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40 Handbook of Intercom Systems Engineering
System Current/System Capacitances/Loading
The system currents are determined by several parameters:
1 The current required to supply standby current for each user station.
2 The current required to supply the dynamic current to generate line signal, headphone
signals, speaker signals and call lamp signals.
3 The current required to start up a system (inrush current) by charging up to (50) 4000
microfarad capacitors or 0.2 farad.
4 The current limit imposed by the power supply to protect itself.
5 The secondary current limit imposed by the power supply when a fault is close to the
power supply (little or no circuit resistance). This limit, called the foldback current,
further protects the power handling electronic devices in the supply and determines the
system start-up time.
Currents 1 and 2 can be calculated by multiplying the number of user stations times the
user station current data in the Complete User Station Specifications. Current 3 is usually
limited by current 5. Currents 4 and 5 are listed in the Power Supply Specifications.
Current 5 can be used to calculate the system start-up time: where:
T is the start-up time (approximated) in seconds.
N is the number of stations.
C is the capacitance per station = 4 millifarads
i is the power supply foldback current
dV is a change in voltage across the capacitors, say 10 volts.
For a 20-station system, a 1 ampere foldback current, and a 10 volt change on the
capacitors:
The actual system start-up time will be longer since voltages in each user station have to
stabilize before audio can be transmitted. This time is on the order of several seconds.
Temperature Range Consideration
All of the elements of the TW Intercom System have been designed to operate over the
temperature range of 0 degrees Celsius (32 degrees Fahrenheit) to 50 degrees Celsius (122
degrees Fahrenheit). The high temperature range is extended another 15 degrees Celsius if
the units are not operating at full capacity or some other worst-case condition. The low
temperature range is extended another estimated 20 degrees Celsius if the full system gain
range is not required. The major operating problem at lower temperatures will be the dew
point and the resultant condensation. If this is the typical operating environment, then it is
recommended that the equipment be opened, cleaned, dried and sprayed with several light
coats of plastic spray. This will lessen the noises generated by leakage currents that occur
when the moisture and any dirt or film combine. Cleaning can be accomplished a rinse of
alcohol, a very mild detergent (saponifier type) wash and 2 or 3 thorough rinses with
distilled water. This routine is to first wash off the nonpolar soluble substances, then the
polar soluble substances.
Cooling Requirements
In general, only the power supplies require cooling consideration. Normally, leaving 2
inches clearance above and below the rack-mounted supplies is adequate. Portable
supplies should not be left in the sun and these supplies should have clearance of 6 inches
from five of the six surfaces. All other elements of the TW Intercom System require no
special consideration. It is important to note that belt packs and other equipment left in the
sun can cause burns to human flesh, due to the large amount of heat transfer possible. The