5 Enclosures, Ducting and Internal Components

288



Electrical installations in hazardous areas



over-pressure relief device in the enclosure so that if there is any unwanted

pressure elevation in service the enclosure is automatically safeguarded.

A requirement for the enclosure to be weather proof is not necessary

as the pressurization requirements will effectively prevent the ingress of

moisture. Prevention of entry of solid foreign bodies, however, remains

necessary as these may still enter due to their nature and, unlike moisture,

will not be easily removed by purging if they enter prior to energization.

A requirement is for the enclosure to have an integrity of not less than

IP40 in accordance with BS/EN 605298 in the absence of the pressurizing

gas. In the case of rotating machines where IP-ratings are defined slightly

differently, due to the nature of such apparatus, the Standard used is EN

60034-5, which is published in the UK as BS 4999, Part 105.9

When considering the IP-rating of the enclosure, the deliberate apertures

for inlet and outlet of the purge gas may be ignored. These, however, must

be sited to ensure adequate purging and the avoidance of dead spots where

any flammable gas or vapour may dwell even after purging. This may only

require careful siting of single apertures in each case but may, where enclosure shapes are complex or components create divisions in the enclosure,

need several inlet and outlet apertures. The choice is one which can only

be made individually but it is not likely that multiple apertures will be

the norm.

Where outlet apertures are fitted to enclosures which are pressurized

by static pressurization or pressurization with leakage compensation, these

must be arranged so that they can be closed or sealed after purging by

devices which form part of the apparatus.

It is recognized that the apparatus will, in common with other types

of apparatus, be fitted with doors and covers which permit access to the

interior of the enclosure. The ideal method of dealing with these in the

case of pressurization with leakage compensation, and pressurization with

continuous dilution, is to utilize an interlock which isolates the electrical

supplies and any other electrical connections (see Fig. 11.2) when the door

or cover is opened. Because this would limit the opportunity for adjustment

it is possible to introduce an interlock defeat system (see Fig. 11.3) which

would permit competent persons to open the enclosures, under a system

of control, for adjustment and similar purposes without electrical isolation.

After such work, when the enclosure is reclosed it is important that the

system goes through its full purge cycle to remove any gas which has

entered, although it is not necessary to isolate the electrical supplies during

this period.

Where a tool or key is required to open doors or remove covers isolation is not necessary, but the purge cycle needs initiating after reclosure.

In this case it will have to be done manually. A device can be fitted which

recognizes opening or removal, and isolates electrical supplies with an override as already described. The tool or key requirement in this case does not

indicate any requirement for 'special fasteners' or the like (see Chapter 8).

In the case of static pressurization, doors and covers will not be provided

for adjustment after pressurization due to the nature of the approach. All



Protection concept pressurization ‘p’ 289



I

I

-1

I



1



Pressurized

enclosure



Other

electrical

circuits



I

I

I

I

I

I

I

I



I



Fig. 11.2 Disconnection of electrical supplies and circuits on pressure failure



Purged

enclosure

Pressure switch

NOT PREFERRED



PREFERRED



Fig. 11.3 Defeating of control system. Note. The preferred method ensures the

repurge of the enclosure after reinstatement of pressure.



290 Electrical installations in hazardous areas



that is required is for any doors or covers to need a tool or key for them to

be opened or removed. Once again, the requirement for a tool or key does

not indicate a requirement for the 'special fasteners' described in Chapter 8.



11.5.2Ducting



As the ducting forms part of the enclosure it needs to satisfy the same

requirements as those for enclosures. With the use of compressed gas, this

is no problem as the ducts will be short, being only in the area of the

enclosure (see Fig. 11.4) - the supply of gas to that point normally being

compressed gas piping, and the outlet ducting being short as it discharges

into the hazardous area. (It is not possible to permit the exit ducting to

exhaust into a non-hazardous area, as to do so would make it a hazardous

area). Therefore, in these cases, all piping and ducting will be at a pressure

above atmospheric pressure and the ducts can form a part of the apparatus.

Where the protective gas is air, and fan supplied (not normally possible

for inert gas), then there are two possibilities. First, the fan and immediate

ducting may form part of the enclosure and be under the control of the

manufacturer. The air must, however, be obtained from a non-hazardous

area resulting in a considerable length of ducting. This, then, becomes an

installer problem. Which makes matters very difficult because the pressure



Pressure



Pressurized

enclosure



Atmospheric pressure



Fig. 11.4 Pressurization using compressed gas



Protection concept pressurization ‘p’ 291



in the supplying duct will be below atmospheric pressure (see Fig. 11.5)

and, as the ducts must pass through a hazardous area, any leak will permit

the possible entry of an explosive atmosphere into the enclosure. This first

approach is not, therefore, recommended and the alternative of the remote

fan with pressurized ducting is preferred (see Fig. 11.6).

Where the exhaust ducting is short and discharges into a hazardous

area any incandescent sparking inside the enclosure can, in certain

circumstances, cause the ejection of incandescent particles into the

hazardous area. A device to prevent such ejection is normally necessary

when the apparatus produces ignition-capable sparking which can give

rise to such particles or contains some other mechanism (for example, a

flame ionization chromatograph), which can in normal operation, and the

exhaust duct discharges into any hazardous area and when even though

the apparatus does not itself produce such particles in normal operation

but the exhaust ducting exhausts into a Zone 1. This latter is to cover the



Hazardous area



Pressure

switch



Non-hazardous

area



Fan delivery

pressure



I



50 N/mz minimum

I



I



Atmospheric pressure

Fan inlet

pressure



Fig. 11.5 Fan pressurizing with fan at enclosure inlet



Non-hazardous

area



1



Pressure

switch

Hazardous area



Pressurized

enclosure



I



Ducting



pressure



-



50 Nlm2 minimum

i



Atmospheric pressure

Fan inlet

pressure



Fig. 11.6 Fan pressurizing with fan in non-hazardous area



292



Electrical installations in hazardous areas



situation where internal electrical fault can cause the incandescent particles.

Table 11.2 produces this in tabular form.

Table 11.2 Requirements for spark and particle barriers



Necessity for spark/particle barrier

Apparatus sparks in

Apparatus does not

normal operation

spark in normal

operation



Zone into



w i h purge

hc

gas exhausts



Exhaust not permitted

Barrier required

Barrier required

Barrier not required



Zone 0

Zone 1

Zone 2

Non-hazardous



Exhaust not permitted

Barrier required

Barrier not required

Barrier not required



worn BSEN 50026)



Spark-arresting devices are usually labyrinthine devices or, for example,

steel wool (see Fig. 11.7).

Finally, it must be noted that the exhaust ducts provide a route for

explosive atmosphere entry in periods when no pressurization is present.

Therefore, some method of preventing or slowing down entry of such atmospheres is necessary as part of the procedure for restricting the rate of entry

of explosive atmosphere on pressurization failure or electrical isolation.

Lengthwise

cross-section



End cross-section



Labyrinth



Exhaust gas b



-abyrinthine

Retaining

gauze



Filled



Fig. 11.7 Spark and particle arresters



1 1.5.3 Internal electrical components, etc



As the internal parts of the enclosure are protected by pressurization, the

requirements for them are normally only those which would be required

for normal industrial use. There are some special conditions however.



Protection concept pressurization ‘p’ 293



Leak of

flammable

matenal



Plume area where

possible flammable

atmosphere is present

during dilution



\

Flammable

material



b

components



No ignition

capable

electrical

components



/

Dilution

outlet

FIammabIe



v



-* material



Pressurized enclosure



Fig. 11.8 Internal release of explosive atmosphere



Where the pressurized enclosure contains other enclosures such as those

normally fitted to the enclosed equipment (e.g., relay cases, television

monitor cases, etc.) it is possible that these could fill with an explosive

atmosphere and not be effectively purged. For this reason the inner

enclosures should either be removed or have large holes cut in to ensure

that the purge is effective. The easier way is normally to remove the internal

enclosure. This is not necessary when the inner enclosure is effectively

sealed by some means. Elastomeric seals in addition to fusion seals are

considered adequate for this purpose.

There is a further possible problem in that where an internal release

of flammable material occurs an explosive atmosphere may occur in the

region of the release before complete dilution of the release has taken

place by the continuous flow of protective gas (if that gas is air or if

oxygen is contained in the release itself). In these circumstances all electrical

components, other than those which are rendered non-ignition capable by

another protection concept, should be kept well away from the dilution

area (see Fig. 11.8). This is normally achieved by ensuring that no unprotected electrical components are near the line of sight from the release to the

enclosure exit aperture. This can be assured by siting the possible source of

leakage between the electrical components and the exit aperture. This does

not apply, of course, to deliberate ignition sources such as flames in flame

ionization chromatographs and their ignitors.



11.6 Safety provisions and devices

It is clear that the security of this protection concept relies to a very large

extent upon the monitoring and control devices which ensure that purging

is effective and pressurization is maintained. As in all cases, except those

associated with pressure maintenance in static pressurization, these are

not always provided by the manufacturer. It appears at first sight to be

somewhat incongruous that a formal Standard is written for a protection



294



Electrical installations in hazardous areas



concept which is less controlled by the manufacturer than others. This is

not, however, true as a close look at any installation using any protection

concept will show that much of its safety is determined by installation

matters (e.g., what electrical protection provisions are made,? what cables

are used?, etc.). Therefore, the situation where the purge and pressurization

sensing devices and their associated control devices are only controlled by

specification is acceptable.

Pressure and purge rate detection devices should, as far as possible, be

sited at or near the enclosure outlet to avoid incorrect identification of the

purge/pressure situation occurring (e.g., a gas leak at the input of the purge

gas causing identification of correct purge when, in fact, gas is leaking before

it enters the enclosure, or identifymg correct over pressure when inlet to

enclosure is blocked). In addition, small bore connections to such devices

should be avoided as far as possible to prevent blockage.

The sequences for purging should be as follows for all three applications

of the technique:

1. Detectors should identify that minimum pressure and flowrate is

achieved before timing of purge is commenced.

2. If pressure or purge flowrate falls below the specified minimum specification during the purge, the purge should cease and the control circuits

should recommence purge when the minimum conditions have been

restored.

3. After purging, in the case of pressurization with leakage compensation

or with continuous dilution, the control system should close purging exit

valves and continue to monitor pressure and, in the case of continuous

dilution, gas flow.

4 On failure of either pressure, or in the case of continuous flow, gasflow

.

provision must be made for an alarm to be sounded or complete electrical

isolation the choice being made by the user.

5. If the enclosure is opened, even if alarm or isolation action is overridden

by maintenance control circuits, the apparatus must go through its entire

purge cycle on reclosure as soon as the purge conditions have been

established.



It is possible to pressurize several enclosures from the same source but

care must be taken in so doing. If they are pressurized in series, it is likely

that failure of pressure in any one enclosure will cause failure in all downstream enclosures. In this case the corrective action on any single failure

must apply to all affected enclosures. Even if this is not the case any single

failure will initiate the repurging of all enclosures. Where the common

pressurization source is pressurizing the enclosures in parallel, this situation does not occur and, provided the failure of any one enclosure can be

uniquely identified, it is only necessary to repurge that one enclosure after

correction.

The reliability of the purge and pressurization control devices is

fundamental to the security of the protection concept. A European Standard



Protection concept pressurization ‘p’ 295



(EN 954)1° is in preparation and will concern itself with the reliability

of safety related circuits and devices, but until this is available there is

no European Standard which can be effectively applied. General safety

approaches (e.g., fail to safety) should govern the design of systems and

if there is any doubt in a particular case, duplication of components or

circuits should be considered.



References

1 CP 1003



Electrical Apparatus and Associated Equipment

for use in Explosive Atmospheres of Gas or

Vapour Other Than Mining operations. Part 2 (1966).

Methods of Meeting the Explosion Hazard Other

Than by the Use of Flameproof and Intrinsically Safe

Equipment.



National Electrical Code (USA). Article 500,

hazardous (Classified) Locations.

3 76/117/EEC (1975) Council Directive on the Approximation of the Laws

of Member States Concerning Electrical Equipment

for Use in Potentially Explosive Atmospheres.

December.

Centre Europeen de Normalization Electrique.

4 CENELEC

Electrical Apparatus for Potentially Explosive Atmo5 B 5501

S

spheres. Part 3 (1977). Pressurized Apparatus ‘p’.



2 NEC 70 (1990)



6 BS/EN 50016 (1996) Electrical Apparatus for Potentially Explosive

Atmospheres. Pressurized Apparatus ’p’.

7 BS/EN 50014 (1993) Electrical Apparatus for Potentially Explosive

Atmospheres. General Requirements.

8 BS/EN 60529 (1991) Specification for Degrees of Protection Provided by

Enclosures (IP Code).

General Requirements for Rotating Electrical Mach9 BS 4999

ines. Part 105 (1988). Specification for Degrees of

Protection Provided by Enclosures for rotating

Machinery.

10 EN 954

Safety of Machinery - Safety Related Parts of

Control Systems (in Preparation).



12



Apparatus with protection concept

increased safety ‘e’

(BS/EN 50019 (1994))



The protection concept (type of protection) of increased safety is one

intended for use in Zone 1 and less hazardous areas. It is a German

development (erhohtesichereit) which explains the use of ’e’ as its concept

symbol. Increased safety did not figure significantly in UK thinking before

1970 and only became important then because of the effect on UK

attitudes produced by the increasing importance of the European dimension

emerging at the time of the European Free Trade Association (EFTA), of

which the UK was a member prior to its entry into the European Union

(EU). It gained further importance in UK thinking with the UKs entry into

the EU itself.

It is a type of protection where, even though the ingress of an explosive

atmosphere into the enclosure is not prevented, the apparatus does

not spark, arc or become excessively hot in normal operation and, in

addition, is of such quality that it is unlikely to become faulty in a way

which would make it ignition capable. This security from fault is further

increased by enclosure protection from its environment, reducing the risk of

environmental conditions adversely affecting its operation. The protection,

however, is much more sensitive to electrical protection devices (fuses,

circuit breakers, etc.) than the other types of protection so far considered

as any fault (e.g., internal short circuit, connection failure, etc.) is likely to

make it ignition capable. Therefore part of the protection depends upon the

length of time for which such a situation can exist prior to the protective

devices operating.

There were many misgivings in the UK concerning the adoption of

increased safety as a protection concept suitable for Zone 1 and less

hazardous areas, particularly for such things a rotating machines and

luminaires, as the requirements for the protection concept in the German

National Standard (VDE 0171) did not, to many, appear to offer a significant

increase in the requirements applied by British Standards to Standard

industrial equipment of these types in UK. There is still considerable

concern in respect of rotating electrical machines and this is dealt with

later in this chapter.

Despite this reservation it was accepted that increased safety would

become a standard protection concept in Europe and the world at large

and to exclude it in the UK for what were, to the majority in the UK,



Protection concept increased safety 'e' 297



unconvincing techrucal reasons (no real evidence existing at the time of its

level of protection being too low, rather than historic UK approaches being

too high) would merely damage UK manufacturing industry. It was also

noted that an International Standard (IEC 79-7 (1969)') was likely to be

accepted by most countries of the world.



12.1 The situation in regard to standardization

The UK accepted the protection concept of increased safety and a National

Standard was produced in 1973 (BS 4683, Part 4*). This Standard was

rapidly overtaken by BS 5501 Part 6 (1977)3 (which was the first edition

of a European Standard, EN 50019) and both of these Standards have now

been overtaken by BS/EN 50019 (1994)4 which is the second edition of

the European Standard and will, in effect, be the Standard to which future

apparatus will be constructed. This chapter will therefore be based upon

the requirements of BS/EN 50019.



12.2 Basic construction requirements

The basic security of increased safety apparatus is in the construction of the

apparatus itself and the limitations of the types of apparatus to which the

protection concept is applicable. These limitations are:

1. The apparatus must not arc or spark or produce ignition capable hot

surfaces in normal operation



2. No apparatus operating at a supply voltage of more than 11kV rms, or

which internally produces voltages in excess of this, can be protected

within this protection concept.

3. Apparatus where component construction cannot be defined in a way

which will permit compliance with the requirements of the protection

concept, or which cannot comply by virtue of their construction (e.g.,

semiconductor devices) cannot be dealt with in this protection concept.

Therefore, increased safety is not normally applicable to instrumentation.

The above means that the protection concept finds its greatest

application in such apparatus as connection boxes, luminaires, rotating

machines, transformers, batteries, resistance heating devices and measuring

instruments (basic meters, etc.).

12.2.1 Construction of enclosures



Enclosures need to comply with the general hazardous area requirements

(see Chapter 8) but because of the nature of this protection concept they

must satisfy additional requirements.



298 Electrical installations in hazardous areas



As previously stated, the protection concept relies to a high degree on

the exclusion of the external environment (keeping out the weather but not

gas) and to this end the enclosures must have a degree of protection of at

least IP54 in accordance with BS/EN 605295 (As elsewhere, the method

of determination of IP-rating for rotating electrical machines is slightly

different and BS 4999 part 1056 is used in place of BS/EN 605295 for

such machines). Where the enclosure contains only insulated conductors

and components, rather than bare live parts, this enclosure integrity may

be reduced to IP44. Further reductions are permissible if breathing or

draining holes are necessary for the reliable operation of the apparatus.

In these circumstances IP44 is acceptable for enclosures containing bare

live parts, and IP24 for enclosures containing only insulated conductors

and components. In these latter cases, to utilize the lower IP-ratings, the

position of the holes is critical and the apparatus will be subjected to

installation conditions which minimize the possibility of ingress of liquid

or solid foreign bodies in its installed location.

The requirements become more complex when this protection concept is

used in conjunction with intrinsic safety (see Chapter 13 which discusses

intrinsic safety) as is sometimes the case when it needs internal monitoring

devices (e.g., temperature sensors in rotating machine windings) to ensure

that early isolation takes place in case of fault. It is clear that the actual

construction of the apparatus must be such that the intrinsic safety is not

compromised but there is a particular point where outside influences can

produce such a compromising effect. This is where the external connections

are made for the increased safety part of the apparatus in the same place

(terminal box, etc.) as the intrinsic safety external connections. In these

circumstances an inner cover of at least IP30 is necessary for the increased

safety connection facilities and a warning is necessary to ensure that this

cover is not removed when the outer cover is off and the increased safety

terminations energized or the intrinsically safe circuits are connected to

external circuits. The inadvertent application of the levels of voltage in most

increased safety apparatus could damage an intrinsically safe installation

in a way which would not be apparent and which may cause danger, not

only in the particular installation but in other intrinsically safe installations

using facilities in common with it.

These enclosures and covers need to be fixed by special fasteners as

described in Chapter 8.

12.2.2Terminals and connection facilities



All electrical connections, both those within the apparatus and those to

permit connection of external electrical conductors, are important in this

concept as any failure, even partial, of a connection can give rise to sparking.

Of the possible electrical connections for external conductors themselves,

only terminals are specifically addressed with specific detailed requirements

which have to be satisfied to ensure they are reliable. The general use

of plugs and sockets is not specifically mentioned in any Standards for