Telephone Intrinsically Safe 1A consists of a telephone unit, a dial pulse converter unit
and a mains powered power unit. All three units are wall mounted.
The telephone unit, when correctly installed with the dial pulse converter and power units,
is designed to meet the appropriate requirements of BS5501 for intrinsic safety. (See also
BS 5345 Part 4).
Intrinsic safety requirements ensure that the equipment shall not generate enough heat
energy nor cause enough electrical sparking to ignite specified explosive gas/air mixtures
that may surround it.
The TIS 1A is not designed for use where explosive dusts may be present (e.g., flour,
sugar, sawdust) nor is it for use where there is a risk of percussive explosions (e.g.
The telephone unit of the TIS 1A may be used in sheltered out-door locations but the dial
pulse converter and power unit are not designed for external use. These should be fitted
inside buildings or in weather proof enclosures.
The TIS 1A should be used for all new work or as a maintenance replacement for existing
'Flame Proof' telephones in locations where telephone service is required and explosive
atmospheres (as referred to in 'Precautions') may be present. See BS 5345 Part 4 Para 3
for definitions and explanation of terms.
(See BS 5345 Part 4. Especially Paras. 3,4,6,8,13,1 5 and 1 6).
The Telephone Intrinsically Safe 1A (TIS 1A) is primarily intended for use as a PBX
extension but can be used on a direct exchange line, if a local protective earth is
provided, it is acceptable to terminate this earth at the main EL & P earth bonding
point, or the PBX earth whichever is more convenient. When providing the earth for this
telephone one should always bear in mind the need for equipotential bonding and the
problems posed by earth leakage and current operated circuit breakers.
The telephone consists of three parts:-
1) Telephone Instrument
2) Dial Pulse Converter Unit
3) Power Supply Unit
THE TELEPHONE INSTRUMENT
The Telephone Instrument is housed in a blue coloured all metal, weatherproof enclosure. A
horn type tone ringer is bolted to the top of the unit, this has a sound output of
typically 90dB SPL (A-weighted). Care should be taken when testing the telephone to avoid
acoustic shock by not standing too close to the telephone when testing the horn. The grey
coloured heavy plastic handset which is attached to the telephone unit by means of a
lightweight black coloured P.V.C. cable is suspended from a side bracket, with associated
gravity switch, when it is not in use.
The PBX recall switch positioned on the opposite side of the instrument to the gravity
switch bracket, is used to provide an earthed loop recall signal when required.
THE DIAL PULSE CONVERTER
The Dial Pulse converter is contained within a protective casing. It converts the voice
frequency, dialling, ringing and busy signals. from conventional to intrinsically safe
THE POWER SUPPLY UNIT
The Power Supply Unit is also contained within a protective casing and is used to convert
the 240v mains supply to 27 volts for use at the converter. The unit contains a 5 hour
capacity battery (charged in situ from the mains) which enables incoming and outgoing
calls to be made during mains power failure.
Only the telephone instrument is electrically suitable for installation within the
The converter and power units are not intrinsically safe and must therefore be sited in a
Special precautions MUST be taken with cabling in both hazardous and safe zones.
The TIS 1A is available for use in industrial sites which by their nature may be
potentially hazardous. To ensure the safety of workers and to minimise the possibility of
workers being responsible for creating a dangerous incident, special safety procedures
have been established by site owners. These are known as 'Permit to Work procedures'.
Permit to Work Rules and Procedures are available from the site owner.
The telephone instrument is suitable for use in hazardous gas atmospheres that are
classified as HAZARD ZONE 1, and therefore as for ZONE 2. The TIS 1A is NOT suitable for
use in either Hazard Zone 0, or a DUST Hazard environment.
TYPES OF HAZARD AREA (See BS 5345 Part 4 Para 8)
A hazard area is an area within which a danger of fire or explosion from the ignition of a
gas/air mixture exists, or may exist.
Hazard areas are classified in three zones as follows:-
Zone 0: (HIGHEST RISK) In which an explosive gas/air mixture is continuously present or
present for long periods.
Zone 1: In which an explosive gas/air mixture is likely to occur in normal operation.
Zone 2: In which an explosive gas/air mixture is not likely to occur in normal operation,
and if it occurs it will exist only for a short time.
(BS 5345 Part 4 1977 Page 1 refers)
It is important to note that it is the site owners sole responsibility to identify and
advise on the hazard categories present on their site.
Before any installation work commences workers must be fully conversant with the
1) Details of site safety procedures.
2) Permit to Work or safety authorisation.
3) Current site plans identifying hazardous and safe zones. If plans cannot be released
for security reasons written confirmation of the site of the telephone, the cable/wiring
and the degree of hazard will be acceptable.
Maintenance on the Telephone Intrinsically Safe 1A is restricted to functional testing,
checking mains supply and cabling, or unit change out. These tests should be carried out
at least every 2 years, or earlier in particularly arduous environmental conditions with
special attention to BS 5345 Part 4 Para's 20, 20.1 to 20.1 0. Refer also to BS 5345 Part
4 Para's 23 to 27.
The Telephone Unit, Dial Pulse Converter Unit and Power Unit are not available as separate
items for replacement purposes, any unit proved faulty should be replaced by a unit from a
replacement TIS 1A. Remove the battery connection and insulate the lead on recovery.
Repair of faulty TIS 1A units out of warranty can be dealt with only by the manufacturer.
Messrs Siemens Ltd
Sunbury on Thames
Telephone Intrinsically Safe 1A (complete - comprising Telephone instrument, Dial Pulse
Converter and Power unit) - Item Code 411108.
NB: Individual units are not available separately.