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The choice of operating voltage for a tramway is a compromise between two main factors:
1) The permissible power loss in the wiring system.
2) The safety factor of the insulation.
POWER LOSS
If you are not familiar with electrical calculations, read IMPORTANT NOTE first
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The voltage dropped is equal to the current flowing multiplied by the resistance of the circuit |
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The power is equal to the voltage dropped multiplied by the current flowing. |
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So power is equal to the current multiplied by the resistance, multiplied by the current |
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From 2. it can be seen that power could be supplied to a tramcar at high voltage and low current or at low voltage and high current or at any intermediate values of voltage and current which, when multiplied together, gave the required amount of power.
However, from 3. it can be seen that the power lost in wiring of any fixed resistance value will be proportional to the square of the current. In order to minimise losses in the distribution system it is better to use a low current, high voltage arrangement.
Low resistance wiring will also help minimise
losses - by increasing the cross-sectional area of a wire, its
resistance is reduced. There is a limit to how far this can be
carried before the value of the material of the wire (usually
a copper alloy) costs more in interest repayments than the value
of the energy saved over the same time period.
Other undesirable effects of increased cross-sectional area include
increased weight, necessitating a stronger and more expensive
support structure, and increased visibility which may give rise
to objections on æsthetic grounds.
SAFETY FACTOR
In the past, the insulating materials available were subject to deterioration under the influence of moisture, heat and high voltages. Modern materials are much better in these respects and often mechanically stronger or less brittle, although some can deteriorate under the influence of the ultra-violet component of sunlight.
Urban air quality has also improved and the heavy soot and chemical deposits which conducted electricity across the surface of otherwise good insulating materials in the past have now become a rarity, even in industrial areas.
These factors now allow the use of higher voltages than previously. The standard tramway voltage on pre-war systems was usually around 550volts; modern practice is to use 750 volts, giving a 45% reduction in transmission losses.
Both voltages could be lethal if touched, so the higher voltage represents very little increased risk.
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