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IndustryArena Forum > CNC Electronics > CNC Machine Related Electronics > Electrical Cabinet Design Questions
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  1. #1
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    Electrical Cabinet Design Questions

    Hi Friends,

    This might not be the right place to post this but it seemed fitting, feel free to move this if necessary.

    I am in the middle building a tangential cutter from what once was a Queen Bee Pro kit. I am using all of the motors and drivers from that kit (ill detail it all below) but I have designed my own tangential cutting head with a servo and I am getting ready to wire everything up. I just finished assembling the electronics cabinet and attached is a breakdown of the components. I have a few questions that I'm hoping someone smarter than me can lend some insight on.

    The kit came with some controls for Mach3 (UC300 and all the stepper drivers) but I am planning on using LCNC and have purchased a Mesa 7i96 to handle the servo control of the tangential cutting head and am recycling the steppers and stepper drivers. I am not sure if that’s relevant but wanted to include as much information as possible.

    Questions:
    - AC Power
    -- This might be a dumb question but its a 20A breaker I have shown and the place I plan to plug it into is on a 20A breaker (on my house) is this just nullified? I also am pretty naïve when it comes to AC and I'm using a 3 lead wire and plan to use this on the live wire then go directly to the PSU with the ground and neutral. I have some din rail grounding terminals, should the AC ground go through one of those before going to the power supply? Can the AC and DC share the same ground? Or am I asking for trouble here?

    - E Stop
    -- I want to wire in an E stop and from what i am seeing i should do that on the AC side, should this be before or after the breaker? Also, the E stop that came with the kit was originally meant to tie into the UC300 that came with the kit. It looks like a standard machine Estop but it functioned more like a button to trigger a software e stop. Can I use this same button on the AC side? Is there anything I should check before I do that?

    -Din Rail Bus Bars
    --I tried to be cute and used din rail terminals for a cleaner look, they have press fit parts that turn them into what I’m calling bus bars. I have 6 leads that will be used but I needed to add 10 to make the press fit parts not hang out exposed. Is this a sustainable way to do this? there is no clamp to hold these bad boys in there.

    -Cooling
    --Should Install cooling fans in the cabinet to pass air over these components i don't want anything to overheat. This wont be running constantly or even have power but i don't want to cook anything.

    Here are the components I am running for what its worth:
    - 4 x TB6600 Stepper Driver
    - 1 x CL42T-V41 Closed Loop Stepper Driver
    - 1 x S-400-24 – 24v 400w Power Supply
    - 1 x DZ47N-63 – 20A Breaker
    - 1 x DDR-15G-5 24v to 5v Buck Converter
    - 1 x Mesa 7i96 LCNC board
    Also might upgrade to a din rail mounted supply because the one that i have in there now is so comically large

    I probably have a lot more questions and/or things I didn’t list that might be important so please feel free to ask.

    I appreciate any insight in advance!!

  2. #2
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    Re: Electrical Cabinet Design Questions

    Quote Originally Posted by jjlee32 View Post
    AC Power
    -- This might be a dumb question but its a 20A breaker I have shown and the place I plan to plug it into is on a 20A breaker (on my house) is this just nullified? I also am pretty naïve when it comes to AC and I'm using a 3 lead wire and plan to use this on the live wire then go directly to the PSU with the ground and neutral. I have some din rail grounding terminals, should the AC ground go through one of those before going to the power supply? Can the AC and DC share the same ground? Or am I asking for trouble here?

    Perhaps I am misunderstanding what you have in mind, but if all the components are powered by that PSU in the photo then the 20A breaker shown in the photo is way too big.

    The 20amp breaker in your house breaker box is sized to protect the wire in the walls of the house. Presumably the wire is 12AWG and 20amp breaker is appropriate. The breaker in your machine should be sized to be somewhat larger (but not hugely larger) than the design load of the machine. If the entire machine is powered by that 400 watt PSU, then a 5amp breaker would be more appropriate for the machine. Add up all the loads, in watts, divide by 120 (for 120 volt AC assuming you are in North America), and add a margin.

    Look at the "input specs" on that PSU to get a better idea of its AC input requirements, It may be that the PSU can ouput 400 watts, but due to overhead inefficiencies the AC load is going to be more than 3.33 amps. So see what that label says, add up any other loads, add a margin to that total to get a breaker size.

  3. #3
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    Re: Electrical Cabinet Design Questions

    Thank you for the quick response.

    I guess I am having trouble specifying the design load of the machine and ill try my best to explain why, which might be a dumb reason. The cutting head has a pizza wheel type cutter on it and *should* always operate tangential to the cutting path. Ideally this will minimize the torque exerted by the X and Y motors compared to profile milling a sheet of MDF for example. The Z load should be relatively constant as i have two air cylinders that will force the head into the material when its cutting and retract it while its moving to a new path or has to reset an angle so other than the providing support for the air cylinders and the back pressure when its is cutting it shouldn't see anywhere near peak torque. Am i thinking about this wrong? How would i go about determining that?

    I just checked the data sheet for the power supply and it lists the input as 6.3A @ 115V. So if i ignore the block of text i just wrote above, a 7A breaker should do it?

    Thanks again.

  4. #4
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    Re: Electrical Cabinet Design Questions

    I agree that a 7amp breaker should be adequate for a PSU with a max load of 6.3amps. In this sense it doesn't really matter what is going on with the cutting head, cutters, motors etc if they are all powered by that single PSU. If there is a high torque / stall type situation where the motors draw more current, it could be that the draw is more than the PSU can supply on a continuous basis. So, to protect the PSU, the breaker would trip. Some might want a higher rating for that breaker, but that risks allowing the PSU to self-destruct (although its possible it may protect itself by shutting down). Some might want a breaker closer to the PSU rating, say, 6.5 amps, but I think 7 amps is a reasonable choice.

  5. #5
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    Re: Electrical Cabinet Design Questions

    Amazing, thank you very much.

    Just a few follow ups before I start wiring everything up. Should I ground the AC ground to though one of the din rail grounding blocks or go straight to the PSU and not worry about it? Also, should the E stop be before or after the breaker?

    Thank you again.

  6. #6
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    Re: Electrical Cabinet Design Questions

    From an electrical safety perspective, the metal frame of the machine should be connected to ground (the AC ground wire). If the 'din rail grounding blocks' are designed to ground to the machine frame then that would be one method. Alternatively, use a bolt hole in the frame to attach the ground wire. I would expect that the ground terminal screw on the PSU is also connected to the PSU frame & shield, so if that is bolted/screwed to the machine frame then that would also be an option. If you want to do that, verify that you have a good ground connection. Don't allow paint or other 'less than good conductor' materials to interfere with the ground path.

    I don't see any reason to put the E-stop before the breaker.

  7. #7
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    Re: Electrical Cabinet Design Questions

    For Earth GND it is normal practice to set up a star point ground bus of some kind on the panel component mounting frame, for one earth central termination point.
    Also see the Siemens publication I have posted a few times before, for equi-potential bonding of the M/C.
    For E-stop, it is required that all power to motorized devices be removed whenever E-stop is activated.
    You can find/search a copy of NFPA79 (Electrical Standard for Industrial Machinery) out there.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  8. #8
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    Re: Electrical Cabinet Design Questions

    Thank you both for the replies. Sorry i haven't had time to work on this in the last few days. I've received the breaker and installed it and i'll upload a photo of the latest revision.

    So based on what I've gathered i will put the estop after the breaker and before the PSU this will cut off all power to all drivers and control components. I did a little googling and found a thread from a different forum on setting up an e-stop with my specific board. ill link it below but it says there are general concerns about just cutting off the power to the motors due to inductive loads and its a good way to get the magic smoke from one of the drivers. The post says that one stepper based machines its best to just signal the Mesa board to do the e-stopping. I am running mostly steppers and one closed loop motor so I am still at a loss for how best to wire up the E stop. Should i have some sort of relay assembly that is powered closed by when the machine isn't in E stop mode then when it is, it sends the signal to the mesa board and LCNC? I am in the blender here.

    Post below:

    https://forum.linuxcnc.org/27-driver...-stop?start=10


    For the grounding I am gong to run the AC ground through the one of the grounding din rail blocks before running it to the input of the PSU then also run the DC ground to back to the little bus bars I've built and use those for the stepper drivers and mesa board grounds. The electrical box i am using is bolted to the wooden tables I've built and they sit on casters on my garage floor. I know that i will need to ground the machine to this bus bar but should I be concerned at all here?
    Attached Thumbnails Attached Thumbnails ElectricalStuff2.jpg  

  9. #9
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    Re: Electrical Cabinet Design Questions

    Hi,
    the 20A breaker is supposed to protect the house wiring....not your machine. If you want various sections of your machine protected then you should put electrohydraulic breakers IN your machine
    cabinet, one for each circuit you wish to protect.

    Typically the spindle is the most power hungry device in any machine. Lets say the VFD that drives it has a max input current of 10A, then get a breaker to match it. You might have a second
    breaker of 5A say to protect the power supply (for your steppers).

    Most people reckon that if the machine Estops that it should kill the power. I disagree. Both my machines, my first mini-mill and my new built machine DO NOT DE-POWER on Estop. I have run both machines
    for over ten years without issue.

    It was common, even mandatory that industrial machines depower to satisfy safety rules as Al has mentioned...but if this is a machine for your personal use then who cares? Second issue is those rules he mentioned
    are for Americans....and who cares what they do....they're all bloody mad!. If I were building equipment for sale I would be applying IEC rules, and they are rather more sophisticated. While in general they do also
    perscribe that the machine de-powers they also allow that certain parts of the machine MUST stay energised in order to effect a safe shut-down.

    I have relied on my controller (Mach3 firstly and for the last nine years Mach4) to stop all axes, apply the Z axis brake and switch the spindle off when an Estop event occurs, it has never de-powered anything.
    I have operated for years that way.......and it has stopped safely EVERY TIME I have Estopped the machine, I mean EVERY TIME.

    I have asked if any people have had an instance where their motion control HAS NOT STOPPED when requested, eg and Estop, and no-one has said 'yes my control does that'....so is depowering necessary?

    Craig

  10. #10
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    Re: Electrical Cabinet Design Questions

    Hi,

    Sorry I didn't elaborate more, its just a 7A breaker just to protect the power supply and in turn protect the drivers and steppers (I think), the max input current of the power supply is 6.2 so i overshot a little but I think it should be okay. I am not running a spindle as of right now and don't plan on it for this machine so I just have the one breaker.

    For the estop, I figured it was best practice to depower but I don't really have a preference as to if it does or not. I think ill continue looking into what the Mesa card says I'm sure LCNC has something similar where I can have tasks execute once an estop is triggered now that i know that I don't have to cut power. I am one of those mad Americans but this is just a 'hobby' machine and wont be in a production type environment i just wanted to make sure that I don't send 6+ amps through myself if looked at it wrong.

    Thanks for the input.

  11. #11
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    Re: Electrical Cabinet Design Questions

    Hi,
    well it just seems that Americans blatantly and obviously break their own laws, and then BS and bluster to get out of it....it makes you wonder whether you should pay
    any credence to their laws at all.

    Next thing to be aware of is that circuit breakers are very poor at protecting equipment.
    If you have a 7A breaker for instance, what's the bet you could draw 8A or 9A for quite some seconds, even as long as a minute before it opened. Circuit breakers do operate quickly under fault conditions,
    lets say your power supply has an issue and draws 20A, then I would expect it to open in short order, maybe 100ms or less.

    Second issue is that a lot of equipment, including CNC will often draw very large currents but for exceptionally brief periods...and that is normal. Motor start-ups are typically like that. If you have a sensitive
    breaker it will open often, a nuisance trip. There are breakers (called D curve) especially for motors which permit short bursts of high current without tripping whereas a normal C curve breaker would trip immediately.

    If your concern is personal injury due to electrocution the I'd fit an RCD (Residual Current Device). They monitor the incoming and outgoing AC current and any difference between the two, called Residual Current, is deemed
    to be dangerous and trips very quickly indeed. In New Zealand, and I imagine elsewhere, they are now standard and required on all domestic wiring circuits. A 30mA residual current will trip in 30ms or less. In places like
    hospitals and schools they are to trip at only 10mA. These devices are much better at protecting people. In New Zealand they are built into the main distribution board....you do not usually have to add them to your equipment,
    but could if you decided the risk warranted it.

    I have just bought a new 400VAC spindle, whereas my machine has always been 230VAC prior. A zap from 400VAC hurts like forty ba*******ds (from personal and painful experience) so I will be reviewing my incoming mains
    distribution within my machine. Its not that I'm scared of 400VAC....but it requires/demands respect. Three phase circuits are not, to my knowledge, protected by RCD's so I will do a little more research before making any
    decisions.

    Craig

  12. #12
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    Re: Electrical Cabinet Design Questions

    Hi,
    well some Googling and there certainly are three phase RCD's. There seem to be two common types, one that trips at 30mA and another at 300mA.

    I would assume that the 30mA version is intended to protect human personal while the 300mA is more about equipment protection.

    I've just had a look and none of the circuits in our workshop, be they single phase or three phase are RCD protected. It may be that this installation was prior to the newer regulations or
    maybe there is no RCD requirement for industrial situations. There is enough room for me to fit a three phase breaker and even an RCD if I want one in the switchboard

    I rather suspect that CNC equipment would often (nuisance) trip a 30mA RCD, whereas a 300mA would not, but 300mA is not much protection for a human!

    My VFD has not arrived yet, although it has cleared NZ Customs having come from the US. May have it as early as tomorrow....so I need to make a decision soon.

    I think a 20A three phase plus neutral and earth socket is the way to go. All my servos are 230VAC, and I don't really want to put a 400V-230V transformer in there so would require
    three phase plus a neutral.

    Craig

  13. #13
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    Re: Electrical Cabinet Design Questions

    Hey Friends,

    I agree most laws here are for the birds, or lining some corporate/ bureaucrats pockets but that's a whole different rabbit hole.

    I also will admit that a lot of the electrical stuff has me over my head. Looking at the breaker i have (link below) you are correct, it would only trip quickly if current is thousands of percent higher than rated, otherwise its going to take at least a minute. Is this even going to protect anything??

    Generally i want to make sure I'm doing a few things things
    - Make sure nothing is designed to fail or kill me if i touch it (nothing that has to comply with OSHA laws or anything, just reasonable safely
    - Make sure the equipment is protected in the case of surges or large current spikes

    Do you guys think that as it stands now I (moderately) safely say that? I didn't mention this but I am running a three prong 120V cord to a surge protector (a basic power strip) then that to the wall. I needed the power strip to power other 120V things like a compressor for the pneumatics on the head and the touch screen display/ RPi I am using to run LCNC.

    https://www.digikey.com/en/products/...102577/1788308

    Thanks again

  14. #14
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    Re: Electrical Cabinet Design Questions

    Hi,

    Looking at the breaker i have (link below) you are correct, it would only trip quickly if current is thousands of percent higher than rated, otherwise its going to take at least a minute. Is this even going to protect anything??
    You are 100% correct. Most breakers and fuses are effective inn high current fault situations but poor in mild overload conditions. 99% of our electrical system and all our appliances are designed and constructed this way
    and yet it does not appear dangerous.

    This sort of breaker:

    https://nz.element14.com/potter-brum...tic/dp/1566490

    is better for protecting circuits and equipment. They have a much better defined trip point, but note that they are not cheap and tend to be used only where required. This is for equipment protection rather
    than personal safety.

    Do you need such equipment to make your machine safe? I rather doubt it. Your washing machine does not threaten to kill you any time you use it does it? Why then would a CNC machine be any different?
    It is when all said and done an 'electrical appliance' like any other. Provided it is designed and built in a manner similar to any other appliance then I see it posing no greater risk.

    400VAC appliances attract a somewhat more stringent set of rules reflecting the increased risk should such an appliance fail in a dangerous manner. A CNC machine with coolant also deserves increased attention
    with respect to electrical safety. Likewise you would be advised to keep electrically conductive swarf out of any electrical bits.....but all this is common sense which you will presumably apply in any event.

    Provided you use standard wiring practices with respect to earthing and insulation requirements I see no reason to expect your machine to be electrically dangerous.

    Protection against overload, say from an axis stall or a crash is another matter. Circuit breakers tend to be pretty 'coarse' protection. VFD's , servo drives and power supplies tend to have built in protection
    mechanisms and they should be your first line of defence against operational faults.

    Craig

    PS: My Delta VFD turned up this morning. It is an 11kW unit despite my spindle being only 3.5kW (rated). It was second hand and so I bought it for about the same as I would have paid for a new
    3.5kW or 5.5kW model....so I thought why not?. If I ever need to get a yet bigger spindle then hopefully this VFD would accommodate the upgrade. It is rated for 27.5A (per phase) input, but I expect to
    use only a small fraction of that so I'm thinking a 20A/phase three phase D curve breaker and a 20A three phase + neutral socket/plug.

    I will probably fit a DC reactor (choke)...the VFD has the terminals for it. I have an AC line reactor on my existing little Delta VFD, and very useful at noise suppression has the reactor proven to be.
    I can use either an AC line reactor or a DC choke with this larger VFD. I'll probably wind my own choke.

  15. #15
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    Re: Electrical Cabinet Design Questions

    Hi,
    just whipped around the corner and bought the new 20A three phase D curve breaker, 20A three phase plus neutral isolator/socket/box.....bloody hell.....hope not to have to buy any more soon!!!
    Close to $400NZD!!!

    Craig

  16. #16
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    Re: Electrical Cabinet Design Questions

    Circuit breakers protect wires, not electronic equipment.

    They are to stop fires in buildings/walls.

    They will not stop a fault within an electronic device from frying that device.

    It might stop a fire in your electrical cabinet if a device fails and draws a large amount of excess current.
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  17. #17
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    Re: Electrical Cabinet Design Questions

    Quote Originally Posted by jjlee32 View Post
    Hey Friends,

    - Make sure the equipment is protected in the case of surges or large current spikes
    I needed the power strip to power other 120V things like a compressor for the pneumatics on the head and the touch screen display/ RPi I am using to run LCNC.

    Thanks again
    Maybe N.A. practices are not perfect, but when I transitioned from General industrial electronics to the inception of CNC control and retro-fits etc, some being custom PC based, Galil etc, and getting in to CNC control panel building, I looked around for a decent reference and found NFPA79, for DIY, you don't have to follow the NEC part of it, but as to earth bonding and grounding etc and other circuit practices, schematic and panel layout, it came in very useful.
    The difference between an Industrial retro-fit package is that, everything is made by one Manuf. and designed to mesh together, with the onset of DIY, parts and devices are most often obtained from various sources, and also often each of these sources specify isolation etc, Device protection maybe?
    But in all of my custom designs, I used the practice of earth ground all power supply commons, never had a spurious LS tripping, compared to several reports a week at one time in this forum from those that chose the contrary isolation method .!
    On the subject of breakers, N.A. is slowly following the European practice of using the safety relay as a breaker,
    Ensure that each of the outlets on the power strip feed a suitably sized protection device for each unit..
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  18. #18
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    Re: Electrical Cabinet Design Questions

    Thank you friends,

    I'll keep the breaker in there for what its worth.

    I guess what i was looking for from this post was a sanity check. Just wanted to make sure that there are no glaring issues with how its going to be wired before I wire it up and start testing everything, sharing AC and DC ground before and after the PSU etc.

    This is my first hobby machine build from scratch so I just wanted to make sure it wasn't designed to fail.

    I am going to wire it up this weekend, thank you everyone for your help.

  19. #19
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    Re: Electrical Cabinet Design Questions

    One other reference that you may want to look up. is the one by Siemens on equi-potential bonding of the M/C grounds, it aims at eliminating ground loops totally .
    The use of which. also allows the use of shielded cable grounded at both ends.
    I posted it here a while back.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

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