Hello, this might be a silly question but I've seen some gear using XLR connectors to transfer DC voltages from an external psu into the actual unit, which I guess it's fine if the DC voltages are within the connector's limits. However, somehow you have to also make sure that the unit is shielded so the earth (gnd?) from the psu should also pass through the XLRs. Is this safe is there's a fault?
There are three different "Grounds" that come into play here;
1) SAFETY Ground. This is the 3rd pin on the AC connector. This MUST connect to the chassis of any device that
also has the live voltage from the line cord in it, so in this case, the remote power supply. It must connect via a separate, dedicated screw, BTW. That's so you should never, in the course of servicing the device, have to remove the ground screw to access another part (which is why it's a no-no to use one of the transformer mounting screws as the safety ground screw, for example). The whole purpose of the safety ground in this enclosure is to ensure that if a live connection were to contact the metal chassis of the device in question, it would short to safety ground and trip the associated fuse/circuit breaker, rather than energize the chassis. Note that this is NOT necessarily the fuse on the device, but the protection circuit in your mains supply, which could be either a fuse/breaker at your service entrance, or simply the breaker on a power bar if you happened to be using one...just the protection device upstream of the power supply. Note also that this safety ground connection is meant to protect against live wires
inside the device from energizing the chassis such as wiring errors, or mechanical issues like you're tightening the lock nut on a round mains switch and it turns 45 degrees and one of the terminals contacts the chassis.
2) "Power Supply Ground", aka "0v" (zero volts). This is the 0v reference for the DC power supply voltages, whatever they may be. This should connect to Safety Ground at one point inside the device, typically as close to the power supply as possible. Note that I'm just laying out the concepts here, there are more knowledgeable folks on this forum that may chime in with more precise detail.
3) CHASSIS Ground. This is the case of the device, which in a unit that was powered by a mains plug would be the same as the safety ground. In a unit powered by a remote power supply, this should still connect to the safety ground in the power supply, BUT, ideally through a separate conductor from 0v.
The best way to accomplish this using, say, a 4-pin XLR for a mic pre-amp that runs on bipolar DC and has 48v phantom power would be to use a four conductor
shielded cable. There's no industry standard (that I'm aware of, anyway) for how the 4 pins would be assigned, but I would do it like this:
Pin-1: 0v
Pin-2: -15v (or whatever the +/- supply rails are, 12, 15, 18, 24 volts, doesn't matter)
Pin-3: +15v (see above)
Pin-4: +48v
Case/Shell: Overall shield
That keeps your 0v and chassis grounds separate until they meet in the power supply, screens the power supply cable and screens the chassis of the device.