9.8 Surge protection for public address systems
(PA systems)
Public address systems are applied as compact
devices with standardised performance characteristics as well as in 19 “ modular design. They are
used for voice, music and signal transmission. For
this the wanted signal is modulated to a carrier
voltage (50, 70, 100 V) and transmitted to the
loudspeakers.
This transmitter transforms the low impedance of
the loudspeaker to a higher value, thus reducing
the signal current. This allows also telecommunication cables (0.6 or 0.8 mm diameter) to be used.
Most different kinds of loudspeakers are used. The
nominal power in the range of fitting or surfacemounted loudspeakers being at approx. 6 – 30 W,
of loudspeaker columns at approx. 20 – 100 W and
of horns at approx. 10 – 60 W. The lowest nominal
power of amplifiers in modular design is about
100 W going up to 600 W and more.
In a line or group, loudspeakers of different power
can be used in common. The minimum power of
the amplifier is equal to the sum of power of the
individual loudspeakers. Not the sum of the loudspeakers‘ nominal power is decisive for the addition, but rather the sum of power levels actually
chosen at the tappings of the transmitters.
Designing of the conductor system of a public
address system is subject to EN 50174-2.
7
loudspeaker 100 V
power amplifier
relay module 100 V
EBB
7
2
loudspeaker 100 V
EBB
3
3
DCF 77 antenna
CD Player
EBB
4
4
8
230 V supply
Tuner
EBB
coax
PC terminal RS 232
1
5
75 Ω
5
EBB
6
central unit with
input slots
intercom with
control and
selection keys
230 V supply
EBB
1 DGA FF TV, Part No. 909 703
3 DGA G BNC, Part No. 929 042
6 DR M 2 P 255, Part No. 953 200
2 DR M 2P 150, Part No. 953 204
(currents > 1 A – 25 A)
or
BCT MLC BE 110, Part No. 919 327
BCT BAS, Part No. 919 506
(currents < 1A)
4 FS 9E HS 12, Part No. 924 019
7 DCO RK ME 110, Part No. 919 923
AD DCO RK GE, Part No. 919 979
(currents < 0.5 A)
5 BXT ML4 BD HF 5, Part No. 920 371
BXT BAS, Part No. 920 300
8 S PRO, Part No. 909 821
Fig. 9.8.1 Public address system in modular design with surge protective devices
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In the following presentation we do not refer to
any further regulations which might be applicable
(e.g. building regulation, public address emergency systems, danger alarm in case of fire and
raid, etc.).
Major PA systems have a 19 “ modular design (Figure 9.8.1) and are in the vicinity of a permanently
occupied working place.
1
EBB
1 BCT MLC BE 110, Part No. 919 327
BCT BAS, Part No. 919 506
(currents < 1 A)
Fig. 9.8.2 Building without external lightning protection and horn in
LPZ 0A protected by combined lightning current and surge
arresters
protective angle
Dimensioning of the surge arresters listed under
and
requires to determine the maximum
current in the corresponding conductor branch.
This is done by the relation I = P/U, with U being
the carrier voltage and P the power of the amplifier to dimension the surge arrester
and the
power of the loudspeaker to dimension the surge
arrester
. In case of several loudspeakers in close
spatial vicinity, P is the sum of the individual loudspeaker’s power.
It is recommended to connect all earthing terminals of the surge arresters
to
with an adjacent equipotential bonding point (mini EB bar).
1
EBB
1 DCO RK ME 110, Part No. 919 923
AD DCO RK GE, Part No. 919 979
(currents < 0,5 A)
Fig. 9.8.3 Building with external lightning protection and horn in
LPZ 0B protected by surge arresters
In the quoted EN standard, surge protection is
described in the installation guidelines under
6.11.3. Beside the mentioned protection of the
conductors, the main focus, namely the protection
of the devices installed in the conductor system, is
pointed out.
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Therefore the existing interconnecting line to the
PC or the site of the intercom is determining for
the use of the surge arresters given under
and
. A protection of the line usually is
required if the distance is > 5 m.
If loudspeakers are positioned in lightning protection zone LPZ 0A (area with hazard of direct lightning strike) of buildings without external lightning
protection system, combined lightning current and
surge arresters have to be installed (Figure 9.8.2). If
here only a lightning current arrester is installed,
the loudspeakers in the building installed in this
track can be damaged.
If loudspeakers are positioned in lightning protection zone LPZ 0B (area without hazard of lightning
strike) of buildings with external lightning protection system, surge arresters have to be installed at
the entrance of the building (Figure 9.8.3).
LIGHTNING PROTECTION GUIDE 263