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12V bus is a de facto standard in cars. Aviation has 28V DC bus as a de facto standard for most of the commerical jet aircrafts. Any particular reason behind the selection of 28V for the DC bus?

JYelton
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KawaiKx
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2 Answers2

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To supplement Brian's answer:

It's partly historic of course. Based on sources of power available in the early days - such as lead-acid batteries and on the needs of components in electrical devices - such as radio receivers/transmitters

Car batteries are usually labelled as 12V. And they consist of six cells using a lead-acid chemistry. The cells actually produce closer to 2.1 volts per cell I believe.

The charging system however produces something closer to 14 V. So whilst the car is running, the electrical power is closer to 14 V.

Aircraft basically have double this. I believe the batteries are 24 volt batteries and the charging system or electric generators produce 28 volts. These are described as 28V systems but car makers would perhaps have called them 24 volt systems.

I read that some aircraft (e.g. SAAB 340?) used 48 V starter motors, so two 24 V batteries were connected in series to produce this voltage. Note that when the engines are not running (and hence neither are generators presumably) you have 24/48 V from the batteries not 28/56 V.

RedGrittyBrick
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    To note, there are 24v car systems, especially RVs. – Passerby Jun 24 '13 at 21:04
  • Back in the days of the high-maintenance lead acid battery, cell voltage was more like 2.3, so at 6x giving 13.8V for the typical automotive battery. Two of those in series give 27.6V, which rounds off to 28. – JustJeff Jun 25 '13 at 01:31
  • @JustJeff: I'm moderately sure that 2.3V/cell is the safe charging level. 2.1V/cell is the open-circuit output of a fully charged cell. [Reference](http://www.progressivedyn.com/battery_basics.html). I think we're in agreement on where the 28V avionics standard originated. – RedGrittyBrick Jun 26 '13 at 14:34
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It's a nice comprise. Higher voltage than 12 V -> lower currents for same power -> smaller wires -> less weight. But still less than 50-60 V so "low voltage" for many safety regulations so less extensive and heavy insulation is required.

Why the specific number and not (say) 30 V or 48 V as used in telecom, I don't know.

Brian Carlton
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  • I feel the limitations on the battery size may also limit the max DC bus voltage. you see in case of emergency, battery has to supply the DC equipments. so they should be able to run on battery, which is about 28V. why battery size is 28V only? Battery is useful only in case of an emergency. otherwise, it is never used much in commercial aircrafts. it is considered a dangerous good due to its content. carriage of dangerous goods has a limitation, that would restrict the size of the aircraft battery too. – KawaiKx Jun 25 '13 at 09:38
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    @Saurabh, the "Carriage of dangerous goods rules" don't apply to the aircraft systems themselves --- passengers aren't allowed to bring jet fuel on board either, but the airline is still allowed to use jet fuel to power the plane. – The Photon Jun 25 '13 at 18:06
  • you are right indeed! didn't think from that angle. – KawaiKx Jun 26 '13 at 05:25
  • 50-60V is also [low voltage](https://en.wikipedia.org/wiki/Low_voltage) for safety regulations. In fact, anything below 1000V AC and 1500V DC is. – marcelm May 25 '16 at 16:25