in 1957 the Telephone No. 702 is a table type instrument with the same physical appearance as the Telephone No. 266, and is suitable for lines of up to
1000 ohm loop resistance. The handset is intrinsically safe, as d.c. does not flow in the handset cord
(Ericsson N 9622). Both the transmitter and receiver are transformer coupled to the appropriate section of the telephone circuit. A rocking-armature receiver is used in this telephone as a transmitter, transmission taking place through a two stage transistor amplifier. A Bell No. 69B must be used with this telephone. For maintenance instructions see Z 5013. Telephone No. 702 supersedes the Telephone No. 266.
Finished in Hammer Grey with a black moulded handset.
Supplied by Ericsson, their Model N1881X1.
Diagram - N802.
Bell No. 69B
The Bell No. 69B is similar to the Bell No. 69A but is the superseding item, and is used with the Telephone No. 702. For maintenance instructions see Z5013. Bells No. 69A and 69B must demounted with the hinge at the left hand side and should be mounted under cover.
Relay unit CD 1982
This flameproof enclosed relay (ordered separately) is required to provide the switching facilities for mains-operated flameproof enclosed apparatus such as
loud sounding bells, hooters and visual signalling devices; it should be connected in circuit as for an extension bell. The terminal chamber is provided with two 0.75in conduit side entries. Overall dimensions of the unit are 10in x 9.5in x 4in, the weight is 27 lb.
The Post Office Electrical Engineers Journal
Vol 50, Part 4 (1958)
Flameproof Telephones and the
Development of a Table Model
By J. L. BELK, Associate I.E.E. and E. WOODWARD
This article outlines the methods of ensuring that
telephones used in situations where dangerous gases and vapours are present
do not cause explosions, and refers to the arrangements for certification of
the degree of safety provided by specific items of telephone apparatus.
The article then refers briefly to existing Post Office flameproof
telephones and continues with descriptions of two versions of a newly
developed table model flameproof telephone.
Although the telephone works at such low power that its safety in everyday
use is never in question, there are circumstances in which great care must
be taken to ensure that a spark at a contact or at an accidental
disconnexion cannot cause an explosion if dangerous gases or vapours should
Oil refineries and chemical plants are perhaps the two main places which
spring to mind when considering where explosive gases might occur, and with
the increasing consumption of both oil products and synthetic materials, and
the consequent increase in the number and size of the industrial plants
concerned, the safety of life and plant from the dangers of explosion is
worthy of all the care and attention that is given to the design and
manufacture, and the installation and maintenance of special equipment.
Telephone apparatus for use in mines, which is not fitted by the Post
Office, is outside the scope of this article. The design, installation
and maintenance of all telephone and signalling apparatus for use in mines
is governed by the Mines and Quarries Act 1954, and the specific
requirements are given in Statutory Rules and Orders No. 797 and No. 1407:
Methods of Providing Safety
Although alternative methods are occasionally used in heavier forms of
electrical engineering, in telephone engineering the two principal methods
of providing safety utilize either a circuit that is safe because it cannot
cause a dangerous spark (intrinsic safety), or a mechanical construction
that will withstand an internal explosion and will not transmit flame
outwards to an external explosive atmosphere (flameproof construction).
These two methods, which are described in more detail in the following
sections, are the subject of practical tests which are imposed on actual
items of equipment by the Safety in Mines Research Establishment at
Sheffield and Buxton. On the results of these tests, a Flameproof
Certificate is issued by the Ministry of Power, or a Certificate of
Intrinsic Safety is issued by H.M. Chief Inspector of Factories, Ministry of
Labour and National Service, or the Ministry of Power, depending on whether
the equipment is for use in industry or coal mines. The equipment is
type tested, i.e. the item that is tested is regarded as fully
representative of the manufacturer's normal production and the certificate
covers this normal production. Any subsequent change to the drawing
mentioned on the certificate requires further certification.
There are cases, however, where it is not practical to use either of these
two methods, and in such cases safety can be ensured as far as possible by
the method and standard of construction. The Factory Inspectorate of
the Ministry of Labour and National Service again issue a certificate after
consultation with the Safety in Mines Research Establishment. An
example of this type of certification is described later when dealing with
the development of the flameproof table telephone (1,000-ohm version).
It has not yet been practicable to make a telephone handset and cord
flameproof according to the requirements of B.S.229 "Flameproof Enclosure of
Electrical Apparatus" and hence, although telephones are commonly described
as flameproof, usually only certain parts have been certified as such.
Consequently it should be realized that in these instances the term
“flameproof" is not strictly correct as a description of the complete
telephone, but it is used as a convenient indication of the type of
installation for which the telephone is intended.
Protection by the method of intrinsic safety is achieved by using a safe
source of supply, either primary cells whose open circuit voltage does not
exceed 24V, or a special transformer of 15V r.m.s., or in the case of a
magneto telephone, by using an approved generator. All these sources
of supply have limited short-circuit currents, the limitation usually being
obtained by using series resistors or in the case of the generator by the
inherently high impedance of the windings in conjunction with a
non-inductive shunt resistor. Furthermore, the supply must not be
earthed. Together with the limiting of the source of supply, it is
usual to fit protective devices in the form of non-inductive resistors or
metal rectifiers across the inductive components of the circuit to by-pass
the inductive energy that would normally appear as a spark at switching
points during the operation of the circuit. By these means any sparks,
which might appear at switching points or at any point of fault, are of such
low intensity that they are incapable of igniting the gas or vapour which
might be present. Intrinsic safety is the statutory method used for
signalling and for telephones in coal mines (Mines and Quarries Act 1954).
The main reason for this is the need for the telephone system to keep pace
with the moving coal face without undue attention to the standard of
With protection in the form of flameproof apparatus no steps are taken to
prevent the sparking that would normally occur during operation of the
circuit or which might occur under fault conditions. The protection is
obtained by enclosing the components in a robust metal enclosure, defined in
B.S.229 : 1957 as:-
“A flameproof enclosure for electrical apparatus is one that will withstand,
without injury, any explosion of the prescribed flammable (The term
“flammable,” now used by the British Standards Institution, is synonymous
with the more common term “inflammable”) gas that may occur within it under
practical conditions of operation within the rating of the apparatus (and
recognized overloads, if any, associated therewith), and will prevent the
transmission of flame such as will ignite the prescribed flammable gas which
may be present in the surrounding atmosphere."
The design of such an enclosure is based on the following data. All
joints in a flameproof enclosure must be either flanged joints, spigoted
joints or screwed joints without the intervention of any loose or perishable
packing. The width of these joints and the maximum permissible gap are
related to the groups of gases and vapours listed in B.S.229. The
flange width in general should be at least 1 in. with a maximum permissible
gap of 0.016 in., although where the gap can be more accurately controlled,
widths of 0.5in. are now acceptable. The maximum gap related to the
0.5in. flange is 0.006 in. for the gases encountered in the petroleum and
The tests to ensure that the maximum permissible gap is not exceeded are
made by checking that each part of the mating surfaces of the flanges does
not vary from a true plane by more than half the stated figure, i.e. for
telephone equipment covered by this article, the maximum variation is not
greater than 0.003 in.
Where an operating rod or spindle passes through the wall of a flameproof
enclosure, it must be of metal and the hole through which it passes must be
such that the effective length of the flame-path is not less than 1 in.,
fitting as closely as operating conditions permit. In no case must the
diametral clearances of the flame-path exceed 0.016 in. and 0.008 in. for
gases in Groups II and III respectively.
For the purposes of flameproof certification, gases are divided into four
groups as follows.
Group I covers requirements for coal mines.
Group II covers, in general, the petroleum, chemical and paint industries.
Prior to 1957, this group was divided into five sub-groups a , b, c, d and e
according to the types of gases met in particular classes of industry.
Group III, divided into two sub-groups, covers ( a ) the more sensitive
vapours of the petroleum industry and ( b) coal gas and coke oven gas.
Group IV. The gases (including hydrogen and acetylene) in this group demand
a maximum permissible gap which is too small to be practicable to justify
certification on the basis of type tests. It is possible to produce
apparatus which will withstand a hydrogen test but it is doubtful whether
the same standards of construction could reasonably be expected in
commercial production. Approval for individual apparatus for use with
gases in this group is usually given in the form of a Test Report issued by
the Ministry of Power.
When apparatus is certified, the certificate quotes the groups of gases for
which the apparatus is considered suitable. The apparatus should not
be used in gases of other groups even though the test conditions appear to
be the same. If the apparatus were suitable for use in other gases,
quite naturally the manufacturer would ensure adequate certification to
increase his sales.
The onus for prescribing the groups of gases which might be encountered in
any telephone installation rests with the subscriber. Care should then
be taken that any apparatus which is used is certified for those groups.
Existing Flameproof Apparatus
Because of the rigid mechanical requirements, flameproof apparatus has
always tended to be strictly functional, and in general the lack of
aesthetic appearance has not been questioned.
The Post Office Telephone No. 149, has been available
for use on automatic and C.B. systems for many years. A similar
instrument, the Telephone No. 153, has also been
available for magneto working. These telephones will continue to be
available for installations where the newly developed table telephone cannot
be used. They are wall-mounted instruments and are weatherproof.
Certificates of flameproof construction which they carry (Groups II and III
for the Telephone No. 149 and Groups I and II for the Telephone No. 153)
relate only to the enclosure of the more dangerous elements of the circuit
such as the dial and the switch-hook contacts.
Development of a Flameproof Table Telephone
The remarkable developments by the oil industry in the
production, refining and distribution techniques of petroleum products have
created an increased demand for telephone equipment. It is now
commonplace to have remote control rooms in which are housed metering and
recording equipment together with the control desks. Telephones are
essential features in these control rooms and, apart from their flameproof
properties, it was considered desirable that they should be similar to the
conventional table telephones if the appearance and function of the control
rooms were to be maintained. Consequently the development of a table
model was put in hand, and it was soon apparent that the design would have
to be based on the following lines:-
(a) The bell should be housed in a separate enclosure due to the difficulty
of finding a suitable arrangement for the bell gongs. The gongs could
not be mounted on the outside of the telephone case if appearance were to be
preserved, and they could not be mounted inside, for there they would not be
(b) The line connexion to the instrument should be via a flexible cable to
permit the telephone to be moved to suit the convenience of the user.
These two considerations led to the design of a unit in which is housed the
bell and also the terminal block for the connexion of the rigid permanent
The bell coils are mounted inside the case and their cores are carried
through the case to actuate the bell-hammers. The bell and terminal
unit can be fitted in a convenient place near the telephone, and connected
to it by a tough rubber-sheathed cable, which is a permitted cable in areas
when only an occasional hazard may exist. Being a separate unit, the
bell and terminal unit can also be mounted as an extension bell if desired.
A range of glands has been developed to enable conduit or any of the
accepted forms of permanent cabling for hazardous areas to be connected
easily and safely to the bell and terminal unit.
The case of the telephone instrument is constructed throughout in aluminium
alloy LM6, the use of which not only permits a great saving in weight,
but, more important, because of its low magnesium content, precludes the
possibility of frictional sparking which might occur with other materials if
the instrument were accidentally dropped.
The case consists of two compartments: (a) the main enclosure in which are
housed the circuit components and (6) the terminal chamber. This
segregation is normal practice with all flameproof electrical apparatus to
ensure that the main enclosure is not disturbed when connexion or
disconnexion of the line wires is required.
The cover of the main compartment serves as the base of the instrument and
is fitted with four rubber feet. It is secured to the body by six
triangular-headed screws, which are recessed to discourage unauthorized
access. A warning notice is embossed on both covers to the effect that
the circuit must be isolated elsewhere before either cover is removed.
The dial is of the latest 'Trigger” type, and the mechanism can be detached
from the instrument case by removing three screws located round the rim of
the dial case. Owing to the extra friction due to the flame-path along
the operating spindle, it is necessary to return the finger plate back to
normal independently of the dial mechanism, so enabling the dial mechanism
to operate unhindered. In this method of operation the finger plate
flies back under the action of its own spring and is locked in the rest
position until the dial mechanism has returned to rest. Otherwise the
dial would be very prone to mis-operation. Provision is also incorporated in
the dial to lock the mechanism for C.B. working.
The remaining elements of the circuit within the case are mounted on a
chassis that can easily be removed for attention.
It was evident during the development of the table instrument that it would
be difficult and impractical to make the handset and cord of flameproof
construction. Hence it was decided to try to make the transmission
circuit intrinsically safe by limiting the maximum short-circuit current at
the microphone to be less than the minimum igniting current as ascertained
from test conditions. Tests using pentane as the representative vapour were
conducted by the testing staff of the Safety in Mines
Research Establishment and were made with the standard 600-ohm circuit
telephone connected to standard Strowger circuits. It was found that
under loop conditions ignition could be obtained with zero line and with
both a.c. ringing current and d.c. transmitter feed current causing a spark
in the microphone circuit with the microphone short circuited. Such a
condition could arise with a sticking F relay in the final selector at the
exchange and, simultaneously, an instrument fault. The chances of such
a condition are extremely remote, but when safety is concerned such are the
conditions of test. It was found possible to reduce the intensity of the
spark to a safe level under these conditions by inserting a 160-ohm resistor
in the circuit. Although this resistor reduced transmission by about 2
db in each direction and reduced the line signalling limits by 160 ohms, the
losses were regarded as unavoidable in the interests of safety.
Development of the Flameproof Table Telephone
With the advent of the 1,000-ohm loop telephone circuit it
was decided that any new flameproof telephone should take advantage of this
latest development if it were to have a useful life without modification.
Further, while the tests on the 600-ohm version had proved the handset
circuit to be safe under the conditions of test, no proof was available that
the circuit would still be intrinsically safe if the instrument were to be
connected to any of the many earlier and different systems used by the Post
Office. A very involved testing program would have been necessary to
check the safety of the circuit in practical tests in conjunction with all
these circuits and circuit elements and, moreover, possible earth fault
conditions were an additional point of doubt. The new table telephone
(600-ohm version) was, however, an improvement on the existing instruments.
Because of all these points it was decided to utilize the 1,000-ohm loop
circuit and in consultation with the Safety in Mines Research Establishment
a different approach was made to the problem of the safety of the handset
As mentioned earlier, an accepted method of providing an adequate degree of
safety under practical working conditions is by ensuring a suitable design
and standard of construction.
The scheme adopted in this instance was:-
(a) To fill the free space where pockets of gas or vapour could possibly
collect in the Handset No. 1, which is used in conjunction with the
1,000-ohm circuit. This was done by fitting a moulded rubber insert at
the back of the transmitter, and fitting rubber plugs into the ends of the
hollow handle to seal the space around the wires. No filling was
required for the receiver end of the handset because the presence of the
lead weight was considered sufficient.
(b) To secure the receiver and mouthpiece caps to prevent unauthorized
entry. The caps on the Handset No. 1 are both of the screw-on type and
it was decided that the use of Allen screws to secure the cups would give
adequate safeguard against indiscriminate removal.
(c) To provide some form of metallic braid to a robust tough-rubber sheathed
flexible handset cord which should be firmly secured at each end. The
type of cord which met the approval of the testing authority is "Stranded
55/.004 in., 4-core, V.I.R. 0.02-in. radial, neoprene sheathed 0.04 in.
nominal, braided with 0.006 in. tinned copper wire and covered overall with
This telephone with the 1,000-ohm circuit has now received its certificates
as follows, the bell and terminal unit and telephone case remaining as for
the 600-ohm version.
Complete Telephone . . . Factory Inspectorate Certificate No. 202
Telephone Case . . . . . . .Flameproof Certificate No. F.L.P. 3651
Bell and Terminal Unit . . Flameproof Certificate No. F.L.P. 3652
These certificates were based on B.S. 229 : 1946 and cover groups 11(a) and
Installation and Maintenance
With all the care and attention taken in the design and certification of
these special telephones, it is obvious that they should be installed
carefully and maintained in good condition. The precaution of
disconnecting a circuit at a point outside the area of risk or at a
certified switching point should always be taken before opening flameproof
apparatus in an area of risk. When covers are replaced, the flanges
should always be wiped clean to ensure that the joint gap is not widened by
any trapped dirt or foreign objects. Any repairs or replacements must
be in conformity with the drawings specified on the safety certificates,
and, because of this, only minor replacements are permitted in the field.
The ultimate success of the standard of safety provided by the apparatus
depends on the standards of installation and maintenance.
The foregoing is but an outline of the certification of telephones for use
in locations where there might be an occasional danger from explosive gases
or vapours. Present day knowledge of the behaviour of explosive
mixtures is the result of years of research and the statistical evaluation
of very many practical tests, and its application to telephones is only part
of the work of the Safety in Mines Research Establishment, where all types
of industrial equipment are dealt with.
In addition to gases and vapours, there are many dusts which ignite easily
and explode with considerable force. These dusts, many of which are of
such common materials as cocoa, flour and sawdust, demand precautions which
are, if anything, more exacting than the flameproof construction that has
been described. In addition to the flameproof construction, it is
necessary to make the equipment completely dustproof. The use of
flameproof equipment for protection in dusty atmospheres is not therefore
justified and the publication of a British Standard on this subject
is awaited with interest.
The authors wish to express their appreciation of the co-operation that they
received in the testing and certifying of the new telephones from the Safety
in Mines Research Establishment and the Inspectorate of Factories.
They also wish to record that the request for a flameproof table telephone
came from the Oil Companies Materials Committee.
The Post Office Electrical Engineers Journal
Vol 54, Part 4 (1962)
A New Flameproof Table Telephone with an
Intrinsically Safe Handset
THE development of a flameproof telephone poses the problem of the safety of the handset and cord, which cannot be of flameproof construction like the telephone case. A new flameproof table telephone has been developed, in which the difficulty has been overcome by making the handset and cord intrinsically safe.
The present flameproof table telephone (Telephone No. 266) was introduced a few years ago to meet the requirements of many industries and in particular the oil industry. In the design of the Mark
II version of the Telephone No. 266 a degree of safety was achieved by adopting a suitable design and standard of construction for the handset and cord without modification to the circuit design. In the interests of further safety, however, the preferred arrangement is an intrinsically safe handset with the line current excluded from the transmitter and cord, thus eliminating the possibility of fire or explosion caused by a spark should the cord be severed or the transmitter connexions become faulty.
For flameproof equipment the components are housed in a specially constructed flameproof enclosure (telephone case in this instance) which will withstand an internal explosion and prevent the transmission from within of a flame that might ignite any prescribed inflammable gas or vapour in the surrounding atmosphere.
Intrinsic safety, which is achieved by circuit design and the choice of components, implies that any sparking that might occur either during normal working or under specified fault conditions is incapable of causing an explosion of the inflammable gas or vapour. Such a handset has been made possible in the development of a new flameproof table telephone (Telephone No. 702) by the use of a rocking-armature receiver as a dynamic microphone and a transistor amplifier associated with a telephone circuit of conventional design and suitable for use on lines of up to 1,000 ohms loop resistance. A standard handset cord carrying speech currents only is used.
A 2-stage amplifier is housed within the flameproof telephone case and is mounted together with the normal telephone components on an easily removable chassis.
Speech currents from the microphone are fed to the input of the first amplifier stage via transformer T3, which both
matches the impedance of the microphone to the base circuit of transistor VT1 and isolates the microphone from the telephone circuit. The first stage is
resistance capacitance coupled to the second stage, VT2, which has an output transformer,
T1, in its collector circuit. Amplified speech signals are fed into a standard
telephone circuit at a point normally occupied by the transmitter. The microphone-amplifier combination provides an output to line approximately equal to that of a
Transmitter Inset No. 13 in the same circuit. The instrument line current, passing through the
68 ohm resistor, provides the operating potential for the amplifier, to which it is applied through a rectifier bridge, thereby ensuring correct polarity with either direction of line current. The transmission loss due to this resistance is minimized by a 20
uF tantalum-capacitor shunt.
As an additional safety precaution the handset receiver is connected to the telephone circuit through a 1:1 isolating transformer, T2.
With flameproof table telephones it is necessary to have a separate bell set which also accommodates the terminals for the line wires. A new bell set (No. 69B) has been introduced for use with the new telephone.
Certification by the Ministry of Power for both Telephone No. 702 and Bell Set No. 69B covers Groups II and
IIIa of B.S.229 : 1957, Flameproof Enclosure of Electrical Apparatus.
Taken from The Post Office Electrical Engineers Journal
Vol 54, Part 4.
U.D.C. 621.395.721.4 614.83