DC Voltage Drop
- Thread starter A-1Sparky
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zog
Senior Member
- Location
- Charlotte, NC
DC Should be less with other factors being the same, no impedance.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
zog
Senior Member
- Location
- Charlotte, NC
Actually no inductive or capactive components of impedance, there is still resistance (Z=R+X).
You say tomatoe
I see your point, I should have said reactance instead of impedance.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
I can't help but wonder if there is any increased corrosion from galvanic reaction because of the DC.
SAC
Senior Member
- Location
- Massachusetts
I'd be quite surprised if there was any real difference between AC and DC for voltage drop in such an application (assuming 60Hz). The reason we use AC instead of DC for our power transmission and distribution system is because of the efficiency of which AC can be easily stepped up to a high voltage for transmission and distribution, and then stepped back down for utilization. While recent technology has made it more feasible to do the same with DC, in general, it is much less expensive to do with AC. I'd suggest there may be some other reason that the airport chose a DC lighting system.
zog
Senior Member
- Location
- Charlotte, NC
I am guessing an airport would use DC lighting so they can have a battery back up power the system in the event of a power loss, simple system for critical application with as few compoents as possible. I agree with SAC that I doubt the reason has anything to do with efficency or voltage drops.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
For smaller cables it isn't a lot different.
This from the cable tables in BS7671 - a British Standard for Electrical Installations sometimes referred to as "The Electrician's Bible".
Single core 35mm^2 is rated at 99A single-phase AC or DC.
Voltage drop on AC is 1.8mV/A/m
Voltage drop on DC is 1.75mV/A/m
The m is metres, not miles...:smile:
On larger cables, the difference can be appreciable. For 300mm^2, the voltage drop with AC is about double that of DC.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Well, you can't go on that. British electrons behave differently. Everyone knows that. :roll:
- Location
- Massachusetts
If I recall most airport runway lighting is wired in series supplied by a constant current regulator and each fixture has a shunt to bypass a dead fixture.
Aye, the are better behaved.
:grin:
broadgage
Senior Member
- Location
- London, England
As stated above, most airfield lighting systems run at constant current, with all the lamps in series and a shunt to bypass failed ones.
For a long circuit with numerous lamps this results in relativly high voltages, DC might be preferable in such cases since a given type of insulation can withstand a higher DC voltage than AC.
If a cable is rated for 1000 volts AC then the insulation must withstand about 1,410 volts since this is the peak voltage of a 1,000 volt RMS supply.
The same cable could therefore withstand, with the same safety margin, 1,400 volts DC. (though this is not allways reflected in either manufactuerers ratings or listing agencies)
Even if not useing cables at the maximum permitted voltage, DC might give a greater safety margin.
For a long circuit with numerous lamps this results in relativly high voltages, DC might be preferable in such cases since a given type of insulation can withstand a higher DC voltage than AC.
If a cable is rated for 1000 volts AC then the insulation must withstand about 1,410 volts since this is the peak voltage of a 1,000 volt RMS supply.
The same cable could therefore withstand, with the same safety margin, 1,400 volts DC. (though this is not allways reflected in either manufactuerers ratings or listing agencies)
Even if not useing cables at the maximum permitted voltage, DC might give a greater safety margin.
Thanks for all the replies, guys. I was told by a questionable source that DC was used in this application for its voltage drop advantage over AC. It sounded a little fishy to me, so I figured I'd run it by you guys. So basically, there really isn't much of an advantage in using DC as far voltage drop goes, unless rather large cables are used. Does that sound about right?
broadgage
Senior Member
- Location
- London, England
Yes you are right no, advantage at all in the case of small cables, and only a very slight advantage for larger cables (unless one is talking about cables much larger than likely to be used for airfield lighting)
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
Thats exactly the way it works.
The constant current is 6.6A, and the voltage across the loop can be up to 5KV either side of ground, so 10KV across the loop in total, for a theoretical maximum load of 66KW.
Because of the need to shunt lamps that have failed many modern systems use transformers for each lamp so if the lamp fails the transformer still keeps the ring intact. Older systems use a coin like thing that goes in the lampholder that ruptures when the lamp fails and then shunts the dead lamp.
The reason that the lamps are in series is that with a conventionally paralleled arrangement of lamps the current increases as you get nearer the source, and calculating voltage drop is an issue, and thus lamps would be at different brightnesses depending on where in the wiring scheme they were. So wire them all in series, use a constant current setup and voltage drop becomes an irelevance.
If only mall car parks were wired this way!
(Actually, street lighting once was, and I believe there is still a bit of LA that has series street lighting)
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