Eaton MTL

MTL Instruments Group

FAQ - Intrinsic safety -

FAQ - Intrinsic safety


What do the various zones represent?

A Hazardous area are divided into zones to indicate the probability of a hazardous mixture of gas (or dust) and air being present. The present IEC standard defines the zones as follows:
Zone 0: A place in which an explosive gas-air mixture is continuously present for long periods,
> than 10 000 hrs/annum.
Zone 1: A place in which an explosive gas-air mixture is likely to occur in normal operation,
>10 and less than 1000 hrs/annum.
Zone 2: A place in which an explosive gas-air mixture is not likely to occur, and if it occurs it will only exist for a short term, <10 hrs/annum.
Zone 20: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.
Zone 21: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.
Zone 22: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation, but, if it does occur, will persist for a short period only.
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What do the various gas groups mean?


Gas grouping is a way of arranging the majority of flammable gases according to the energy required to ignite them.

Most countries mark all surface industry equipment with the Roman numeral II: the gas groups are then sub-divided into IIA (propane), IIB (ethylene) and IIC (hydrogen) – IIA being the least incendive. The names given in parentheses are representative gases; frequently used to describe the gas group.

America and Canada chose to use different group names when marking equipment but, of course, the ignition energy remains the same.

Test gas & ISA
RP12.2 Group
IEC Groups
SFA 3012
SFA 3004
EN 50 014
American NEC
Article 500
Flameproof Group
Propane IIA D 180
Ethylene IIB C 60
Hydrogen IIC B 20
Acetylene IIC A 20

What do the various temperature classifications mean?


Gas-air mixtures can be ignited by contact with hot surfaces; consequently electrical equipment used in hazardous atmospheres is required to be classified according to its maximum surface temperature.

From the table below it can be seen, for example, that an item classified as T4 will not exceed a surface temperature of 135°C.

These classifications are based upon an ambient temperature of 40°C, unless otherwise specified on the device.

If the equipment is to be used in ambient temperatures higher than this, the temperature class must be reassessed.

Class Maximum surface temperature oC
T1 450
T2 300
T3 200
T4 135
T5 100
T6 85
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How do the “simple apparatus” rules relate to the current “well defined” parameters?


The relevant clause in the standard IEC 60079-11 is 5.4, and states in section b) “sources of stored energy with well defined parameters, for example capacitors or inductors, whose values shall be considered when determining the overall safety of the system”.

This was introduced because the 20 microjoule figure of the previous definition is difficult to defend. Adding 20 microjoules to a IIC system, which may already be on the acceptable limit more than removes the safety factor. Another problem is that a thermocouple delivering 10 mV and 1 mA generates 20 microjoules in 2 seconds, hence the energy figure should have had a time assosciated with it.

The new clause does allow a further degree of freedom, particularly if IIB and IIA gases are being considered. The analysis should assume that all the capacitors and inductors within an apparatus are added together and considered as appearing directly across the terminals of the apparatus.The draft system standard requires that the sum of ALL the relevant parameters of ALL simple apparatus within an IS circuit shall be taken into account. Implicit in the same document is that a justification of why a piece of apparatus is considered as “simple” should be included in the safety documentation. The redrafted clause does give greater flexibility,but is not significantly different from considering the capacitance and inductance of cables.

The clause does allow the construction of some complex circuits, but whoever makes the claim of “simple apparatus” must document the analysis, sign it, and accept responsibility for it. It is a usefull solution to the occasional difficult short term problem. However, if a product is to be sold in significant numbers it will almost certainly have to be certified. Care should be taken when analysing simple apparatus to apply the full clause including the restrictions of the second part. This means that any complex apparatus can only be analysed by someone who is familiar with the whole standard and is knowledgeable in the field of intrinsic safety. Such persons are VERY RARE!

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What is an IS Earth?


Protective earthing is a vital part of the safety of almost all electrical systems where potentially explosive gases may constitute a hazard. Correct earthing is important for all protection techniques, including intrinsic safety.

Adequate earthing, though not spelled out in the particular installation codes, is absolutely vital to the safety of all mains-powered electrical equipment, whatever the protection technique. Section 22 of the BS 5345 Part 1 lists no special requirements for the earthing of hazardous area systems but is a convenient cross reference to other documents.

BS 5958: Part 1: 1980, which is a code of practice on the avoidance of static hazards and contains some sound practical advice, should be read as good background information on which to build.

Consider a conventional site with a local distribution transformer and with the neutral star point connected to the standard earth mat. The primary purpose of this mat is to provide an earth return path for any faults that may develop in the distribution system, where the conductivity of the soil may provide a possible current path. Conventionally, each electrical installation provides a return path to the neutral star point in its wiring, via either the cable armour or a specific conductor which is capable of operating its protective network. This is supported by the interconnection of the equipment to the metallic structure which generates a web of structural interconnections to the various electrical equipment return paths. Further support is usually provided by the structure and the antistatic bonds which are normally present. There is nearly always a third parallel path through the soil, but this indeterminate path is not usually relied upon for first line safety connections.

The more probable source of direct invasion of intrinsically safe circiuts is within the safe area shown in Fig. 9.2. The first essential of the dafety earth on the barrier busbar is to provide a return path of low impedance so as to prevent any significant proportion of the faule current entering the hazardous area. This fault current is returned to the neutral star point bond and hence back to the distribution transformer.

The current flowing though the bonding conductor generates a potential difference between the IS earth point at the barriers and the neutral star point. The outside of the field-mounted instrument is bonded to the neutral star point, and the internal circiuts of the instrument are connected to the barrier busbar. The potential difference between IS earth point at the barriers and the neutral star point is therefore transferred to the hazardous area. It is normally safe because this internal circuits are isolated from the instrument housing, but this potential difference should be minimised so that if there is a local insulation failure no danger can arise. The installation conditions of barriers, screened transformers etc. nearly all call for a return path impedance less than 1 ohm. A figure of 0.1 ohm is normally achievable and is more desirable. It is important to remember that the resistance involved is that of the return conductor between points IS earth at the barriers and the neutral star point and the resistance of the earth mat is not important for this purpose. These principals are normally applied within the UK.

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