STANDARDS & FORMULAS

Public Address

Public address (or voice alarm): zones, numbering, the 100 V line check and the schematic.

Standards and regulations

BS 5839-8
Voice alarm systems - design, installation, commissioning and maintenance
Speaker spacing, A/B circuits and the voice alarm (PAVA) link to the fire alarm.
IEC 60228
Conductors of insulated cables
The copper core resistance (Ω/km at 20 °C) for the loop and 100 V line checks.
BS 8434-2
Fire-resisting cables for emergency circuits (enhanced)
The voice alarm’s cable note (PH120).

Formulas

Speaker current

I = P(tap) / V(line)

V(line) = 100 V (or 70 V)

Voltage at speaker k

Vk = V − Σ I(i) × (2 × r / 1000) × min(L(i), L(k))

r = the core’s resistance (Ω/km)
L = distance along the cable from the rack (m)

Level at the last speaker

ΔL = 20 × log10(Vk / V) ≤ 1 dB

Design rules it applies

Voice alarm (PAVA): A/B circuits in every zone, a monitored link to the fire alarm panel and the fire microphone
Taps by kind unless the speaker has its own: ceiling 6 W, wall 6 W, horn 30 W

For reference. MO.tool works these out to help your design; the engineer remains responsible for it. Always check the edition of each standard your project uses and the requirements of the local authority (DEWA, ADDC, Civil Defence…).

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