Thread: Earthing?
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03-09-2012 #1
Hi again.
I need advice on earthing, generally I am ok, but being new to electronics I thought I would get a second opinion.
My question is this,my PSU, Drivers etc; are enclosed in a metal cabinet. I ordered an On/Off switch for the cabinet, it arrived with 2 poles only. The cabinet is powered from a 3 pin mains socket, can I use the earth cable of a 3 wire circuit, to earth the metal cabinet, by attaching the cable through a hole in the cabinet wall, and attaching with a bolt/screw?
This is what I have now.
Regards
Ray
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03-09-2012 #2
Your main earth wire should go to a central (star) ground point within the box, and should be permanently connected.
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03-09-2012 #3
Hi
Thanks for reply, excuse my ignorance what is a STAR?
Regards
Ray
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03-09-2012 #4
Hi Ray,
Star means all AC Earths come from one point "Star point". This is very important so you don't introduce electrical noise into the system thru creating ground loops.
The signals coming and going from control software thru the parallel port to drives or sensing switchs like home switchs etc are very sensitive to electrical noise. This has the affect of tricking the motors, sensors etc into false moves or switching when shouldn't and cause real head scratching problems.
Mostly when people have missed steps or false e-stops etc it often turns out to be bad earthing practices. This is also why it's important to use sheilded cables for stepper motors and spindles ETC and correctly route back to the star earth point. (See quick pic showing)
NOW don't confuse AC Earth with the " - " or negative of the DC side of the system. While they ultimately all go back to the same Earth point they should be treat has separate and any DC negatives are best taken from a DC negative point. IE from the BOB Com or " - " pin or the " - " negative of the DC side of a PSU.
Often I'll run a single wire from the Main PSU DC negative to a distribution rail where I then pick up all the other DC negatives I need for drives ETC. . . . I do the same for DC positive as well.
All shielded cables should only be earthed at one end and that taken back to the main earth star point.
Hope the rough sketch helps.?
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The Following User Says Thank You to JAZZCNC For This Useful Post:
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03-09-2012 #5
A bit more information required. Are you directly connecting your power supply for the machine to the socket circuit or are you connecting it via a 13amp plug?
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03-09-2012 #6
Hi
Thanks for reply, i am connecting via 13 amp plug.
Regards
Ray
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Earth loops are always trying to sneak in and act like shorted out transformers. For example, an AC electric motor has an earth connection that goes to it's casing and that casing connects to the machine body which has a seperate ground connection. That is an earth loop with some major magnetic field activity in one corner and current starts to flow. Not only does the motor earth wire heat up but so do any other loops in the system, especially if they pass through the machine body. The Volts may be low but the current can be enormous.
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03-09-2012 #8
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The Following User Says Thank You to JAZZCNC For This Useful Post:
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03-09-2012 #9
The guys going to be confused if he stays around here much longer anyway
Jazz got it with his description of earthing. If you follow that you should be ok. Your original drawing made me think that you were connecting directly to the ring final circuit.
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The Following User Says Thank You to Web Goblin For This Useful Post:
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03-09-2012 #10
To all that have helped me so far, many thanks, it as restored my faith in human nature. It's great to know strangers can help each other.
Best Regards
Ray
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