Grounding in a multiple power supply system - Page 1
Forum section: electronics / beginners

author: danderson
author_rank: Contributor
timestamp: September 12, 2015, 08:56:49 pm
content: I'm building a custom computer rack (https://goo.gl/photos/hzJTNFKFeDotwGFw8if you're interested - hit the info bubble for descriptions and details). I'm now at the stage where I want to build a custom power distribution unit for the whole thing, and I could use some advice on grounding.The basics: the whole frame is aluminum. There are 10 machine trays, also aluminum. The machines need a 12V supply. I have two 12V@30A industrial PSUs providing isolated 12V rails, which I'll be merging with an LTC4370 current balancer (http://cds.linear.com/docs/en/datasheet/4370f.pdf) for redundancy.Now, I need to ground this whole thing cleanly. The mains cables coming into the PSUs are the obvious "uplink" towards the service panel ground. With that in mind, my current proposal for grounding ishttps://goo.gl/ef24hn. I'll install an insulated grounding plate to act as the star point for the rack, and wire the power supply grounds to it, as well as grounding pigtails connecting to any piece of metal that lacks a solid path to ground.The part I'm not sure about is how to connect the plate to the mains earth wires. In that diagram, I've hooked up the ground wires from both electrical circuits. The diagram clearly shows that this forms a loop. But is that loop problematic? I'm reasoning that the mains earth wires for both circuits follow the same path back to the panel (literally inches apart in conduit), and there are no current generators between the rack's star point and the panel earthing point. So... Doesn't that just make this a harmless redundant path, which is completely fine and dandy?I also vaguely recall seeing in one of Dave's videos some piece of equipment that had a pair of pigtails connecting the chassis to the mains earth reference - same start and end point, just redundant. If I'm not misremembering, is my diagram electrically equivalent to that?This is my first foray into systems-level grounding, so I'm quite sure I've got it wrong, and would appreciate some wisdom and experience.

author: mariush
author_rank: Super Contributor
timestamp: September 12, 2015, 10:25:50 pm
content: I would have installed the motherboards rotated 90 degrees on each metal plate with the connectors on the left side, because this way you could install fans at the end of the rack blowing air over the grooves on the heatsinks unobstructed.By making a small cut on each plate on the left side, you could slide a network cable through that metal cut from the switch above to each motherboard and you'd still have room to connect keyboards or whatever you want on the side.You would still have room on the right side for power cables to each mini power supply.Don't feel like commenting on the grounding issue but,,, honestly, I would have just bought a 750-850w atx power supply which would give you real 60-70A of 12v, such power supplies have enough 12v cables in the cpu connectors and the pci express connectors that you could use 2 of those 12v wires to connect each mini psu. Would also solve the grounding issue and make it easy to replace power supplies if it fails.

author: danderson
author_rank: Contributor
timestamp: September 12, 2015, 11:20:25 pm
content: Quote from: mariush on September 12, 2015, 10:25:50 pmThe back of the rack is open, and supports the installation of whole-rack blowers if needed. With that said, there is no need for any fans, the whole thing is passively cooled and and is perfectly happy even in 33C ambient and full compute and I/O load. The component TDPs were specifically picked for fanless operation.QuoteWhy? That's more machining, forces you to unplug everything whenever you want to slide a tray out for inspection, and requires either using flimsier rails, or increasing slot height by an inch or so.QuoteSuch an ATX power supply would cost more than the two PSUs I got put together, require additional control wiring to actually power on, and would not provide power redundancy.That said, if you don't like my plans, you're welcome to build your own, and we can compare notes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 12, 2015, 11:24:42 pm
content: Can you check if the motherboards are grounded to their screw holes?If nothing else, any grounded device (like USB, video, serial/parallel..) connected to a motherboard will share that ground, so if you have more than one board connected in such a way, you'll have ground loops there as well.Single point grounding should be fine. Using the chassis structure as ground should be fine, too (but make sure the panels make contact, say by using star washers on both sides of the bolts holding everything together). If nothing else, you'll want the chassis grounded anyway for safety. Whether the low voltage circuits are single-point or multi-point grounded depends, but it probably doesn't matter. The ground loops formed between chassis and power wiring won't be large, and the possibilities for RF noise will be similarly small (but do keep the runs neatly tied along the chassis).Obviously, if the motherboards are grounded, you'll have no choice. Mmm, I forget if drives are grounded, but I should think so. So you have zero choice there anyway..Tim