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Proper switches circuit breakers & fuses for a 60VDC system

Discussion in 'Power Electronics' started by Functional Artist, Nov 11, 2018.

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  1. Functional Artist

    Functional Artist

    Jul 21, 2018
    I am working on a go kart that will have a 60V propulsion system.

    I just found/bought a Boma 60V 2,000W brushless motor & controller combo from Alfa Wheels. ($250.00)

    I have worked with 12V, 24V, 36V & 48V systems, but 60V is new to me.

    I have used (12V) automotive fuses many times without problems, but to use them on a 60V seems to be "pushing it".

    I have also used marine style circuit breakers, but they are usually rated @ 48V max.

    The current specs. on this 60V controller says, 35A.

    What is the Rule of thumb or guidelines on the proper & safe use of fuses & circuit breakers? tips? or suggestions?
    SAM_2854 (2).JPG
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    No rule of thumb, a necessity: All components used must be rated to at least the max. expected currents and/or voltages. A good design leaves a safety margin on top of that, e.g. >= 10 %.
    I assume you use a battery. Note that a battery with a nominal voltage of 60 V can go much higher during charging. The whole electrical system must be fit to support that overvoltage.

    Also note that ratings for AC and DC often vary considerably. This is due to the different behavior of AC and DC in case of a failure. Most notably this is important for switsches, relays and fuses, everywhere where a mechanical contact is involved. For an example see this relay datasheet. This relay can switch 8 A at 250 V (AC), but only 0.2 A at 250 V (DC).
  3. kellys_eye


    Jun 25, 2010
    Simple fuses will work and automotive HRC fuses (as used in high power amplifier fittings in cars).

    Voltages up to 60V DC are considered 'ELV' and any equipment specified for use with ELV will work.

    Since all you need to do is 'protect' rather than 'control' isn't the on/off function provided by the control electronics?

    If not, and you want to use a physical switch, look into the type used in fork lift equipment.
  4. Functional Artist

    Functional Artist

    Jul 21, 2018
    Thanks for the replies.
    Yes, Harold my kart will use 12V 12AH SLA batteries that are 60V nominal (~66.5V fully charged)
    So, I guess sizing up (for safety), I should be looking for stuff rated for 72VDC.

    * Doing a bit of research, it looks like up to ~75VDC is still considered 'ELV' (Extra-Low Voltage)

    Yes, Kellys_eye it's mainly to "protect" not "control"
    I want/need to install a fuse between the battery pack & the speed controller.
    I just want to get the right or a "proper" one.

    I also need a main cut-off switch, to use for maintenance/storage & some low amperage switches (like 1A) for the on/off switch (to turn the speed controller on & off) & for lights etc.

    Any ideas on where to get 72VDC stuff?
  5. Functional Artist

    Functional Artist

    Jul 21, 2018
    Thanks again for all of your guys help.
    I am still working out a few details.

    So far, I have found a few 72VDC rated switches like this:

    An on-off switch (72VDC 16A) for the (power lock) ~$10.00
    an on-off-on switch (250VDC 15A) for the (3-speed function) ~$13.00
    ...& still looking for a compatible off-(momentarily) on switch for the (reverse function)

    But, they seem way "over built" to control simple "signal" circuits.

    Yes, these circuits are "pack voltage" (60VDC) but, they should not carry much (if any) "actual" current.

    This speed controller should only draw maybe ~250mA
    ...& I don't think there should not be much of an amp draw from the reverse & 3-speed circuits at all.

    Only the "big power" wires will ever carry the high amperage. (up to 35A)

    The thick red (+) & thick black (-) "Power" / "Battery" wires, "carry the current" from the battery pack to the speed controller
    ...& the thick blue, thick yellow & thick green "Motor" / "Power" wires, "carry the current" from the speed controller to the motor.

    So, "thinkin' it thru" & "outside of the box" even, I was thinking, what is the "worst case scenario" if any one of these switches ever failed?

    1.) On/off switch:
    If it failed in the "open" position, the speed controller would not turn on.
    If it failed in the "closed" position, the speed controller would not turn off.

    2.) Reverse switch:
    If it failed in the "open" position, the speed controller would not activate the reverse function.
    If it failed in the "closed" position, the speed controller would be stuck in reverse mode.

    3.) 3-Speed switch:
    If it failed in the "open" position, the speed controller would operate (in default mode) at med speed.
    If it failed in the "closed" position, the speed controller would be stuck in either low or high speed.

    I don't see where, if any of these switches failed, it would or could cause an unsafe or "runaway" situation.

    But, I figured I'd run it by you guys to get some input from electronic experts.

    Thanks again, Kevin
  6. kellys_eye


    Jun 25, 2010
    If the control switches are really only operating 'functions' and not controlling the main power feed (in or out) directly then they can be 'any' switch that provides the required function.

    Only the 'power' and 'motor' cables carry high currents.

    Use an isolator switch and a fuse for the power (from the battery) and connect the motor directly to the control unit. You could even use a large contactor to switch the power on/off (defaulting to off when power is removed) which would be easier to source than a switch at those ratings. The contactor itself would be controlled by a small switch. This would even allow you to fit an emergency stop button (big red mushroom) in the event of problems or even act as a 'dead mans switch' by virtue of a cord-operated switch to shut off power if the driver falls out etc.

    The remaining control voltages are just that - control - and you can use any switch (such as those rated at 12V, 125V or 250V etc) as they carry minimal current.
  7. Functional Artist

    Functional Artist

    Jul 21, 2018
    Yup, that's kinda what I was thinkin'.
  8. Functional Artist

    Functional Artist

    Jul 21, 2018
    While doing this research, I asked this same question (see above) to the guys on DIYelectriccars (an electric car forum)

    ...& Elmoto (an electric motorcycle forum)

    Both are forums that I belong to. I wanted to get input & info from their point of view or field of expertise.

    Now, everyone on both forums are "freakin' out" that I am being reckless & spreading bad &/or false info.

    I even reminded them that were just talkin' about very low current control/signal switches & even "if" there were to fail it could "never" be catastrophic. Just particular function would just NOT function.

    I don't see a problem, is there something that I'm missing?

    Thanks, Kevin
  9. kellys_eye


    Jun 25, 2010
    No forum likes cross-posting on the same issue. Either seek and get your advice from one of them or cease asking. You're setting (or trying to set) one forum against another when the advice (as I read the comments) seems to be identical across all of them.

    I note that someone reverse engineered the schematics and found that many of the control signal lines are 60V. Are these 'lines' switched to ground to detect the function or do they pass the 60V through the switch to some other part of the circuitry?

    Either way you'll need a switch to suit the voltage and most small toggles switches will handle 120/250V @1A quite easily (just make sure you get the right one of course). Switches rated OVER the used voltage are just as good as any - you don't need to find one with the 'next appropriate voltage' rating...... a 72VDC switch???? ridiculous. Just use a 125V/250V rated one.

    Your repeated concerns over failure situations is not credible either. You can't count for ALL possible failure scenarios and a simple 'dead mans switch' (big red mushroom) to switch off ALL power in the event of any issues would suffice. Make it a remote-operated one if you're that concerned. One presumes the kart will be used under adult supervision anyway?
  10. Functional Artist

    Functional Artist

    Jul 21, 2018
    I am sorry, please forgive me, I am not trying to cause problems.

    I have been working with, researching & gathering information on electric go karts for years.
    ...& then sharing the information that I found, with others
    I thought that's what these forums are for, gathering & sharing info.

    I was taught to NOT get all of my information from (1) source & then, try to verify any info or get some type of consensus from other sources, before accepting it as fact & then, sharing that info.

    I cross-post because,
    The DIYelectric car folks know about building electric cars from scratch. (electric go karts are "mini" electric cars)
    The Elmoto guys know a lot about mounting electric motors & stuff into small packages.
    The Solar guys are experts at working with DCV systems & charging.
    The Electronics guys (you guys) are experts at working with circuits & switches.
    The go kart folks are usually teens & parents (who don't have a clue but, want to build one or fix their kids toy)
    ...but, no one seems to know everything.

    * I always use the properly rated fuses/circuit breakers & main cut-off switches for the "power" circuits.
    …& will use properly rated switches for these "signal/control" circuits as well.

    Thanks again for your help & hopefully your understanding.
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I see nothing particularly wrong with the advice you were given on the other forums.

    The general advice is that voltage and current ratings for switches must exceed the voltage and current being switched (other than allowing a safety margin, any additional is irrelevant).

    For fuses the voltage rating needed is calculated similarly to switches, but you need to consider the breaking current in power applications. Your 12V batteries and your wiring may allow several hundred amps to flow. Your fuse must be able to break that current.

    Posting in multiple locations pisses people off because we don't want to put the effort into answering questions you've already had answered. If you want a second opinion, find an electrical engineer and pay them for it.

    Were you banned from those other 2 sites? Because getting banned there then coming here and badmouthing them is not likely to win you friends.

    I will close the thread because your problem seems solved and any questions I asked were purely rhetorical.
    kellys_eye likes this.
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