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General Mach Discussion / Re: GROUNDING - what is the correct way to ground a CNC machine?
« on: November 20, 2008, 01:55:35 PM »
I copied most of the following from another post of mine on the forum:
OK, 'grounding'...'(earth)ground IS NOT equal to DC common. They do not mean the same thing, unfortunately we all throw around the term 'ground' very loosely. 'Ground' is this context refers to 'Earth Ground' which is a safety device. The incoming mains voltage is referenced to ground (through the 'ground' rod(s)) and so is your equipment. This provides a path of least resistance to shunt the voltage away from important things like people in case something goes wrong. 'Ground' SHOULD NEVER carry any current, it IS NOT a common return path for all circuits. That is the job of DC common, and your system may have more than one DC common which is not a big deal. Generally speaking you should not tie your DC common(s) to (earth) ground anywhere. Some power supplies may do this internally with a small resistor to pull the DC common up off the ground plane noise.
The problem is that folks think 'ground' is this universal reference for EVERYTHING in a system and try to measure voltage to it from any given point, which is wrong. Think of it this way, if I were to nail three pieces of wood together at odd angles and ask you to measure their length how would you do it? Would you pick the bottom of the closet piece of wood and measure from there to every other piece? Or, from the floor (ground) to each piece? Nope, because that would not tell you a thing. You would run your measuring tape from end to end on each piece of wood (so your reference is the beginning of each piece of wood and you are measuring from that reference to the end of the wood to find its length.) Measuring voltages is the same idea, you are measuring from a reference point (common) to some other point in the same circuit (same piece of wood).
It is very important that you tie your machines (Earth)ground to your mains earth gound, even if you have a local ground rod. This prevents nasty shocks from things being (earth)grounded at diffrent physical locations. A differnece of 100' could create a 50V potential difference.
OK, 'grounding'...'(earth)ground IS NOT equal to DC common. They do not mean the same thing, unfortunately we all throw around the term 'ground' very loosely. 'Ground' is this context refers to 'Earth Ground' which is a safety device. The incoming mains voltage is referenced to ground (through the 'ground' rod(s)) and so is your equipment. This provides a path of least resistance to shunt the voltage away from important things like people in case something goes wrong. 'Ground' SHOULD NEVER carry any current, it IS NOT a common return path for all circuits. That is the job of DC common, and your system may have more than one DC common which is not a big deal. Generally speaking you should not tie your DC common(s) to (earth) ground anywhere. Some power supplies may do this internally with a small resistor to pull the DC common up off the ground plane noise.
The problem is that folks think 'ground' is this universal reference for EVERYTHING in a system and try to measure voltage to it from any given point, which is wrong. Think of it this way, if I were to nail three pieces of wood together at odd angles and ask you to measure their length how would you do it? Would you pick the bottom of the closet piece of wood and measure from there to every other piece? Or, from the floor (ground) to each piece? Nope, because that would not tell you a thing. You would run your measuring tape from end to end on each piece of wood (so your reference is the beginning of each piece of wood and you are measuring from that reference to the end of the wood to find its length.) Measuring voltages is the same idea, you are measuring from a reference point (common) to some other point in the same circuit (same piece of wood).
It is very important that you tie your machines (Earth)ground to your mains earth gound, even if you have a local ground rod. This prevents nasty shocks from things being (earth)grounded at diffrent physical locations. A differnece of 100' could create a 50V potential difference.