Jeff,
(With appologies to Simpson36 for hijacking his thread)
I think we have some
common ground in this discussion.
and I thank you for your considered reply.
I agree with you that the case should never be considered as a reference point to measure circuit volts - a hangover from the days of early radio and television where the chassis was used as a common signal return path. Much confusion exists with regard to using the terms ground and earth interchangeably. One even sees the earth symbol used in automotive applications!
Your references were very interesting but I do not see the statement that under no circumstances should the 0 volt line be connected to earth. My problem is your adament statement that connecting the low voltage DC 0 volt line to earth is "absolutely wrong." Your third reference Figs.7 and 9 actually show this connection. My guru in this aspect of circuit design was a M.I.E.E and as far as I was concerned his word was law.
Practical experience, designing and building switch mode power supplies, a touch sensitve electronic piano and power audio amplifiers, taught me the advantages of star earth points and earthing the 0 volt star point from several internal power sources, but the overriding point is safety, the transformed, rectified voltage should never be allowed to float up and this is avoided by connecting it to earth. Floating secondaries are allowable where the equipment is double insulated or, as one of your references stated, the circuit is designed so that finger contact is impossible, not really practical with the circuits we construct to drive our CNC machines.
Bernard Babani publish many books in the UK for the amateur circuit builder and one their most prolific authors is R.A.Penfold. (Never sure if he actually writes all of the books attributed to him!) in his "Power Supply Projects", without exception, all transformer DC 0 volt lines are connected to earth.
You are quite correct in emphasizing the importance of correct ciruit design and the isolation of control signals from noisy wires like VFD's and even microstepping power lines to stepper motors. Metal cases and braided tubular copper shielding for control wires should be considered essential to minimise these problems.
You raised some valid points and interesting references but we really could do with an Authority or Electrical Regulation that would give people reading this thread the confidence to build circuits that are safe. (Not me, I am only a impecunious machinist!)
Ian
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, I might be able to keep them out of the way!)