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  1. #37
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    Re: Size for ground wires

    Good info Al.

    Have you noticed safety relays being the heaviest maintenance item in modern control panels? Maybe semi modern. Maybe it is the implementation of the specifier but I have machines that all electrical failures over the years, inside the control panel, have been safety relays.

    Quote Originally Posted by Al_The_Man View Post
    While on the subject of safety, this is what NFPA79 has to say about E--stop, at least as far as N.A. is concerned.
    The electrical code allows for a controlled stop, where the controlled stopping of a VFD can be done according to code using a Safety Relay, these can be obtained with all kinds of features such as issuing a stop input to the VFD PLC input followed by a disconnect via the supply contactor.
    Safety relays are typically one device that can achieve this and have been in used on European machines for some years, they are slowly becoming more popular and eventually mandatory for N.A.

    The E-Stop process required by industrial machinery is spelled out in NFPA79 (Electrical Standard for Industrial Control).
    There are three categories of stop outlined, 0,1 & 2.
    Cat 0: Is stopping by immediate removal of power to the whole machine, i.e. an uncontrolled stop.
    Cat 1: A controlled stop with power to the machine actuators available in order to achieve a the stop and then power removed, the controlled stop is where a Cat: 0 may cause damage to both machine and personnel.
    Cat 2: Basically is a the same as Cat1 but power is left on to actuators such as input devices such as PLC inputs etc which will still allow monitoring their current state.
    Al.

  2. #38
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    Re: Size for ground wires

    Quote Originally Posted by millhouse_ View Post
    I'm form germany and the whole machine must get inspected if it's going to run in a shop. And it is planned to do so. But on my budget it would be good if this only needs to be done once.
    A few comments:

    A lot of the responses here are reflective of North American practices. Since most of us know little about code specifics in Germany you need to take comments here under consideration.

    Second I’d strongly suggest working with a local panel builder if this build needs to be compliant. Again they will be able to correctly build for local codes. Yes this costs money but if you really want to do it once then you have little choice because I don’t think you are at the point right now where you can DIY a compliant build.

    By the way things like color codes for wiring and other considerations for wiring, should not be dismissed. In some cases screwing up color coding can fail an inspection. In the same way electrical codes have all sorts things that must be complied with. Depending upon the machine there may be other organizations that need to approve the build or your build must meet regulatory requirements. So it isn’t just an issue of meeting electrical code requirements. For instance we have tools with high power RF sources and thus must comply with FCC regs. I have no idea what might come into play in Germany.

    As for grounding I do not like Din Rail grounding solutions at all. They seem to be routinely installed poorly and frankly grounding to steel doesn’t make a lot of sense to me. This may be old fashion but the engineer that steered me this way was pretty old ??, we used copper grounding blocks and in some cases aluminum grounding bars that you might see in a load center. One reason for this approach was the capability for visual inspection. Of course you effectively force a star ground instead of grounding terminal blocks spread all over a high impedance terminal mounting rail.

    There is a bit of a side discussion about leaving DC supplies floating. There is considerable danger from floating DC supplies that go to field, as such I don’t recommend the approach. In most cases you want negative bonded to ground.

    One other point if the panel / wiring project doesn’t look good and by that I mean professional workmanship, expect the inspector to go over the machine with a fine tooth comb. At least locally you will get a very complete inspection if your build looks like an amateurs effort.

  3. #39

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    Re: Size for ground wires

    Quote Originally Posted by wizard View Post
    There is a bit of a side discussion about leaving DC supplies floating. There is considerable danger from floating DC supplies that go to field, as such I don’t recommend the approach. In most cases you want negative bonded to ground. .

    When did you last check the electrical code for this, I think you will find that it is not recommended, ( Old methods don't apply anymore ) DC Negative is never floating it is the return path if you want extra noise /voltage on your Grounds then it's your choice how you want to terminate it but the electrical code says different

    Din Rail Grounding is completely to Code and can be more affective than what you are suggesting
    Mactec54

  4. #40

    Re: Size for ground wires

    Most supplies have a grounded neutral, so the DC negative wouldn't appreciate being grounded, you'd pop the breakers and possibly damage the PFC / rectifier. Not something to try!

    Some installations such as safety critical applications like escalators (and steel mills, paper mills etc with multiple drives) are designed with their own isolation transformer, which allows for the DC bus to float. The reason for that is to allow tolerance to a single short circuit to ground. It is possible to detect that short (usually an insulation breakdown) and flag it up as a warning without causing or forcing the system to shut down uncontrolledly. You can imagine the carnage if a steel mill tripped out at full speed - if it's capable of continuing to operate and shutting down in a controlled manner, you can see the benefit in terms of safety and cost in doing so..

  5. #41
    Community Moderator Al_The_Man's Avatar
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    Re: Size for ground wires

    Quote Originally Posted by Muzzer View Post
    Most supplies have a grounded neutral, so the DC negative wouldn't appreciate being grounded, you'd pop the breakers and possibly damage the PFC / rectifier. Not something to try!

    .
    Every DC supply I have ever used or built was isolated from the AC supply, they are either 50hz/60hz transformer and bridge etc, or SMPS, all typically isolated.
    Therefore in all the above examples, there is no problem earth grounding the DC supply common.
    Of course, in order to do this, the transformer secondary itself cannot be, and is usually not, grounded, this allows the DC common to be.
    A good example is a tower or desk top PC, the power common is connected to earth GND via the screws securing the MB that are through the board ground plane to the earthed chassis.
    where the service ground is also terminated.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

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

  6. #42
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    Re: Size for ground wires

    Quote Originally Posted by mactec54 View Post
    When did you last check the electrical code for this, I think you will find that it is not recommended, ( Old methods don't apply anymore ) DC Negative is never floating it is the return path if you want extra noise /voltage on your Grounds then it's your choice how you want to terminate it but the electrical code says different

    Din Rail Grounding is completely to Code and can be more affective than what you are suggesting
    Din Rail grounding maybe to code but that doesn’t mean that it is wise! There is huge variability in the quality of the din rail grounding systems I’ve seen. Combine that with the way some builders splatter those grounding blocks all over the panel and you end up defeating the concept of a star ground. Being code approved and inspiring confidence are two different things.

    As for the comment on DC supplies I’m not sure if we are on the same page there.

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