Joe's 1000x600x200 steel and aluminium router started

[Neale]

There are a few reasons for the galvanic isolation. That's actually just a posh name for saying that there are no connections from the live/neutral terminals to the output terminals. I mentioned one reason for this earlier - so that the same smps could be used as a +24V source or a -24V source with one output terminal grounded. Another reason (and someone else mentioned this in a post a little while back) for industrial equipment is that if the 24V supply rails are floating with respect to ground, then you can put in fault detection mechanisms that check if either rail does get shorted to ground under fault conditions; the machine can then be stopped before any magic smoke is emitted. Or, maybe, you have a motor speed controller that needs a 24V supply, but is connected to the mains supply to the motor with no isolation. However, if neither smps output terminal is grounded, you really need to make sure that none of the control electronics wiring or terminals are touchable (due to the whole thing floating at some undetermined but possibly dangerous voltage with respect to ground).

On a practical note, this also means that a simple electrical touch-off system for tool height setting is not going to be easy as this depends on the touchplate being connected to one controller input and being shorted to ground by the tool tip via spindle/machine frame, etc. If the controller is not referenced to ground, then this ain't going to work, or you will need to isolate the spindle from the machine frame and provide an additional connection to it - which will all be floating at "it tingles a bit" voltages.

All these issues disappear if you just take the SMPS 0V connection to ground. I cannot see any downside to this.It is what just about any other home-built machine does.

External, e.g. PC, connections? Valid point, and might be an issue in an industrial environment where different bits of kit are connected to different supplies. In a domestic situation where all the equipment is probably connected to the same ring main, this is a non-issue. In addition, an RJ45 ethernet connection will be galvanically isolated and I believe that the USB connection spec calls for galvanic isolation as well.

[devmonkey]

There are a few reasons for the galvanic isolation. That's actually just a posh name for saying that there are no connections from the live/neutral terminals to the output terminals. I mentioned one reason for this earlier - so that the same smps could be used as a +24V source or a -24V source with one output terminal grounded. Another reason (and someone else mentioned this in a post a little while back) for industrial equipment is that if the 24V supply rails are floating with respect to ground, then you can put in fault detection mechanisms that check if either rail does get shorted to ground under fault conditions; the machine can then be stopped before any magic smoke is emitted. Or, maybe, you have a motor speed controller that needs a 24V supply, but is connected to the mains supply to the motor with no isolation. However, if neither smps output terminal is grounded, you really need to make sure that none of the control electronics wiring or terminals are touchable (due to the whole thing floating at some undetermined but possibly dangerous voltage with respect to ground).

On a practical note, this also means that a simple electrical touch-off system for tool height setting is not going to be easy as this depends on the touchplate being connected to one controller input and being shorted to ground by the tool tip via spindle/machine frame, etc. If the controller is not referenced to ground, then this ain't going to work, or you will need to isolate the spindle from the machine frame and provide an additional connection to it - which will all be floating at "it tingles a bit" voltages.

All these issues disappear if you just take the SMPS 0V connection to ground. I cannot see any downside to this.It is what just about any other home-built machine does.

External, e.g. PC, connections? Valid point, and might be an issue in an industrial environment where different bits of kit are connected to different supplies. In a domestic situation where all the equipment is probably connected to the same ring main, this is a non-issue. In addition, an RJ45 ethernet connection will be galvanically isolated and I believe that the USB connection spec calls for galvanic isolation as well.

I get your point on the convenience of probing Neale, however if you don't connect DC ground to AC ground and you are using an SMPS there is no current path to AC ground at all, there is no shock risk. You get the tingle from the Y capacitor in the SMPS EMI filter there are usually two, one coupling AC live to DC+ and the other AC neutral to DC-, the capacitors are not allowed to be big enough to carry enough charge to hurt you and they are required to fail to open circuit. If you take any 5v USB charger and hold the shield whilst touching AC ground you will get the same tingle.

I'm not against connecting AC ground with DC ground but it does come with the risk that an AC live fault in the machine enclosure will fry all your electronics as there is a current path between AC live and DC ground. My last machine didn't have this connection so when probing a temporary DC ground connection was placed on the spindle then removed afterwards. I'm not sure which is more correct.