The Telephone Intrinsically Safe No. 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. munitions factories).

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 No. 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 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 is contained within a protective casing. It converts the voice frequency, dialling, ringing and busy signals. from conventional to intrinsically safe energy levels.

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.

Important Note:-

Only the telephone instrument is electrically suitable for installation within the hazard zone.

The converter and power units are not intrinsically safe and must therefore be sited in a safe zone.

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 following:-

  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
Siemens House
Windmill Road
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.

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Last revised: March 13, 2023