3 Roofs Without Walls or Associated With One, Two or Three Walls

Area classification in areas which are not freely ventilated 131



hipped roof (dutch-barn construction) which is more common in industrial

loading and unloading areas. This latter may be with vents or without vents,

although, in most cases, vents are present particularly in cases where the

flammable gas or vapour is lighter than air.

Where no walls are present the effect of a flat roof is dependent upon

the proximity of the roof to the edge of the hazardous area which would

be created if the release were in free air (see Fig. 5.6). If the roof is more

than 5 m beyond the extent of hazardous area predicted by Fig. 3.4 and 3.5

(in Chapter 3) the roof is unlikely to have any effect but in other cases the

hazardous area is expected to reach the roof. Where a hipped (dutch-barn)

type roof is present without walls and if the top of the hazardous area

defined in the absence of the roof is within 5m of the roof, the entire area

contained by the roof will become a hazardous area o the same severity,

f



Roof



lea+kad



Source of



release



Fig. 5.6 Effect of flat roof within 5 m of free ventilation limit of explosive atmosphere



132 Electrical installations in hazardous areas



Hip



Ground



E = Extent of hazardous area neglecting roof,



a=lm

Fig. 5.7 Effect of hip roof within 5 m of E



provided that in cases where the flammable material is not heavier than

f

air, vents must be provided in the apex o the roof. Figure 5.7 shows this

situation and suggests a notional 1m of hazardous area around the vents.

This latter is difficult to predict but, at 1m, is felt to be sufficiently large for

most normal situations.

5.3.2 Roofs associated with one wall



Where a wall is present in addition to the roof, the situation is further

complicated and the hazardous areas described in Fig. 5.1 and 5.2 become

modified to give the situation shown in Fig. 5.8. The main thing to notice

here is the extension of the hazardous area over the roof for a small distance,

given by the difference between the extent of the hazardous area produced

when the wall is present and the length o the wall. The length is used in

f

preference to the height as the overlap will only occur if the wall length

is less than the horizontal extent of the hazardous area, which is already

distorted by being extended up to the roof level. The situation is similar in

respect of the situation with a hipped roof, but here the effects shown in

Fig. 5.7 must also be taken into account.

5.3.2 Roofs associated with two walls



Where two walls are present, then the entire area covered by the hazardous

area shown in Figs. 5.3 and 5.4 will form part of the hazardous area up to



Area classification in areas which are not freely ventilated 133



Wall



Ground



!

Source o

release



Roof



E = Extent of hazardous area taking account of wall

proximity but in absence of roof

V = Height of source of release

X = Distance source/wall

a = E -- Y (with a minimum of 0.5 m over roof)

b = 2y - 2a

Note: Where b is less than E hazardous area

extends behind and over roof to F



Fig. 5.8 Effects of roof over single wall



the roof and down to the ground (see Fig. 5.9). As for one wall, the extents

of hazardous area above the roof line will be determined using Fig. 5.3

and 5.4.



5.3.4 Roofs associated with three walls



Where three walls exist, the situation is entirely different and the area

should be treated as a room as described later in this chapter, giving a

hazardous area as described in Fig. 5.10, which is based on Table 3.2 in

the same way as would be the case for an opening, such as a door in

a room.



134 Electrical installations in hazardous areas



E = Extent of hazardous area taking account of walls if roof is not present

C = Distance from source to end wall

D = Distance from source to furthest corner of roof

F=E-D

a=E-C

Fig. 5.9 Effect of roof over two walls



5.4 Rooms above ground

As previously stated, an above-ground room with no attempts made to

give ventilation will have a very long retention of any release of flammable

material. A method of calculating this is given in Chapter 4 based upon a

proposed new British Standard3. It is difficult, however, to be sure that all

pockets of a room will be swept clear within this specified time by crack

ventilation, and retention must be assumed to be capable of removing the

source of hazard/classification relationship. It is not considered possible

to countenance a room with no specifically included ventilation requirements being used for any purpose which means that it will contain sources

of release more onerous than secondary grade sources of release. In such

circumstances the interior area classification will, because of the paucity

of ventilation, give an internal classification of Zone 1, which will clearly

introduce significant difficulty in the use of the room.



Area classification in areas which are not freely ventilated 135



Roof



Ground



a = Taken from Table 3.2 on the basis of maximum release

(minimum figure 0.5 m)



Fig. 5.10 Effect of roof on three-wall system



Any door which enters the room will be considered as a source of release

equivalent to the quantity of vapour released within the room and Table 3.2

should be used to determine the extent of the hazardous area outside

the room, around the door or other opening. Where liquids above their

flashpoints but below their boiling points are contained within the room,

drainage should be provided to ensure that a significant pool cannot form,

and the external hazardous area should be based on Table 3.8 with a minimum of 0.5 m in cases where no mist is assumed. This latter is a safety factor

to take account of unknowns. Where the room contains a liquid contained

above its boiling point then the area outside the room (which is a hazardous

area) should be based on Table 3.10. These areas outside the room will, be

Zone 2 in the case of secondary grade sources of release (the only sources

of release considered as acceptable in these circumstances) even though the

retention problem is likely to lead to the classification of the room as Zone 1.



5.4.1 The application of additional general ventilation



Where a room contains sources of release then it is almost certain that

additional ventilation will be necessary. A room may be provided with

additional ventilation in one of three ways.



136 Electrical installations in hazardous areas



Openings (normally louvered) may be fitted to create more airflow

in the building



Where openings are provided they do increase ventilation, but in no way

as significantly as artificial ventilation unless they satisfy the criteria set out

earlier in this chapter. Such openings are not in many cases practicable

unless louvred, however, as they remove much of the protection from

the environment which is often the reason why a room is necessary.

The effect of such additional ventilation is to reduce the retention of

explosive atmosphere in the building to a level where the source of

hazard/classification relationship (i.e., secondary grade source of release

gives Zone 2) is maintained. Because of the limited effect which they

have, however, they do not allow parts of the building to be delineated

as particular risks (e.g., parts Zone 1 and parts Zone 2) and the entire

building will adopt the classification produced by the most onerous source

of release contained within it. They must also be strategically placed to

take maximum account of the internal situation. In buildings where internal

activities produce temperature gradients of in excess of 3°C between the

interior of the building and the outside air, then significant chimney effects

can occur and movement of explosive atmospheres within a building will

be affected to some extent by their density relative to air. The results of

these considerations generally result in louvered openings being placed at

both the top and the bottom of the building.

In addition, care must be taken to ensure that the effect of any air entering

through louvered openings is general, rather than limited, to specific areas

as the airflow is not sufficient to ensure that releases do not contaminate

all of the building. The layout of equipment within the building is also

important as it could produce blindspots. In most cases considerations such

as this tend to create a scenario where louvres are necessary all around the

building, as shown in Fig. 5.11, at both high and low level which is not

always ideal. The result will still usually lead to the entire interior of the

building being classified as Zone 1 if a primary grade source of release is

present which, because of toxic and asphyxiant considerations, may often

mean that access to such buildings is severely restricted.

To sum up, buildings which merely have openings to allow enhanced

natural ventilation without meeting the criteria described in Section 5.1.1

normally require ventilation openings as shown in Fig. 5.11 and, in the

main, will only be suitable for secondary grade sources of release. Even in

such cases, layout inside the building is of critical importance.

Where buildings ventilated naturally by openings not complying with the

criteria for unrestricted ventilation, provided the ventilation openings have

been provided with sufficient thought to internal airflow, secondary grade

sources of release within the building will produce a Zone 2 classification

and external Zones 2 at the openings, based upon the maximum single

secondary grade release and using Table 3.2 as a basis, with a minimum

of 0.5 m.



Area classification in areas which are not freely ventilated 137



V = Vents (louvres or otherwise)

a = Extent of hazardous area (Table 3.2) with a minimum of 0.5m

Hazardous area

Not acceptable

Zone 1

Zone 2



-



Source of release

Continuous grade

Primary grade

Secondary grade



Fig. 5.11 Hazardous area is a room with openings (with or without louvres)



Where a building (as described above) contains primary grade sources of

release, unless local forced ventilation is fitted in a way which removes the

release separately, the entire interior of the building will be Zone 1, and

a Zone 1 will exist at all ventilation openings based upon the maximum

number of primary grade sources of release which are considered as

releasing at the same time, based upon Tables 3.1 and 3.2, with a minimum

of 0.5 m (not normally acceptable for personnel entry).

It is not considered as acceptable to have any continuous grade sources

of release releasing into such a building without additional precautions.



Buildings provided with forced ventilation



General forced ventilation is the most common way in which the building

may be scoured with air. The objective is normally to ensure that the

building is swept with air while at the same time, ensuring that the

forced ventilation provided does not create hazardous areas around the

building. To do this it is necessary to first determine the maximum release

of flammable material from any one secondary grade source of release (it

is not considered as necessary to consider more than one secondary grade

source of release releasing at any given time) and, if greater, the maximum

release from any possible combination of primary grade sources of release



138 Electrical installations in hazardous areas



considered possible from the application of Table 3.1 (if it is decided that

it is acceptable in any given circumstance to have a building which is all

Zone 1).

If it is not considered as acceptable for the entire building to be Zone 1,

then each primary grade source of release must have local ventilation

applied to remove the material released before it can access the generality

of the building. In no circumstances is it considered possible to allow any

continuous grade source of release to freely release into the building where

only general ventilation is present.

Having determined the maximum quantity of flammable material

released into the building, it is then necessary to ensure that the amount

of air supplied by the ventilation system is sufficient to ensure that, if a

release occurs when the ventilation is on, the mixture of air and flammable

material exhausted from the room is below the lower explosive limit and

it is wise to apply a safety factor here. Chapter 4 provides equations which

will allow this to be done. When the ventilation has been properly designed

it can be accepted that, provided that early repair of the ventilation system

is executed so that it is not off for a long time (say repair is to be completed

well within a shift), then a secondary grade release will not occur while

the ventilation is off and there is little likelihood of the building filling with

explosive atmosphere and then exhausting when the ventilation is switched

on, creating an explosive atmosphere outside the building. Unfortunately,

this does not hold for primary grade sources of release and these must be

expected to release at times when the ventilation has failed but, as failure

of the ventilation is abnormal, such releases can be considered as secondary

grade giving rise to Zone 2 within the building and not Zone 1. There

remains, however, the problem that when the ventilation is repaired and

switched on, an explosive atmosphere will be exhausted from the building.

This, together with the fact that if input forced ventilation is used there will

be some leakage of explosive atmosphere from within the building at all

openings, demands that any ventilation system used is extract ventilation

so that the pressure in the building will be very slightly lower than outside,

resulting in release of explosive atmosphere to the outside only at the

ventilation exhaust. Figs. 5.12 and 5.13 show how this situation exists in

practice.

Where only secondary grade sources of release exist with general extract

ventilation, the inside of the building will be Zone 2 and no external

hazardous area will exist. Even in these circumstances, however, it is

advisable to have an indirectly operated fan with the drive motor outside

the ducting and clear of the ducting end, as if the ventilation fails for a long

time the fan will be necessary to remove a possible explosive atmosphere

from within the building. Although this is so unlikely as not to be within the

area classification considerations it is clearly not acceptable to use a source

of ignition to try to clear an explosive atmosphere, however unlikely that

atmosphere may be.

Where primary grade sources of release exist without local ventilation,

as described earlier, the inside of the building will be Zone 1 and a Zone 2



Area classification in areas which are not freely ventilated 139



/ Buildina



Extract

aidlow

Ihlnt.3 3)



bE(

Crack

+-- (Note



(Note

Door



Air inlet _ _ .

(Note I )



Ground



Fig. 5.12 Secondary grade source of release with forced ventilation. Notes: (1)

Extract ventilation ensures that airflow at all apertures will be into building.

(2) Extract airflow sufficient to dilute maximum secondary grade source of

release to required fraction of LEL



Extract

air



+



(Note 2)



Air inlet



-(Note



1)



Air inlet

1)



-(Note

Ground

a = Table 3.2 but 0.5 m minimum



Fig. 5.13 Primary grade source of release with extract ventilation. Notes: (1)

Ventilation ensures airflow is into the building in normal operation

restricting Zone 1 to inside building. (2) In abnormal circumstances

explosive atmosphere may leak from extract vent in addition to building

openings



140



Electrical installations in hazardous areas



(based upon Table 3.2 taking account of the maximum release of the source

of hazard, but with a minimum of 0.5m) will exist where the ventilation

exhausts into the outside air. In this case, the fan motor will need to be

considered as previously to allow for repair delays. This will have already

been considered as the fan motor will be in a Zone 2 i in-line.

f

As previously, the use of extract ventilation will mean that all leaks of air

will be into the building and so no external Zone 1will occur. Any air inlets,

however, will be surrounded by Zone 2 based upon the maximum leakage

rate and should be sized in accordance with Table 3.2, with a minimum

of 0.5 m.

Continuous grade sources of release are not considered as acceptable in

normal circumstances within this type of building.

5.4.2 The application of additional local ventilation



Section 5.3.1 describes the situation which arises when general ventilation

only is provided for. It is possible to additionally arrange for local ventilation at a single point of release or location to limit the extent of hazardous

areas produced by specific sources of release in areas which are not generally freely ventilated; be they unventilated or provided with general natural

or forced ventilation. There are two basic ways of doing this.

Provision of specific ventilation for individual sources of release



Where a particular source of release is provided with individual ventilation then, provided that such ventilation is properly conceived, the size

of the hazardous area around that source may be described, even though

the area in which it is sited is not generally freely ventilated (e.g., is in

a building). The ventilation provided here must be local to the source of

release and must be sized to ensure that the airflow in the area of the release

is greater than the release velocity. It must also ensure that the quantity

of ventilation air provided is sufficient to dilute the maximum release of

the source in question to some fraction of the lower explosive limit of the

flammable material released, so as to ensure that the gas vapour/air mixture

exhausted from the area is below the lower explosive limit. Chapter 4

provides a method of defining the quantity of air necessary, but determination of the velocity of airflow depends upon detailed knowledge of

the geometry around the release and the release velocity. This approach is

generally only effective for extract point source ventilation as in other cases

the effect of the ventilation would be to push the released gas/vapour into

the effectively enclosed area. Also, the nature of secondary grade sources

of release, in that they are often large and at high velocity, means that

the approach is only effective in limiting Zones 0 and 1 caused by continuous and primary grade sources of release. In such cases, however, it has

significant value in that it can be used to maintain a general Zone 2 classification in such places as buildings containing continuous and primary



Area classification in areas which are not freely ventilated 141



(Note



Continubus grade

source of release



Fig. 5.14 Effects of point source ventilation. Notes: (1) Interior of building and exterior around openings are Zone 2 to take account of ventilation failure. (2)

Immediate area around continuous (or primary) grade source of release

is Zone 0 (Zone 1). (3) Where source is a point source and ventilation

velocity local to it is 2m/s Zone 0 (Zone 1) is restricted Table 3.2 with a

minimum of 0.5 m



grade sources of release. The effects of tlus ventilation may also be considered when deciding on the additional level of general ventilation necessary

for such a location. These situations are described in Fig. 5.14.

Where a continuous grade source of release is present in an area which

is not freely ventilated, local extract ventilation (e.g., for an oil/water

separator in a building) may be provided locally to the source of release to

remove any gas or vapour which is normally released. Unless the source of

release has additional modes of release it is, in such cases, reduced, as far

as the area in question is concerned, to a secondary grade source of release

on the basis that it will release into the area only when the local ventilation

fails. This does suppose that the ventilation is quickly repaired if it does fail

and here repair within the hour will be necessary. If the source can behave

in other modes then these must be separately considered. Where the source

of release can be described as a point source then, provided that the airflow

around the source as a result of the ventilation is at least 0.5 m/s, the Zone 0

will be less than 0.5m.

Where a primary grade source of release is protected by local ventilation,

the situation is similar to that existing for a continuous grade source of