5 Apparatus Producing Arcs, Sparks and/or Ignition-Capable Hot Surfaces

444



Electrical installations in hazardous areas



Poured seals which are used as permanent seals and not subject to

separation in normal service must have a melting point which is at least

20°C higher than the maximum temperature which the device is expected

to reach in service at maximum ambient temperature.

The internal free volume of both enclosed break devices and nonincendive components must not exceed 20 cm3.

Ratings

There are also limits on the rating of these devices and these are as follows.

First, enclosed break devices shall not be rated at voltages exceeding 660V

and which are rated for currents in excess of 15A.

Second, non-incendive components have similar but lower absolute limits

w i h are 250 V and 15 A. In this case, because of the nature of the device,

hc

the circuit feeding any sparking contacts is important as flame quenching

is the possible means of protection. They are usable only in particular

circuits and testing for ignition must be in circuits typical of those in

which the component is intended to be used. For this reason non-incendive

components normally appear only as parts of other apparatus and not as

apparatus in themselves - unlike enclosed break devices which can more

readily stand alone.

Type tests



To execute the necessary type tests the devices are filled and surrounded

with the explosive atmosphere specified in Table 14.5.

In the case of enclosed break devices the atmosphere within the enclosure

will be ignited by the contacts within it or by the internal hot surface and

ignition verified by some means (e.g., thermocouples or ionization detection). The external atmosphere must not be ignited. The test will be repeated

three times using renewed explosive atmospheres in each case and the ignition will be by the contacts operating at their maximum rating or the hot

surface at its maximum temperature.

In the case of non-incendive components there is no need to confirm

internal ignition. The enclosure is filled and surrounded with the explosive



Table 1 . Test mixtures for flame transmission in

45

enclosed break devices and non-incendive

components

Sub-group



Test gas mixture



IIA

IIB

IIC



6.5 f0.5% ethylene i ah

n

28.2 f 2% hydrogen in air

34 f 2% hydrogen117 f 1%oxygen in nitrogen



(from BS 6941)



Protection concept



'N ('n')



445



atmosphere as before and the contained hot surface or contacts operated

in the most onerous condition in the circuit in which they are used. In the

case of contacts they will be preconditioned by being operated 6000 times

at a rate of around 6 operations per minute at their rated load (not the

circuit load). The contacts are then operated at their maximum circuit load

50 times at around the same rate when the explosive atmosphere is present

and ignition of the external explosive atmosphere must not take place. No

time criterion is given for hot surfaces but allowing the test to proceed for

10 minutes is acceptable. These tests also must be repeated three times with

new explosive atmospheres each time.

I,

I

The apparatus is, in this case, sub-grouped I A IIB or IC depending

upon the test mixture used. A typical use for an enclosed break device is

shown in Fig. 1 . where it is used as an automatic isolator for a screw

43

lampholder. A microswitch which is a typical non-incendive component is

44

shown in Fig. 1 . .

74.5.2 Hermetically sealed devices



Hermetically sealed devices are devices which are permanently sealed by

means such as fusion. This type of seal is used in normal equipment for

such things as contacts and relays to allow for the production of a controlled

atmosphere around contacts to give maximum efficiency and longevity. In

Type 'N, however, they can be used to exclude an explosive atmosphere.

End electrical

connection



Side electrical

connection

Lamp



Fig. 14.3 Automatic isolation of lamp on removal using enclosed breawnonincendive component technique



446 Electrical installations in hazardous areas

Operating



rod



Switch

enclosure



Fig. 14.4 Microswitch using enclosed breaklnon-incendive component technique.

Note: The shroud is removed for testing



Construction



Seals are made by such arrangements as glass/metal fusion, welding,

brazing or soldering. Seals made using gaskets cannot be termed as hermetic

seals. Where hermetic sealing is used the seal must not be damaged in

installation of the device in apparatus and must not be damaged by the

normally applied impact and, if appropriate, drop tests.



Type testing



Once the seal is made it is not possible to carry out any measurements

inside the enclosure and the only test which can be carried out is a leakage

test. This can be carried out by placing the device in hot water to expand

the internal gas (water at 75 "C to 85 "C is used and the initial apparatus

temperature is between 15°C and 35"C), or by evacuating the air above

the water in which the device is immersed to encourage leaking (in this

case the temperature of both test item and water are between 15°C and

35°C and the pressure over the water is reduced to an absolute value of

5 x 103N/m2absolute).

The differential temperature test is normally for 2 minutes and leakage

is observed by noting a single large production of bubbles at the device

surface, or a train of bubbles for at least 20 seconds. Normal air release

from the surface on initial immersion should be ignored even though it

gives similar indications for a short time. In the case of reduced pressure

a continuous train of bubbles will be noted until the enclosure reaches the

pressure of the air over the liquid.



Protection concept ‘N’ (‘ny 447



74.5.3 Sealed devices



Sealed devices are similar in operation to hermetically sealed devices but

use sealing gaskets and poured seals, and are often secured together by such

things as rivets. Like hermetically sealed enclosures, they are not intended

to be opened in service and by the use of such things as rivets or adhesives

they are destroyed by opening.



Construction



Sealed devices are limited in size, unlike hermetically sealed devices, to

100cm3. Their gaskets are conditioned as for enclosed break devices but

in situ and they must be protected against mechanical damage or environmental damage in the same way. Poured seals are also subject to the same

limitations.



Type testing



The type tests applied to sealed devices are identical to those for hermetically sealed devices.



14.5.4 Energy limited apparatus and circuits



This technique is very similar to intrinsic safety in that it relies on the

limitation of power and energy in the circuit in question to levels that,

even if released as a spark or delivered to heating of a surface, will not

cause sparks or hot surfaces which are ignition capable. The approach is

much more simple.

There are two types of energy limited apparatus: the first type, ‘apparatus

containing energy limited circuits’, is that which is connected to a circuit

which contains ignition-capable energy where limitation is within the

apparatus where sparking contacts or possible hot surfaces are present; the

second type, ‘Apparatus for connection to energy limited circuits’, is that

which is connected to a circuit usually initiated in a non-hazardous area

where the entire circuit in the hazardous area - both within and outside

the apparatus - does not contain incendive levels of energy.



Construction



Apparatus containing energy limited circuits is fed from a supply which

is not energy limited and will be either limited by means described earlier

for non-sparking circuits or by the mains. It will normally contain further



448 Electrical installations in hazardous areas



voltage limiting devices, such as a transformer if mains fed, or zener diodes.

Further current limiting by such things as resistors will normally also be

present, the objective being to define clearly the power and energy which

can be fed to sparking contacts or potentially ignition-capable hot surfaces.

In the case of sparking contacts the sparks are always there (unlike sparks

caused by faults) and the likelihood of coincidence of a spark and the explosive atmosphere is thus increased. This means that the maximum voltage

and current let-through of the limiting devices, in the case of mains with

a 10 per cent elevation, become important and form the basis of evaluation of safety for both spark ignition, and hot surface ignition rather than

the normal operational limiting circuit parameters. Thus the voltage and

current are in all cases based upon the tolerances of the voltage and current

limiting components in the apparatus. In many cases these elements will

not form a part of the operational circuit and thus, if voltage limiters such

as zener diodes are used and their failure is not identified by maloperation

of the circuit, they must be duplicated if they protect a sparking contact.

This also applies to such things as resistors used in a shunt mode to limit

voltage but not to such devices used in series. Where such components are

used to suppress inductances their connection to the inductance must be

made reliable by placing them as close to it as possible with arrangements

to limit the possibility of their becoming disconnected without the simultaneous disconnection of the inductor, unless their disconnection is identified

by maloperation of the apparatus.

Apparatus for connection to energy limited circuits may or may not have

within it limiting devices to prevent overheating or incendive sparking but,

they will be based upon the energy limited source specified for connection

to the apparatus. In this case such devices will be predominantly for temperature limitation or for energy limitation in cases where storage within the

apparatus can occur, such as included inductors or capacitors.

The important thing here is to specify the maximum open circuit voltage,

short circuit current, and power transfer capability required in the circuit to

which the apparatus is connected, together with the maximum inductance

and capacity (with L/R ration as a possible alternative to inductance)which

may be present in that circuit. It is then up to the installer to ensure that

these parameters are complied with. This will often mean the apparatus in

the non-hazardous area containing voltage and current limitation to a level

similar to that required in this type of apparatus.

It must be remembered here that use of the non-incendive circuit

approach, where the entire circuit is non-incendive, is intended to allow

live working to a similar degree as that for intrinsic safety (see later in

this book).

Testing



Testing of non-incendivity is carried out at points where sparking will

occur using the spark test apparatus and the same test mixtures as for

intrinsic safety (see Chapter 13) but in this case no safety factors are used.



Protection concept ‘ N (n)

‘‘



449



The test gas mixtures and calibration currents for the spark test apparatus

are repeated in Tables 14.6 and 14.7, with additional calibration currents

for resistive circuits. The presence of resistive calibration currents stems

from the historic position in intrinsic safety where it was felt that the difference between inductive calibration and resistive calibration was sufficient to

warrant separate calibration for resistive and capacitive circuits to that used

for inductive circuits. This difference has now been accepted in intrinsic

safety as not significant and only the inductive calibration is used. While it

may be expected that the same will be the case in Type ‘ N it is not currently

so and thus the two different calibration currents still exist. Another difference here is that the test may be carried out with a t n disc in the spark

i

test apparatus rather than the normal cadmium. This gives higher levels of

ignition power and energy because it does not have an ignition enhancing

effect. It is not permitted for intrinsic safety because of the difficulty of

exclusion of cadmium, zinc and magnesium, all of which have this ignition

enhancing effect. In Type ’N’, however, where the risk is more remote it is

concluded that the level of attention to adequately ensure the absence of

these materials may be less rigourous and thus possible. Notwithstanding

ths, however, it is recommended that the cadmium disc (i.e., the more

sensitive) test is used where possible.

Assessment of sparking circuits is possible for type ’N’ using the curves

given in Chapter 13 (Figs. 13.5, 13.6,13.8 and 13.10).The curves can be used

directly as there are no added safety factors as in intrinsic safety.



Table 14.6 Test gases for non-incendive

sparking

Sub-group



Test gas



IIA

IIB

IIC



5.25 & 0.25% propane in air

7.8 0.5% ethylene in air

21 k 2% hydrogen in air



~~



*



~



~



~~



~



Table 14.7 Calibration currents for spark test apparatus

Sub-group



Inductive circuits



Resistive circuits



Cadmium

disc

mA

IIA

IIB

IIC



Other

disc

mA



Cadmium

disc

A



Other

disc

A



100

65

30



125

100

52



1.0



2.75



0.7



2.0



0.3



1.65



450 Electrical installations in hazardous areas



14.5.5 Restricted breathing enclosures



This is a somewhat unusual type of protection relying not on total sealing

but on the limitation of entry and exit of gas (breathing). Historically it

has only been used on certain lighting fittings in the UK and has not been

considered as acceptable for sparking equipment. In Switzerland, however,

where much more use of the technique has been made, quite the reverse is

the case. The technique used by the Swiss does not permit hot surfaces and

only allows sparking contacts. Its use is much more prevalent, for example

it can be applied to large switchboards.

The historic UK approach, which is still current, is the use of restricted

breathing for luminaires and its exclusion for sparking contacts. This stems

from the view that luminaires are not regularly switched on and off and

that, even if some breathing occurs, it is unlikely that anything like an ideal

explosive atmosphere will ever exist in the enclosure. For this reason it

is considered that the approach is acceptable for Zone 2 and, indeed, the

historical use of this approach without problems supports this. While the

approach has been considered as suitable for luminaires which are not regularly switched on and off (i.e., not subject to duty cycling) the support for

any other use where temperature cycling within the enclosure occurs more

regularly does not exist. This is not a problem for the UK because restricted

breathing has in the past been restricted to luminaires and this continues

to be the case. The following is the situation for restricted breathing as

practiced in the UK.

Construction



There are no constructionalrequirements for restricted breathing enclosures

other than that thermoplastic and elastomeric seals are preconditioned, as

for those in enclosed break devices, before any type testing is carried out

and that they need to be protected against mechanical and environmental

damage in the same way. Likewise poured seals are subjected to the same

requirements as those in enclosed break devices.

Restricted breathing enclosures are required to have facilities for

execution of the restricted breathing test in service and there has, therefore,

to be a facility to allow the connection of a test device.

Testing



Type testing for restrictive breathing enclosures is quite simple as it is solely

a test of the inward breathing of the enclosure (the outward breathing is

not important). The enclosure is reduced in pressure to less than 0.97 x

lo5N/m2 absolute (approximately 0.97 atmospheres) and then the time

taken for ingress of air sufficient to change the pressure from 0.97 x lo5

to 0.985 x lo5 is measured. This time must not be less than three minutes.

It is possible to determine this time with an internal overpressure only



Protection concept ‘N’ (‘n’) 451



where it can be shown that the performance of the enclosure is the same

for overpressure as for underpressure.

This test may need to be carried out after reclosure of the apparatus,

sometimes after each opening, and provision needs to be made in the

construction to allow this to be done. Usually this is by having an entry

device for a simple evacuating device which can be sealed with a valve or

a screw and gasket after testing.



Other approaches to restricted breathing enclosure

The other country which has historically used restricted breathing as a protection concept is Switzerland where the concept was used in a quite different

way. Here the concept has been used for equipment which may spark in

normal operation, such as switchgear, and it has been limited to apparatus

where the internally dissipated power is not sufficient to raise the internal

temperature by more than 10°C. Thus it has not been possible to use it for

luminaires. While the basic constructional requirements have not differed

greatly the testing has. One of three tests have been applied as follows.

1.A diffusion half-time test is applied whereby carbon dioxide is introduced

into the enclosure without raising its pressure until the level of internal

carbon dioxide concentration is 25 per cent. The internal concentration of

carbon dioxide is then monitored for sufficient time to allow a straight line

curve showing reduction of concentration with time to be plotted. This

curve is then extrapolated to estimate the time necessary for the internal

concentration of carbon dioxide to fall to 12.5 per cent which must not be

less than 80 hours. Where the pressure measuring device adds significant

volume to the enclosure volume (more than say 10 per cent) then the time

given above needs to be multiplied by the total volume divided by the

enclosure volume.

2. Alternatively, where the enclosure has a volume of less than lolitres,

it should be raised or lowered (alternatives) to a pressure of 500N/m2

above or below atmospheric pressure and the reduction in pressure differential measured. The time taken for the difference to fall from 400 N/m2 to

200N/m2 must be at least 80 seconds in this case. The same multiplication

of time as for the carbon dioxide test is applied where the measuring device

has a significant volume.

3. A third alternative is to pressurize the enclosure to 400N/m2 above

atmospheric pressure and calculate the volume of air required each hour to

maintain that overpressure. That volume, divided by the enclosure volume,

should not exceed a value of 0.125.



Use of the concept

As with the UK approach the Swiss approach, as defined in IEC 79-1511, has

been limited to apparatus that is not subject to duty cycling, which can cause



452 Electrical installations in hazardous areas



significant temperature changes. In addition, the technique in its Swiss/IEC

guise is limited to gases and vapours which have restricted breathing factors

(s)of less than 20. This factor is related to the boiling point (bp), the molecular

weight (m) and the lower explosive limit (LEL) of the gas or vapour in

question and is derived from Fig. 14.5. The boiling point is identified on

the left-hand side of Fig. 14.5 and the molecular weight on the right. These

are then joined by a straight line which creates a point Z on the centre line.

The LEL of the gas or vapour is then identified on the right-hand side of

Fig. 14.5 and a line drawn to the left-hand side via the point Z on the centre

line. The restricted breathing factor can then be read off on the left-hand side.

While this approach to restricted breathing is not currently used in the

UK, the imminent production of a European Standard for type of protection

' N may include it as the approach is in IEC 79-1511 which will form the

basis for the European Standard. The presence of such a European Standard

may create a situation where it may become incumbent on the UK to utilize

this type of apparatus and for this reason it is included. The safety of such

equipment may be called into question but its long use in Switzerland

produces no evidence to support any such doubts.



Fig. 14.5 Determination of restricted breathing factor (S). Notes: (1) Draw a line

between the material boiling point end its molecular weight. This crosses

the centre line at Z. Draw a line from the LEL via Z to determine S -the

breathing factor. (2) Where boiling temperatures of less then 40 "Coccur,

40°C will be assumed for use of figure



Protection concept ‘N’

(‘n’) 453



References

1 BS/CP 1003 (1967)



Part 3. Division 2 Areas.



2 BS 4137 (1967)



Guide to the Selection of Electrical

Equipment for Use in Division 2 Areas.



3 BS 5345



Installation and Maintenance of Electrical Apparatus for Use in Potentially

Explosive Atmospheres. Part 1 (1976).

Basic Requirements for All Parts of

the Code.

Council Directive on the Approximation of the Laws of Member States

concerning electrical Equipment for

Use in Potentially Explosive Atmospheres. 18th December.



4 76/117/EEC (1975)



5 94/9/EC (1994)



Directive of the European Parliament

and Council on the Approximation of

the Laws of Member States Concerning

Electrical Equipment for Use in Potentially Explosive Atmospheres. 23rd

March.



6 BS 4683



Electrical Apparatus for Explosive

Atmospheres. Part 3 (1972). Type of

Protection N.



7 IEC



International Electrotechnical Commission.

Electrical Apparatus for Explosive Atmospheres with Type of Protection N.

Specification for Rotating Electrical

Machines of Particular Types for

Particular Applications. Part 16 (1972).

Rotating Electrical Machines with Type

of Protection N.



8 BS 6941 (1988)

9 BS 5000



10 BS 4533 (1976)



Luminaires: Part 101, Specification for

General Requirements and Tests;

Part 102 Particular Requirements: (Section 51) Specification for Luminaires

with Type of Protection N.



11 IEC 79-15



Electrical Apparatus for Explosive Gas

Atmospheres. Part 15 (1987). Electrical

Apparatus With Type of Protection n.

Comite Europeen de Normalisation

Electrique.



12 CENELEC



454 Electrical installations in hazardous areas



13 BS/EN 60529 (1988)



14 BS 4999



15 BS 5901 (1980)



16 BS 5917 (1988) (withdrawn)



17 IEC 34



18 BS 5042 (1987)

19 BS 6776 (1990) (withdrawn)



20 BS 6702 (1991) (withdrawn)



21 BS 3772 (1980)

22 BS 2818 (withdrawn)



23 BS 4782 (1971) (withdrawn)



24 IEC 61 (1969)



Specification for Classification of Degrees of Protection Provided by Enclosures (replaced BS 5490).

General Requirements for Rotating

Electrical Machines. Part 105 (1988).

Classification of Types of Enclosure.

Method of Test for Determining the

Comparative and Proof Tracking Indices of Solid Insulating Materials under

Moist Conditions.

Specification for Conformal Coating

Material for Use on Printed Circuit

Assemblies. This Standard has been

replaced by the following: BS/EN

61086-1 (1995);BS/EN 61086-2 (1995);

BS/EN 60086-3 (1995).

Rotating Electrical Machines. Part 5

(1991). Classification of degrees of

protection provided by the enclosures

of rotating machines.

Specification for Lampholders and

Starter Holders.

Specification for Edison Screw

Lampholders: this Standard has been

replaced by BS/EN 60238 (1992).

Specification for Lampholders for Tubular Fluorescent Lamps and Starter

Holders: this Standard has been replaced by BS/EN 60400 (1992).

Specific for Starters for Fluorescent

Lamps.

Ballasts for Tubular Fluorescent Lamps.

This standard has been replaced by the

following: BS/EN 60920 (1991); BS/EN

60921 (1991).

Specification for Ballasts for Discharge

Lamps (Excluding Ballasts for Tubular

Fluorescent Lamps). This standard has

been replaced by the following: BS/EN

60922 (1991); BS/EN 60923 (1991).

Lamp Caps and Holders Together with

Gauges for Control of Interchangeability and Safety.