10 Infallible Components, Assemblies and Construction Elements

386



Electrical installations in hazardous areas



not reach this level are considered to fail in any event if such failure makes

the circuits more ignition capable. There is a third level to be considered

which is that level where construction and rating is of such a high level

that failures can be ignored. In components, assemblies of components, and

connection elements this level of excellence warrants the award of the title

of infallible component, assembly or connection. Such things as separations

can also reach a level where failure need not be considered (see Table 13.7)

but in this case they are termed as ’not subject to fault’ and Infallible is

not used. In any event, the results are the same and when carrying out a

fault-counting exercise specific failure modes in any of these components,

assemblies, wiring, connections and separations need not be considered.

In all cases the infallibility of such components, assemblies, wiring,

connections and separations is limited to the particular mode of failure

which their construction permits to be ignored (e.g., the separation of a pair

of printed circuit tracks may make them such that connections between

them can be assumed not to occur, but it will not have any effect upon

consideration of their breakage).

While it is confusing to have the two terms ‘infallible’ and ’not subject to

fault’ applied to particular failure modes of components and other circuit

elements, the use of either term is understood to impart the same information and no danger should result.

Infallible components, assemblies and arrangements which are considered as not subject to particular fault conditions need to comply with

specific requirements which are additional to the basic constructional

requirements of the protection concept which give countable fault status

(see Sections 13.7 and 13.8).

13.10.1 Infallible transformers



Clearly, mains transformers, used in associated apparatus to reduce voltage

to the normal low levels used in intrinsically safe circuits, are important as

failure of the windings associated with that circuit to the mains winding or

other auxiliary windings on the transformer associated with other intrinsically safe circuits or non-intrinsically safe circuits, would lead to a situation

where intrinsic safety was compromised. Such a transformer needs to be

infallible against such interconnections both in normal operation and in

failure conditions. In addition, where failure of windings associated with

the intrinsically safe circuit to the core of the transformer, which is normally

earthed, could produce an ignition-capable condition, the transformer must

be infallible against that failure also. The requirements for such transformers

are as follows.

Mains transformer construction



Several modes of construction are envisaged for mains transformers in

BS/EN 50020 and are given type numbers. The simplest of these is the



Protection concept intrinsic safety 'i' 387



Table 13.11 Minimum dimensions for wire or foil used

for transformer screens

Rating of

protective fuse



Thickness o foil

f

forming screen



or circuit



screen



breaker



(Note 1)



Amps



mm



0.1



0.05

0.05



0.5

1.o

2.0

3.0

5.0



Diameter o wire

f

forming layers of



0.075

0.15

0.25

0.3



(Note 1)

mm

0.2

0.45

0.63

0.9

1.12

1.4



(from BSEN 50020)

Note: 1 The above figures for wire and foil dimensions are nominal

figures and a tolerance of up to 10% manufacturing tolerance is

permitted provided that in all cases the variation in dimension may

not exceed 0.1 mm.



type where the transformer is permitted to exceed its insulation ratings but

a failure between non-intrinsically safe circuit windings and other windings is prevented by an earthed metal screen of thickness complying with

Table 13.11. This is termed a 'Type 2b transformer and its principles of

construction are shown in Fig. 13.23.

Type 2b transformer

The Type 2b transformer is normally a 'shell type' transformer (see

Fig. 13.23) with its windings over one another on a single bobbin, or

without a bobbin, but consolidated in both cases on a single core so

that any separations are reliable and repeatable. The screen must be the

full width of the winding area to ensure that no possibility exists for

interconnection across the screen except at the outer edge of the windings

where Table 13.7 applies. In either case the insulation or other separation

between the windings associated with intrinsically safe circuits and the core,

the screen and the other windings must ensure that the transformer satisfies

the following as a minimum:

1. The composite separation between the winding mass of the separate

windings around the screen must satisfy the requirements of Table 13.7.

(In most cases this will require a composite calculation of separation

distance - see Figs. 13.20 and 13.21).

2. The screen must be of either a single layer of foil which comprises at least

one turn and overlaps but is insulated so that the ends of the screen cannot

become interconnected, or of two layers wound in insulated (varnished)

wire. For a foil screen this requirement is to avoid the screen ends becoming



388



Electrical installations in hazardous areas

Windings (Note 5)



Insulation

(Note 4)



I



Screen

(Note 2)



Insulation

(Note 3)



Fig. 13.23 Type 2b transformer. Notes: (1) Core to be provided with earth

connection (interleaved in laminations if appropriate) unless core

material is non-conducting. (2) Screen to be provided with two earth

connection wires if foil; both layers to have one end brought out if wire

layers. (3) Insulation to normal industrial requirements. (4) Insulation to

withstand 500V test unless earth faults not acceptable when Table 13.7

applies, as appropriate. (5) Windings to be consolidated



connected to form a short circuited turn which will cause the transformer

to fail. For a wire screen, insulation between each turn of wire and between

the two layers is necessary for the same reason. Because of this, the only

insulation requirement between layers is that the insulation will not fail

when subjected to an insulation test at an applied voltage of 500Vrms ac

or 700 V dc. The voltage is applied by raising the applied voltage steadily so

that the specified value is attained after 10 seconds and then maintaining

that value for 60 seconds. The leakage current should be less than 5mA

and must be constant or reducing as any increase will be assumed to be

advance warning of insulation failure.

3. The transformer must not create a condition where the winding/s

associated with a particular intrinsically safe circuit become connected to

other windings. To prove this a test is applied where one (any one) of the

output windings of the transformer is short circuited and then all other

output windings are subjected to a load which will produce maximum

rated current from that winding at rated voltage. The transformer then is

subjected to application of its rated voltage at the input winding or such

smaller voltage, as will give l.71n into the transformer. This condition is

applied for 6 hours or until the transformer fails. The criteria for insulation

is that when a voltage of twice the transformer rated voltage (U,) plus

1OOOV with a minimum of 1500V is applied between the windings in

question, the current which flows should not exceed 5mA and should be



Protection concept intrinsic safety ‘i’ 389



constant or falling. The transformer may fail during this test but must not

burst into flames as burning may introduce further consequent failures.

4. Where failure to earth may cause problems for the intrinsically safe

circuit, failure of the transformer must not occur and the transformer must

not exceed its temperature rating during the above test. This can be achieved

by the use of in-built thermal trips as already described or by the inclusion

of a limiting resistor to limit current in the transformer. Where automatically

resetting trips are used, rather than non-resetting trips, the test is extended

to 12 hours to allow for repeated trip operations. Where a resistor is fitted

it will be mounted in the input circuit of the transformer and must satisfy

the separation requirements of Table 13.7, be of adequate rating (see earlier

in this chapter), and be infallible as described later. The separation of the

winding/s supplying the intrinsically safe circuit and other windings and

the screen must in this case satisfy Table 13.7. The possible effect of failures

to earth is shown in the example in Fig. 13.24.



5. Where the screen is of two separate layers of winding each layer should

be individually connected to earth and provision made for this on the

transformer. Where the screen is a foil it must have two separate wires

connected to it to allow the same security of earth connection and care is

necessary to ensure that connection of these wires does not introduce a

partial short circuit turn on the transformer.

6. The core of the transformer must also be earthed and its construction

should allow for this. This is usually done by interleaving a conducting

strip in the laminations and then providing a terminal on it where it

exits the laminations. Some transformer materials, such as ferrites, may

be insulating in themselves and in such cases the earth connections

are not necessary. It would not be acceptable, however, to insulate a

conducting core and not earth it unless the insulation thickness satisfied

Table 13.10 and the transformer was prevented from overheating as

previously described.

7. The separation of the connection facilities should also comply with the

requirements of Table 13.10 to prevent interconnections at the termination

points.



Type la, 7b and 2a transformers



The other types of transformer envisaged are the Type la transformer, the

Type l b transformer, and the Type 2a transformer. The Type la transformer

has its windings wound side by side on one limb of the transformer and

the Type l b transformer follows the same construction but has the separate

windings on separate limbs of the transformer, but still as separate entities

(not one over the other).

The Type l b transformer is more common as it uses a ’shell type’ core

where one limb goes through the winding and the two other limbs surround

it, rather than the Type l a transformer which normally has a ‘core type’ core

with one limb through each winding and usually no surrounding limbs.



390



Electrical installations in hazardous areas



The Type 2a transformer is similar to the Type 2b transformer, but in

this case there is no screen with the windings being on top of one another

and having insulation between them. The insulation must, of course, be the

full width of the winding area (see Fig. 13.24) to prevent the presence of

a path through it except around the outside edge of the winding where

Table 13.7 applies.

Insulation

(Note 2)



A



Insulation

(Note 5)



Windings

?Notes 4 and 6)



Insulation

(Note 3)



Fig. 13.24 Type l a transformer. Notes: (1) Core to be provided with earth connection (interleavedin laminations if appropriate) unless core material is nonconducting. (2) Insulation to normal industrial standards. (3) Insulation

to withstand 500V test unless earth faults are not acceptable when

Table 13.10 applies. (4) If necessary to prevent overheating, thermal trip

embedded in either primary or secondary (as appropriate). (5) Insulation

to Table 13.10. (6) Windings to be consolidated

Core

(Note 1)



\ Windings (Notes 4 and 6)



Insulation

(Note 2 )



Insulation

(Note 5 )



Insulation

(Note 3)



Fig. 13.25 Type l b transformer. Notes: (1) Core to be provided with earth

connection (interleaved in laminations if appropriate) unless core

material is non-conducting. (2) Insulation to normal industrial standards.

(3) Insulation to withstand 500V test unless earth faults are not

acceptable when Table 13.7 applies. (4) If necessary to prevent

overheating, thermal trip embedded in either winding (as appropriate).

(5) Insulation to Table 13.7. (6) Windings to be consolidated



Protection concept intrinsic safety ‘i’ 391

Windings

(Notes 4 and 6)



Insulation

(Note 2)



I



Insulation

(Note 5)



/



Core

(Note 1)



Insulation

(Note 3)



Fig. 13.26 Type 2a transformer. Notes: (1) Core to be provided with earth

connection (interleaved in laminations if appropriate) unless material

is non-conducting. (2) Insulation to normal industrial standards.

(3) Insulation to withstand 500V test unless earth faults are not

acceptable when Table 13.10 applies. (4) If necessary to prevent

overheating thermal trip embedded in primary or secondary winding

(as appropriate). (5) Insulation to Table 13.10. (6) Windings to be

consolidated



In all these cases the transformer must have its windings consolidated

and must not fail when subjected to type tests described for Type 2b transformers, which almost invariably means that a thermal trip or limiting

resistor is needed as in the case of most Type 2b transformers. The thickness of insulation between each winding and others, or the earthed core

of the transformer (if earthing causes a problem) must satisfy Table 13.7.

Detail of the construction of Types la, l b and 2a transformers are shown in

Fig. 13.24, 13.25 and 13.26.

As transformers being wound and generally assembled components

are subject to a higher degree of variability than other more critically

controlled components a routine test is necessary and this test is described

in Table 13.12.

Construction of transformers which are not mains transformers



Within intrinsically safe circuits it is common to use transformers for other

purposes (e.g., to produce galvanic isolation between parts of the circuit).

Such transformers must be constructed in accordance with the mains

transformer requirements except that the 5-hour test need only be done at

rated transformer load. Where such transformers are connected between



392



Electrical installations in hazardous areas

Protective

fuse

Transformer

Transformer fault (Note 1)



limiting resistor



LP



Ij,



Mains



(Note 1)



A (Note 2)



I

I

I

I

I



Fig. 13.27 Effect of transformer windingkore or screen faults. Notes: (1) Field earth

fault can be expected as normal operation and thus secondaty/frame fault

in transformer (either as single countable fault or non-countable fault) will

bypass current limiting resistor destroying intrinsic safety. (2) If this point

is infallibly earthed transformer fault shown will merely blow protective

fuse leaving intrinsic safety intact



Table 13.12 Routine test voltages for mains transformers

Application points



rms applied voltages



Between the mains input winding

and all secondary windings

Between all the windings (both

input and secondary) and the

transformer core.

Between the Intrinsically Safe

circuit winding(s) associated with

a particular intrinsically safe

circuit and all other secondary

windings.



4 U,, or 2000 V rms



2 U,, or 1000V rms

2 u,, + 1000v rms

with a minimum of

1500V rms



Note: The criteria for the test is a current of less than 5 m A w i h is

hc

steady or falling during the test w i h i applied for 60 seconds the

hc s

voltage having been raised steadily to the test value in 10 seconds.



intrinsically safe and non-intrinsically safe circuits there is, however, a

further problem in that the non-intrinsically safe circuits are not evaluated

and it must be assumed that mains can break through to the transformer.

In such cases it is necessary to introduce measures to prevent this causing

damage to the transformer. Such precautions could be those applied to



Protection concept intrinsic safety 'i' 393



mains transformers of the same type or, alternatively, by placing a shunt

zener diode across the affected winding and then protecting the diode with

a suitably rated fuse to ensure that the mains breakthrough is removed

before the transformer or zener diode can be damaged. The requirements

of Table 13.7 need to be applied to this fuse/zener combination to ensure

it cannot be by-passed.

Transformer protection



To ensure that no failures occur it is essential that the transformer cannot be

operated in such a way as to allow those failures. This necessitates protection of the transformer in a way which allows its worst case conditions to

be defined. All mains transformers have to be supplied with an input fuse

to remove the mains supplies if failures occur either in the transformers

themselves or the connected circuits.

The basic level of protection is to fuse the input circuit so that the maximum

design operating conditions of the transformer specified during its design

cannot be exceeded. As the transformer can handle these without causing

problems in the intrinsically safe circuit, it can be considered as infallible.

The fuse must comply with the requirements already described in

Section 13.6.2 for fuses mounted in associated apparatus, which means that

the maximum current which the transformer can continuously draw is, with

the presence of such a fuse, considered as 1.7 times the rated fuse current

(In). A suitably rated circuit breaker may be used in place of a fuse and the

circuit breaker need only comply with a recognized standard of construction

for such circuit breakers. The 1.7 I, figure is retained and in this case is 1.7

times the rated current at which the breaker will operate. Therefore, the

transformer must be capable of withstanding this current without damage,

which could adversely affect intrinsic safety.

The basic requirements of circuit breakers is that they comply with a

recognized Standard which normally is taken to mean a European Standard,

a harmonized International Standard or, failing the presence of either of

these, a National Standard.

The use of the fuse or circuit breaker described above is usually sufficient

for transformers of Type 2b construction unless winding faults between

the transformer windings and core or screen are not acceptable when, in

common with Types la, l b and 2a transformers, the current which the fuse

will pass (1.7 I,) must not additionally allow the winding temperatures

to exceed their insulation rating. In circumstances where this can occur a

thermal trip will be necessary to ensure this does not occur.

Where such a thermal trip is used to disconnect the transformer from its

supplies before winding temperatures are exceeded it must not be accessible. It must be embedded, in or consolidated with, the windings so that

confidence is present that it will reliably disconnect before any winding

or other insulation within the transformer exceeds its rated limit. The trip

may be non-resettable or automatically self-resetting but not manually reset

as this implies external intervention which would not be acceptable as it



394 Electrical installations in hazardous areas



would permit resetting before the transformer could cool, and the possibility exists of the temperature limits of the construction being exceeded.

Only one such device is necessary.

The constructional requirements for such a thermal trip are that it should

be to a recognized Standard for such devices. (Recognized Standard in this

context usually means a European Standard where one exists, a harmonized

International Standard or, failing either of these, a national Standard).

Where the transformer is possessed of a suitable earthed screen it is

not essential to ensure that insulation temperature limits are not exceeded

if earth faults within the transformer do not cause problems. It must be

remembered that, in the UK, the normal mains supplies are earthed as a

matter of policy for personnel protection by connection of the neutral pole

of the supply to earth at the point of supply origination. This means that in

the case of a fault in the transformer any supply pole could be earthed. To

ensure that the supply is effectively isolated on such failures all non-earthed

electrical supply conductors must have a fuse or circuit breaker satisfymg

the already specified requirements.

In cases where it is not considered necessary to include transformer

winding/earth faults in the fault scenario and the power supply is not

referenced to earth (which is the case in some other countries), only one

protection device in the input circuit may be necessary as failure to earth of

a single supply conductor does not cause the same type of problem as it may

where the supply is referenced to earth. An evaluation of the possible fault

scenarios shows that, even if the fault combination envisaged within the

intrinsic safety concept does not include a situation of by-passing protection devices, the more remote possibility of this occurring is enough to

produce a recommendation that all non-earthed supply conductors have a

protective device, regardless of the type of transformer used and its supply.

13.10.2Damping windings

Damping windings are used to minimize the effects of inductance in

cases such as relays. They are effectively short circuit turns and reduce

the apparent inductance to the leakage inductance which represents the

inverse of the efficiency of coupling between them and the main winding.

If such windings are used to minimize inductance for the purposes of

intrinsic safety, they must be reliable so that they can be considered as

infallible against open circuit failure. The typical way to ensure this is to use

uninsulated copper tubes or wires which are continuously short circuited by

soldering or welding. Any winding used for this purpose needs to achieve

this level of security against open circuiting.

73.10.3Current limiting resistors

Current limiting resistors are typically used to limit short circuit current to a

hazardous area, to limit the current which can be fed to an inductor, to limit



Protection concept intrinsic safety ‘i’ 395



current to a component for temperature classification purposes, or to limit

discharge rates of otherwise ignition capable capacitors. These resistors need

to be infallible against failure to a lower resistance value but may increase in

resistance or become open circuit. In order to achieve this the resistors must

be of particular types of construction as follows. First, carbon or metal film

type, where the resistance element is effectively an insulated layer of pure

carbon or metal deposited on a substrate which is normally ceramic or similar

insulating material, helically etched to give the required resistance, and then

protected by insulating enamel or varnish to prevent external contact with

the element; second, wound in wire on a ceramic former or a former of

similar material and then consolidated by enamelling, varnishing, etc., to

prevent the wire from unwinding on breakage or being contacted from the

outside of the resistor; and third, printed resistors used in hybrid circuits

where the resistor is printed on an insulating substrate using conducting

inks, produced by metal deposition, or constructed in a similar manner and

then covered with an insulating conformal coating or encapsulated.

The commonly available carbon composition resistors are not suitable for

this use as experience has shown they can fail in a way which produces

lower resistance values due to the impurities introduced into the carbon in

their construction.

13.70.4 Blocking capacitors



Where capacitors are used to prevent the transmission of dc and are

required to be infallible it is possible to use only two capacitors in ’ia’

circuits to achieve this, provided the capacitors are of sufficient reliability.

This effectively excludes electrolytic and tantalum types and restricts the

choice of capacitor to those with solid dielectrics such as ceramic, paper,

polyester and similar materials. Where two capacitors of acceptable types

of construction are connected in series they are considered as infallible and

the effective capacitance will be that present if either of them fails to short

circuit. Thus a single capacitor failure is equated to two countable faults.

Apart from their construction type each of the two capacitors needs also to

be proof against the following situations.

First, each capacitor must withstand, without breakdown, a 50Hz rms

ac and dc voltage of 2 U + 1OOOV with a minimum of 1500V. The test is

applied as previously described for withstand tests (i.e., raising the voltage

steadily over 10 seconds and then maintaining it over 60 seconds) and also,

as previously, the maximum current which may flow is 5mA which must

be either falling or stable during the 60 seconds of the test. For the ac test

the current passing will, of course, be dependent on the reactance of the

capacitor but increases during the test are taken as a sign of failure.

Second, where the capacitors are connected between the intrinsically safe

circuit and earth, or separate intrinsically safe circuits, and failure to earth

could create an unsafe condition by, for example, by-passing safety components, the test voltage above should be reduced to 2 U with a minimum of

500 V.



396



Electrical installations in hazardous areas



In both of the above cases the mounting of the capacitors must be such as

to comply with Table 13.10 if the assembly is to be considered as infallible

against direct interconnection of the two circuits separated by the capacitors.

The voltage U, specified in the electric strength tests above, is the

maximum voltage which can appear across the capacitor combination in

service in the case of capacitor combinations.



13.10.5 Shunt safety assemblies



A shunt safety assembly is one which typically limits discharge of inductors

(using normally reverse biased diodes) or limits voltage across parts of

intrinsically safe circuits so as to ensure safety. Such limiting action must

be infallible and such assemblies are considered as infallible against open

circuit failure. The mounting and connection of such assemblies needs to be

such that open circuit of connections can be ignored (see Section 13.10.6).

Shunt safety assemblies are limited to semiconductor devices as it is

necessary to have confidence that the devices themselves have a low rate of

failure to open circuit and, almost uniquely, the mode of failure of a semiconductor junction is a short circuit or lower resistance as it results from

degradation of the junction in the semiconductor. This permits the number

of elements of the shunt protection to be reduced from three to two in ‘ia’

circuits.

The particular use of thyristors presents a problem due to their operating

time and resultant let-through energy. Where such devices are used as safety

shunts this let-through energy must not exceed the following limits:



IJA = 160pJ

IIB = 8 0 ~ J

IIC = 20pJ



Shunt safety assemblies which limit voltages within intrinsically safe

circuits



Shunt Safety Assemblies which limit voltage are those which are interposed between power supplies and intrinsically safe circuits, between other

circuits (e.g., output receptors) and intrinsically safe circuits, or used to

further limit voltages within an intrinsically safe circuit where the principal supply voltage has already been defined. In the two former cases the

external circuits must be assumed to contain transients unless their power

supplies are fed from the mains via an infallible transformer or are batteries.

Where the voltage limiter in question is not the prime voltage limiter (e.g.,

it has a further infallible voltage limiter between it and the external circuit

or power supply) it can also be assumed that transients do not occur.