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I built up a LM317 power supply which is working great.

I have moved it into a large case as I am now powering it off 240VAC (fused) down to 18VAC then rectified into the power supply. Which is going well. I also put a much bigger heat sink on it (1.4C/W). The plan is to set up another one in there as well and have dual variable supplies.

Now the question is should I connected the ground pin off the 240V connect to the case? The case is a 19" rack mount. It is aluminium?

I am pretty sure that is what most people do but thought I would ask. It would stop the case from becoming live if something shorted to the case would it not?

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2 Answers 2

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I think a good starting answer is

(1) - It would stop the case from becoming live if something shorted to the case would it not? :-)

(2) BUT, not only stop it becoming live, but also provide the intended means of operating the provided protection equipment if a phase to case fault occurred.
aka "blowing the fuse" or "tripping the breaker" as the case may be.

The operation of fuse or breaker in the case of a fault is probably more important than keeping the case from reaching mains voltage at all. Both achieve much the same result in terms of case potential above ground BUT breaker/fuse tripping both tells you there is something wrong and removes a potentially lethal problem which may otherwise manifest in some different manner.

(3) Along the way it may stop nuisance shocks which occur from current through any X and Y caps that may be fitted (probably none in your power supply).

Also ground referencing your supply stops the whole assembly wandering off to a semi-random voltage of it's choice relative to ground that has nothing to do with the power supply proper. eg in some cases an ungrounded case may be driven by electrostatic charge from the effect of carpet on clothes etc and gain a voltage of thousands of volts (very literally) relative to ground. Touc it when grounded and you may feel a small or not so small kick as the stored energy "discharges". What feels small to you may be the last thing that your circuitry ever "feels".

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  • \$\begingroup\$ Grounding the case yes, but I would not ground reference the supply ( but which side of it the + or - ) I would add a ground binding post so the user could ground either side if desired. \$\endgroup\$
    – russ_hensel
    Commented Dec 19, 2011 at 14:48
  • \$\begingroup\$ Thanks for the answers I thought that would be the case (Haha), and yes it would allow for the RCD's to work as they check for earth currents if i am right. I will scrap away some of the paint and attach the earth to the case and check for a good connection. \$\endgroup\$
    – Ashley Hughes
    Commented Dec 19, 2011 at 22:20
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Yes. 110% definately! Infact depending on where you are you are probably required to do it by law.

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  • \$\begingroup\$ I am not sure about the laws, but I do know we check all work equipment and tag it that it is safe. I will properly take it in to work in the new year and check it and tag it so I can use it at work if I need. I'm in Australia BTW \$\endgroup\$
    – Ashley Hughes
    Commented Dec 19, 2011 at 22:24

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