AC versus DC amperes: Are they the same?

ILFEILFE Solar Expert Posts: 364 ✭✭
I am basically wondering if you can use AC circuit breakers if you do not have DC amp breakers available.

Can you use AC breakers in place of DC circuits in a solar system?

PV Panel Array > Circuit Breaker > Controller
Battery > Circuit Breaker > Controller
Controller > Circuit Breaker > Load

Also, can you use an AC Ammeter to read DC Amperes?
Paul

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: AC versus DC amperes: Are they the same?

    Short answer: no.

    Long answer:

    Technically an Amp is an Amp whether DC or AC. But the meters used to measure them are different because one is current flowing in one direction and the other is current flowing in either direction, changing rapidly (50 or 60 times per second).

    The characteristics of this make controlling DC current more difficult. AC "shuts off" with every cycle as the Voltage crosses zero when the current direction reverses. As such any type of contact rated for AC will not necessarily stand up to the same Voltage & current DC; the contacts may weld shut so that they can't be disconnected or fry apart so that they do not conduct.

    Some breakers are rated for both, but it is important to read the specs for the particular unit as the DC rating may be at lower Voltage and/or current.
  • BB.BB. Super Moderators, Administrators Posts: 29,523 admin
    Re: AC versus DC amperes: Are they the same?

    In general, the answer is no, you are not supposed to use AC breakers in a DC application.

    The reason is that DC current sustains and Arc very nicely vs AC current. If you look at switches (and some breakers) you will find that they have both AC and DC ratings and the DC ratings (maximum current/maximum voltage) are usually much lower than the AC ratings.

    In the US, we have some home AC panel breakers that are also rated for DC use (something like 48 VDC maximum vs the normal 120/240 VAC ratings)--But I don't know what you have available there.

    Fuses also have DC and AC ratings.

    Lastly, there is a value called the AIC rating (maximum Ampere Interrupting Current ratings--or something like that). Basically, that is the maximum current the fuse/breaker can interrupt with a dead short current through it. Turns out that a a medium sized DC battery bank can output thousands of amps into a dead short (heavy/short cable, etc.).

    This Youtube video is an example of an breaker for US house wiring switching switching off relatively high DC voltage (~290 Vmp array @ 12 amps)... and you can see that it breaks out in fire

    http://www.youtube.com/watch?v=HLX9cdB4TFQ

    This video (2nd half) shows what 300 amps at 12 volts can do to a nail/screw/bolt in a short circuit:

    http://www.youtube.com/watch?v=Avg5mJw0qHc

    So--What is the take away from all of this? More or less, arcing starts to be a problem around 12 VDC... And becomes a larger problem at higher voltages. Does this tell you that a 12 VDC battery is safe on an AC rated fuse/breaker--No. Does it mean that it is safer than a 290 VDC circuit on an AC fuse/breaker--probably.

    It sounds like you can get hardware by mail--So, if you look around you can probably find some decent DC rated fuses/breaker/switches. You may be able to find 24 volt rated devices for marine/large trucks too.

    Can an AC meter measure DC current... It depends. The "typical" AC meter cannot accuratly measure DC current unless it has a DC measurement setting.

    Some better meters can if they are "True RMS" (true Root Mean Square) type meters. Basically there is a micro processor inside the meter that measures the current (or voltage) waveform and calculates the RMS value of that wave form. You may have to experiment/check... Many meters simply use a capacitor to block DC voltage measurements--So those will only read AC values.

    Then there is connecting a meter to wire/cable to measure the current. With a standard meter, you have to cut the wire and put the meter in the circuit. For smaller current values (less than 10 amps for most general purpose digital multi-meters (DMM) and lower voltages--That is what is done.

    For higher current circuits, for higher voltage circuits, and where you do not want to cut the wire/insert the meter into the circuits, there are AC and DC current clamp type meters (and clam+DMM meters).

    For AC only reading meters, the clamp is part of a transformer that opens and clips around the wire you want to measure current in. Transformers only work with AC current--So they cannot measure DC current.

    Then there are AC+DC current clamp meters. Instead of a transformer, they use a Transistor sensitive to magnetic fields. The only draw back to these types of meters/sensors is that the transistors do drift over time and temperature and have to be rezero-ed every so often (every few minutes in some cases) for accurate DC current measurements (and there are True RMS version of these meters too).

    I really like using and suggesting people buy a DC/AC Current Clamp DMM--They are getting pretty cheap now for ones that are "good enough" for our needs (working on solar power systems). And they can measure 40/400 amp (or so) maximum current range--They are much more useful and safer than using typical current meters when you have to open the circuit to insert the meter.

    If you want to permanently measure DC current flow (such as with a Battery Monitor--A very nice instrument to have--[URL="http://www.victronenergy.com/battery-monitors/bmv-600s and bmv-602s/"]Victron[/URL] is another good brand from Europe), then you would use a precision power resistor with "Kelvin Contacts" (little screws on the side where you put a precision volt meter to measure voltage drop across the resistor). This eliminates the DC drift problem with DC clamp sensors.

    wind-sun_2269_13640931

    Deltec 500 amp, 50 millivolt current shunt

    Sorry for the long answer for a "simple" question.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ThomThom Solar Expert Posts: 189 ✭✭✭
    Re: AC versus DC amperes: Are they the same?

    They feel very different ! 120v dc to 120v ac.

    Thom
    Off grid since 1984. 430w of panel, 300w suresine , 4 gc batteries 12v system, Rogue mpt3024 charge controller , air breeze windmill, Mikita 2400w generator
  • westbranchwestbranch Solar Expert Posts: 5,142 ✭✭✭✭
    Re: AC versus DC amperes: Are they the same?

    If you have access to Square D QO breakers they are rated for up to 48V DC.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
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  • mike95490mike95490 Solar Expert Posts: 8,380 ✭✭✭✭✭
    Re: AC versus DC amperes: Are they the same?
    BB. wrote: »
    ..
    This Youtube video is an example of an breaker for US house wiring switching switching off relatively high DC voltage (~290 Vmp array @ 12 amps)... and you can see that it breaks out in fire

    http://www.youtube.com/watch?v=HLX9cdB4TFQ

    This video (2nd half) shows what 360 amps at 12 volts can do to a nail/screw/bolt in a short circuit:

    http://www.youtube.com/watch?v=Avg5mJw0qHc

    So--What is the take away from all of this? More or less, arcing starts to be a problem around 12 VDC.....
    -Bill

    Awesome videos thanks for the links
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