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Thread: Earthing equipment questions

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    Diamond Member adrianh's Avatar
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    Earthing equipment questions

    I need a bit of advice. Two of my machines have to be earthed but it seems that the earths have to go directly to ground as opposed to the earth (Let's call it common earth) that forms part of the supply cable.

    I need to understand a couple of things:
    1. The purpose of the earth is to sink electricity that shows up on the body of a piece of equipment directly to common ground - right?
    2. The earth leakage breaker trips if there is even a minor amount of power being sinked to common ground?
    3. Why then is there a need to sink the machine to a local ground rather than thought the common ground?
    4. What is considered to be a local ground, a 1m stake in the ground but what to do if you are in an enclosed building having four walls and a cement floor?
    5. People often use water pipes as a local ground, is this dangerous because if a fault occurs a person may get shocked as the power travels through the pipe?
    6. Should the common ground and the local ground be connected together on the machine?
    7. Should each piece of equipment go directly to ground vs daisy-chaining (I suppose daisy-chaining the ground would force the fault current to pass from machine to machine before going to ground)?
    8. Please explain the term ground loop and what can be done to avoid it. Not only in mains circuits but also in low power DC circuits?
    9. When you have long computer cable runs it is said that the screen should only be connected to the one side (is this to avoid ground loops)?
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    Diamond Member AndyD's Avatar
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    Quote Originally Posted by adrianh View Post
    I need a bit of advice. Two of my machines have to be earthed but it seems that the earths have to go directly to ground as opposed to the earth (Let's call it common earth) that forms part of the supply cable.

    I need to understand a couple of things:
    1. The purpose of the earth is to sink electricity that shows up on the body of a piece of equipment directly to common ground - right? That's the function of a protective earth. The earth is also functional and is used by surge arrestors to sink energy from voltage spikes, it can be used in its functional capacity by certain types of comms arrangements as well
    2. The earth leakage breaker trips if there is even a minor amount of power being sinked to common ground? Yes and no. It will detect fault currents and certain types of functional leakage. There are faults that the earth leakage can't see if they're on the other side of a transformer for example.
    3. Why then is there a need to sink the machine to a local ground rather than thought the common ground? Could be a requirement because of the communications system it uses. Could be a number of reasons
    4. What is considered to be a local ground, a 1m stake in the ground but what to do if you are in an enclosed building having four walls and a cement floor? The main earth terminal (MET) of the incoming supply.
    5. People often use water pipes as a local ground, is this dangerous because if a fault occurs a person may get shocked as the power travels through the pipe? Building pipework should already be bonded to the MET in nearly all installations to prevent the possibility of shocks. The pipework and its bonding should not be used as a protective or functional earth for electrical final circuits or for machines etc
    6. Should the common ground and the local ground be connected together on the machine? Usually there's no reason the MET shouldn't be used as both.
    7. Should each piece of equipment go directly to ground vs daisy-chaining (I suppose daisy-chaining tte ground would force the fault current to pass from machine to machine before going to ground)? As long as the current carrying capacity of the wire meets the requirements of the adiabatic equation you can 'daisy chain' but keep the bonding wiring as one continuous length if possible.
    8. Please explain the term ground loop and what can be done to avoid it. Not only in mains circuits but also in low power DC circuits?
    9. When you have long computer cable runs it is said that the screen should only be connected to the one side (is this to avoid ground loops)? I'm short of time and ground loops is an involved subject. Yes they can be a consideration in certain designs.
    Can you give some details about the two machines? How are the connected together, also the comms arrangement between them? I'm short on time but I'll revisit later and try give some answers that are focused and less general if you can give more details.
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    Diamond Member adrianh's Avatar
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    Both my lasers have additional grounding connectors on their chassis. The laser power supplies output 25KV. I haven't been able to find anything specific regarding this but it seems that the Chinese try to cover themselves by forcing users to ensure that the equipment is properly grounded. I've read that some countries are quite lax about applying proper electrical standards and as such grounding isn't always done properly.

    I'm still trying to wrap my brain around the the relationship between, grounding, ground loops, ground potential differences and communication problem. (I don't have a problem, the machines are running perfectly, I am just fascinated by the subject)

    This is a really good read:
    What I found interesting is that local grounding can exacerbate communication problems.

    http://www.copper.org/applications/e...grounding.html
    How easily someone is offended is directly proportional to how stupid they are.
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    Silver Member bones's Avatar
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    my laser has two plugs 1 with the earth neutral and live
    the 2nd plug with only an earth wire connected to it
    i never had the power trip

    i dont think the second earth does much more then the
    first one but i got it second hand and that is how it was
    wired so that is how i use it

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    Diamond Member AndyD's Avatar
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    What the Americans call grounding we refer to as earthing. The kinds of issues outlined in the case study you linked to Adrian are typical in a large manufacturing environment where equipment on the same network is supplied by different DB's often in different buildings. I'm guessing your set up is somewhat smaller and everything on your network is supplied from a single DB so many of the issues discussed won't be relevant to you.

    In short I'd suggest one of the most important things you could check would be the N-E voltage when the electrical system is under load ie when your machines, hot water geyser, lighting etc is all on. For machine networks and also EPOS installations we like to see less than 1V N-E, at a push we'll settle for slightly higher but anything over 2V we usually recommend remedial action. The other thing that would be important for your set up would be a 'good earth' or low impedance earth at the main supply as well as at each individual machine. Testing the earth impedance isn't something you can really do, it requires a special tester. Depending on the earthing arrangement of the incoming supply there's no reason you shouldn't have a sub-ohm earth impedance in many cases. Often the earth impedance (Ze) and the N-E voltage are related and improving the Ze also assists with reducing the N-E voltage.

    I recommend that all cable shields are connected to earth at both ends but I definitely wouldn't recommend local earth rodding of individual machines. If your machines have an extra earth connection point on their chassis you could link them together with an earth wire that runs back to the main earth terminal of the installation but it's usually completely unnecessary if the electrical circuits supplying the machines were designed and installed correctly in the first place.

    The earthing of industrial and even commercial installations can be a complex subject and the implications of the design are far reaching, it's not just about the safety of the equipment for the user, there's also functional considerations which can become a balancing act in some circumstances.
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    Diamond Member adrianh's Avatar
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    Thanks Andy.

    Another question; why do you sometimes get a sense of a large amount of static electricty when you run your finger lightly over the body of a machine?
    How easily someone is offended is directly proportional to how stupid they are.
    ~GS Elevator

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    Diamond Member AndyD's Avatar
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    Is it a machine with a metal chassis and metal covers or is it plastic?
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    Diamond Member adrianh's Avatar
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    The entire machine is metal and has no plastic. I feel it when I lighty move my finger over the bare metal parts.
    How easily someone is offended is directly proportional to how stupid they are.
    ~GS Elevator

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    Silver Member bones's Avatar
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    Quote Originally Posted by AndyD View Post
    Is it a machine with a metal chassis and metal covers or is it plastic?
    not to chip in but

    lasers use a massive amount of energy
    to work and need to discharge when it
    is done so basically it cannot do that
    on the power supply side so i was told
    that the second earth is to get rid of
    the stored energy and static energy
    buildup while it was running it is also
    why you get static discharge

  10. #10
    Diamond Member AndyD's Avatar
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    @ Bones, regardless of the internal functions and internal voltages used in a machine it shouldn't be using the earth (CPC or protective conductor) to discharge any substantial amount of energy. Functional earth currents should be well below 10mA otherwise their supply circuit would require what we term as a high integrity earth. TBH a large hobby type or smallish commercial laser cutter shouldn't even be close to 10mA functional earth current especially if it was supplied with a standard 3 pin plug that fits a standard 16A socket.

    @Adrian, I'm seriously wondering about your 'static' question now in conjunction with what you stated earlier;
    Quote Originally Posted by adrianh View Post
    Both my lasers have additional grounding connectors on their chassis........
    Firstly I'd bet you money that the static feeling you're getting from the metal covers or chassis isn't actually static. In fact nearly all the times that I hear about static shocks from electrical equipment or household fixtures etc it's not actually static, it's always caused by the item being at an elevated voltage with respect to earth because of electrical faults.

    I use the plural 'faults' because in order to get a shock or a tingling sensation from something there is usually two faults required to be present simultaneously to cause this. There needs to be an insulation fault that's allowing an earth leakage current but this on it's own doesn't cause a shock if the earthing is good because the low earth impedance keeps the chassis at earth potential or 0 Volts. To cause a shock you need a poor earth as well which allows the voltage of the chassis to rise and this chassis voltage then causes an earth leakage current flow through the machine operator.

    How much current flows ie how big a shock he/she gets is down to how big the insulation fault is and how poor the earthing is. With the static type shock you're getting, a fault current of maybe a milliamp or two would cause this so it's possible it's not an insulation fault because 1 or 2 mA is in the realms of acceptable functional leakage for this type of machine so it's possibly just one fault that's causing it. Poor earthing.

    I'm suspecting that the earth terminal on the chassis that you thought was there as an optional extra in fact must to be connected to the supply earth (CPC). Obviously without any first hand knowledge of the machines or your electrical installation I may be incorrect but if you're feeling a static type shock I can guarantee you something is amiss and there's a potential hazard that should be addressed.
    _______________________________________________
    I am special and so is Vanash.
    _______________________________________________

  11. Thanks given for this post:

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