin which you can't get in to!)


Intrinsic Safety

Code: Ex ia, Ex ib, Ex ic

Ex ia: suitable for zones 0 and 1 and 2. Equipment Category 1G

Ex ib: suitable for zones 1 and 2 (not zone 0). Equipment Category 2G

Ex ic: suitable for zone 2 (not zones 1 and 0). Equipment Category 3G

The equipment is designed such that the electrical power available within the circuit is below the minimum ignition level for the gas group concerned. There will be some Associated Apparatus (such as a zener barrier or galvanic isolator) located in the non-hazardous area to ensure that this low energy state remains true even under fault conditions.

Intrinsic safety 'ia' is probably the single safest (non-ignition-capable) type of protection.

The Associated Apparatus will have the code in square brackets, for example [Ex ia] IIC would indicate associated apparatus suitable for an 'ia' circuit loop and whose output could go to an intrinsically safe circuit in gas groups IIa, or IIB or IIC. (Note that the "IIC" is outside the brackets, indicating the output can go to IIC. Also note the code for associated apparatus does not have a T-Class. (Since the associated apparatus is not going to be installed in the hazardous area we are not concerned about hot surfaces igniting the hazardous atmosphere!)

Because the power available to the intrinsically safe circuit in the hazardous area is limited by the function of the associated apparatus, it is permitted to use certain uncertified "simple apparatus" in an intrinsically safe circuit. Simple apparatus is limited to non-energy storing equipment such as junction boxes, thermocouples, switches etc.

Because an intrinsically safe circuit may contain uncertified apparatus and because it is important to ensure that the intrinsically safe circuit is not jeopardised by coming into contact with other circuits, intrinsically safe circuits should always be clearly identified. This is commonly done by using cable with a light blue outer sheath for intrinsically safe circuits.

Intrinsic safety is used for instrumentation, measurement and control functions. It clearly cannot be used for anything where the equipment requires more than around 30V or more than around 40mA. (Minimum ignition curves are used to determine the permissible current at differing voltages.)

 Intrinsic safety in one of the three most popular and widely used types of protection.

 

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