DC Currents - Do you know where they go?

[venquessa]

Scenario:

I have a 48V 10 Amp DC power supply. Connected to it are two DC/DC converter electronics bench supplies. These are CC/CV units.

One of the PSUs is outputing 19.50V and is powering a laptop docking station setup. The other is outputting 12.50V and is running 2 USB hubs (charger ports etc.)

The problem is this. The 19.50V supply says it's outputting 0.5 Amps. So does the 12.50V one. However I know this is incorrect. The laptop docking station is drawing just over 1 Amp and about 22 Watts. What gives?

A current clamp and some selected unplugging reveals there are problems.

The + output of the 19.50V supply shows 1.1 Amps. The - output of the 19.50V supply shows -0.6 Amps. The clamp around both leads shows a positive 0.5 Amps. Where did the other half an amp go? Unplugging the HDMI cable from the docking station and the PSU jumps back up to 1.1 Amps.

Okay. So we have the DC ground on the HDMI cables and DP cables linking the DC current through the monitors and the PCs onto, ultimately the USB ground. However, the current does not show up on the 12.50V supply either. It is not returning via it. There are only 3 things connected to that 48V power supply. The two PSUs in question and ... the PE.

I have tested this by connecting a multimeter between the copper radiator pipe and the - lead for the 48V power supply and low and behold, 0.5 Amps flows from the radiator to the PSU.

What's even more disturbing is the 48V power supply is 10 meters away in the garage! That DC current, if my multi-meter was not helping it has to go all the way through the PE in the gaming PC, through the PE wiring to the consumer unit and from there out via the garage plug circuit PE to the inverter and then to the Earth on the DC power supply where it finally completes back to the earth referenced DC power brick where it started.

Throwing 1 or 2 Amps around this way is "curious" and I'm not overly concerned. Just annoyed I have no current limiting control and no valid current measurements from these PSUs. However. That 48V power supply is capable of 480W. The Victron Multiplus charger is capable of 16A and around 500W. It would be "losing" ground currents for those Amps which would concern me. Basically which ever device/path has the least resistance to return via is where the current will go. If that path happens to be something which will not take 16As of DC current, then it's going to get very, very hot and probably burn out.

In a similar vein my Victron multiplus seems to be the favoured DC return path for all the "admin" stuff like the raspberry PI, wifi access point and a few ESP32s for data monitoring/collecting. The MPPT controllers "load" is outputing about 200mA, but only random noise is returning and the rest favouring the RaspberryPI USB to the Victron and from there to the battery.

I don't think there is a solution to this, other than maybe introducing a few selected isolated DC/DC converters. However, as soon as you connect two devices together which share a ground, that isolation is broken.

Any thoughts? Anyone been down this rabbit hole?

Acronyms:
CC/CV constant current, constant voltage
PSU - Power supply unit
MPPT - Max power point tracking (solar charger)
PE - Protective earth (the third pin!)
ESP32 - Wifi/Bluetooth gizmo
HDMI/DP - Monitor cables

 

[venquessa]

Here is an article I found which describes at least part of the problem.

https://control.com/technical-articles/negative-effects-of-grounding-earthing-a-dc-power-supply/

Some points I made are re-iterated and expanded upon. The DC return path may bypass any fuses or circuit breakers placed in the DC circuit.

Also of note. "How can I have 0.5 Amps of current flowing through my consumer unit earth without triggering a ground fault?"

Two answers. One, DC current probably won't be detected by an AC residual current trip. Two, the DC current can flow via the earth block unimpeded. The N and L currents remain perfectly normal.

In my case the RCD (ground fault device) is not in the consumer unit, but at the house wide isolator. This DC current does not need to pass through it to get to the garage circuit and back to the battery.

I think it unfortunately highlights that you can't just mess around with low voltage DC circuits loads and supplies. They are not entirely that safe.

 

[Ampster]

In response to the title of the thread I do not know where my DC currents go. I measure all my loads on the AC circuits because my DC currents on those circuits are trivial. One exception is my battery powered chipper shredder in which I get good info from the BMS on that device. It takes 50 to 100 Amps at 32 volts depending on load. The other exception is my hybrid inverter which gives me good information in addition to the BMS. In that case I view most of that data in kWs.

 

[Vigo]

Since you mentioned a copper radiator pipe, you may be interested to look up some of the possible electrolysis cooling system issues in vehicles.

I get the conceptual irritation with what's going on but it sounds like it's mostly an inefficiency at the voltages you're talking about.

In general adding low-resistance paths between various points without having a guarantee that either a: the device needs it to function or b: it will definitely make it safer, to me just feels like adding variables that Murphy's Law can use to try and kill me later.

 

[Johno1066]

If you have any stray current issues with your DC system it would come from the source of supply having one of the legs grounded and possible grounding in the appliance you are using the DC on. Stray DC currents can do damage to metal in the ground due to stray current corrosion which will slowly erode the metal where the current leaves it in the ground. This is not to be confused with disimilar metal corrosion where any two different metals in a solution will cause electron flows between those metals causing the lower on the noble scale to lose metal.

 

[venquessa]

I get the conceptual irritation with what's going on but it sounds like it's mostly an inefficiency at the voltages you're talking about.

I think for the amps I am "losing down the back of the sofa" it it just an irritation. However, the underlying issues are not just inefficiencies and can be genuinely serious.

I don't think it's a matter of "if" either. I can almost guarantee you have many earth referenced DC power supplies. The average office can have half a dozen and they are interconnected. Normally the currents that flow between them are small and are mostly a pain in the neck because they create a huge amount of common mode noise on those DC rails.

The common'ing interconnects are the "meant to be short range" links like HDMI, DisplayPort, USB. USB the ground is optional as the data lines are a differential pair (USB1/2). If you want USB power obviously you take the common ground. Ethernet on the other hand is "usually" fine and transmitters "usually" use tranformers such that the ethernet cables do not create a galvanic link between devices. However, of course, the CAT5/6/7 cables also get used for many other things, such as RS-485 which carries DC ground and Power over ethernet obviously aims to carry up to 50V and 2 Amps!

Surprisingly the AC power supplies have very little coupling beyond their coupling to a common "neutral". Any "wayward" currents would get "tripped" out pretty fast by any protective equipment.

"We" as solar off grid enthusiasts have many complex DC circuits interconnected. Obviously in some instances those cables are 25-50mm sqr! In others they are 26AWG USB cables or RS-485 lines. The risk is, what happens when the current which you assumed would return to the battery via the big heavy 0AWG cables, actually returns via the 26AWG RS485 cable? What if the thing supplying that current is regulatig it back on the return current on that 0AWG cable and has no idea about the current melting the RS485 cable.

The "Ground" or PE (Protective Earth) is continuous throughout our houses in the UK. Every socket, every radiator every single cable, pipe, bathtub, shower anything exposed metal all connects to "Earth", you are really, really not meant to have current flowing on it. Having any current using the PE for passage should really be considered a safety liability and I should be questioning my setups protective equipment as I'd prefer this did cause a fault and it was caught.

In part this has less concern for me going forward as I moving away from a DC only system and using an inverter for primary power. So as @Ampster says, it probably won't matter anymore. Isolated, individual DC pwoer supplies supplying individual devices and everything is "normal and expected", it's those devices "natural" habbitat.

I was hoping, maybe to centralise the DC power supplies into one to make it more efficient, but I think this DC return current issue is the final nail in the coffin for "DC-ification" for me. It's a shame as the inverter does waste a whole bunch of watts producing 100W which mostly ends up back as DC in the end anyway.

 

[TacomaJoe]

I would look closely at the CC/CV units and what isolation exits on the positive and negative rail. What you want is common negative. Next to review would be the PE of the different equipments and see how PE is referenced to the positive and negative inputs. In the ideal world, PE should not be tied to either rail in the equipment and just provides a point to bond the units together. You don't want stray currents flowing on your HDMI, keep in mind that stray current results in stray voltage drops.

 

[venquessa]

Well, mostly it becomes academic for me now. The DC circuit is gone and I have a lovely new off grid island 240V circuit in it's place.

I'm still running a DC power supply on that, but only one. The other DC loads can go back on their respective DC bricks and plug into the 240V solar circuit.

 

[Vigo]

Well, wed have to pick a topic between human safety, safety of tiny wires, or noise/interference in device function to really prioritize what the solution would be.

In general i take a skeptical eye to intentionally creating a low-resistance path between a bunch of touch points unless im pretty dang sure it facilitates the function of some RCD or other, and on the DC side of things unless you installed it it probably doesn’t exist. So at some point you might consider whether having a bunch of floating housings and a different floor covering to stand on wouldnt be safer than actually grounding all of it and assuming youll experience a dc shock that actually falls within the wheelhouse of the ac RCDs to interrupt (doubtful). But now im probably being an ass. /rant

 

[CoolWill]

I don't trust DC clamp ammeters below 10 amps. Use a shunt or fall of potential measurement to verify the currents you're seeing.

 

[Vigo]

I think what needs to be said there is that clamp meters are not ‘one size fits all’ and different clamp attachments (or different meters if it is built in) are typically used for measuring small or huge currents. Auto techs frequently use clamps to measure currents way below 10a, but it is not the same clamp used to measure starter current or battery load test currents of 150-600a. Using a huge clamp on a tiny current is definitely sketchy.