Intercom and PA Systems

How a PA/GA system works

Central amplifiers drive constant-voltage loudspeaker lines, usually 100 V, divided into zones. Each area is fed by two interleaved loops, A and B, from separate amplifiers, so losing one cable or amplifier leaves half the speakers working. The system continuously supervises lines and amplifiers and raises a fault alarm on an open circuit, short or earth fault. The fire and gas system triggers alarm tones automatically, and access panels on the bridge and in the control rooms carry announcements. A priority order puts the general alarm above emergency speech and routine paging. High-noise areas such as the engine room and shaker house get flashing beacons as well.

Intercom systems range from talkback stations on a dedicated exchange to IP stations on the unit's network. Sound-powered telephones need no power at all and often link the bridge, steering gear and engine control room.

Analogue and IP designs

Analogue systems route audio from central racks over speaker lines and are simple to fault-find. IP systems send audio and control over Ethernet, expand by adding nodes and often merge PA, intercom and telephony. Both work only with parts from the same system family.

Key specifications

Standards and what a used system must meet

SOLAS chapter III regulation 6 requires a general emergency alarm, and section 7.2 of the LSA Code sets the performance of that alarm and of the public address system. The alarm must reach at least 80 dB(A) and 10 dB(A) above ambient noise in the spaces it covers. The IMO Code on Alarms and Indicators sets alarm tones and priorities. The 2009 MODU Code adds general alarm and internal communication provisions for drilling units. Class societies type approve PA/GA equipment and approve the installed design, including loops, power supplies and the cause and effect with the fire and gas system. Speakers, beacons and stations in hazardous areas need IEC 60079 certification.

That design approval is the key point for a used purchase. Adding approved spares of the same model and firmware to an approved system is simple. A complete used system on a different unit needs new drawings, sound calculations and class approval.

Failure modes and inspection

• Horn speakers on open deck corrode and their diaphragms fail.
• Cable insulation degrades and earth faults appear.
• Amplifiers fail, often in the output stage or power supply.
• Ex cable glands and housings corrode, which voids the rating.
• Backup batteries lose capacity.

A survey runs a sound pressure level check in each space against the required levels, measures line impedance and insulation resistance, simulates line and amplifier faults to prove supervision, tests the fire and gas cause and effect, and checks battery autonomy.

Documentation

• Class type-approval certificate and firmware list.
• Line diagrams, zone list and cause and effect matrix.
• Last sound pressure level survey.
• Ex certificates for hazardous-area devices.
• Configuration backup for IP systems and exchanges.

Integration with your unit

Check that new speakers match your line voltage and that added load stays within amplifier margin. Interfaces to the fire and gas system, radios and the telephone exchange must use the same signals and audio levels. IP systems need their own network segment and power over Ethernet budget. Power must come from the emergency switchboard with battery backup.

Refurbishment, used or surplus

Used amplifiers, cards, speakers and stations make sound spares for an approved system of the same make and generation. Unused surplus Ex speakers and stations are a good buy. A full used system makes sense where you run the same system on a sister unit and can reuse its design approval.

Related equipment

Radio traffic patches in from VHF and UHF radio systems, and IP systems ride on data communication networks. Naventr can request the Seller's line diagrams and type-approval certificate on your behalf.