Limits to use an off-the-shelf grounded AC-DC power supply with a two-prong cable

Question

We are selling a product using an AC-DC power supply manufactured by CUI INC (SDI24-5-U-P5 datasheet). We sold the product in a few European countries but we are currently talking with a new distributor in Japan.

The distributor recommends us to use a cable with a type A plug (cable which could be used) because most power outlets in Japan do not seem to have Earth ground.

The device which is powered by this external power supply does not have any metal parts (if that matters), it is independent (no other devices can connect to it), there is no chassis ground on the PCB either. The PCB can only be connected to the outside world with the power supply DC output.

Even though I believe that this AC-DC power supply is still supposed to work without Earth ground since some houses have terrible Earth grounding (if not inexistent), I have a few questions:

• Without Earth ground, does that only mean that the DC output of the power supply will be floating? Or do some filters or other things require the Earth ground to be effective for the power supply to work properly?
• Is there any safety concerns using such type A cables (with only two prongs) on a grounded AC-DC power supply?

I think that we could change this power supply to an ungrounded one in the future, such as SDI24-5-UD-P5 datasheet, but we would like to stick a bit with the first one since it was used for CE marking and that we have stocks. But obviously, safety concerns go first !

Thank you!

Answer 1

I think these two have the answer you're after.
Grounding conductor meant to protect electrical devices?
How to safely ground a switching power supply with floating outputs?

Your power brick has isolation between the input and output so removing the ground wire isn't a concern for human safety unless they're cutting into the power brick or the AC power cable. In those instances a ground wire isn't going to help them anyway.

Having the ground may help reduce noise in the output line. That appears to be the main usage of the ground pin according to the second question.

Your output DC power is always floating, regardless of whether you have the ground wire or not. That's what the 1500 to 3000V isolation is all about. The last thing you want is the ground wire connected over on the DC side (as that would break isolation and increase risk of lightning hitting the user). Note if you had the ground wire for the power brick, it's only connecting to the input side of the power brick, not the output side (except possibly through high isolation capacitors).

Answer 2

As is the case with many power bricks, this one has a floating output whether or not you connect its ground. The datasheet confirms it by listing input to output isolation as 3000V. That means you can connect one output terminal to 3000V from input ground, current flowing in that connection will be negligible, and the voltage between the + output and the - output will still be 24V.

You can verify it by plugging it in and measuring resistance between an output terminal and earth with a good multimeter, or by verifying that it works with earth disconnected.

This property is useful when you want a negative voltage. For example, I recently made a device that required -5V supply and its ground had to be connected to earth ground. I used a regular USB power brick, its +5V as the device's ground and its ground as the device's -5V.

All that said, I don't see a class I or class II appliance class marking in the datasheet. Make sure that regulations allow its earth unconnected.

Answer 3

• Without Earth ground, does that only mean that the DC output of the power supply will be floating? Or do some filters or other things require the Earth ground to be effective for the power supply to work properly?

No, the output will not be floating. This is due to the negative conector. The output voltage will be:

$$ V_{DC} = V_{positive} - V_{negative} $$

where \$V_{DC}\$ is the rated output of your converter. Without the earth, though, \$V_{negative}\$ might float.

As for the converter behaviour, I do not think that the absence of Earth will make it not work properly.

• Is there any safety concerns using such type A cables (with only two prongs) on a grounded AC-DC power supply?

Yes, there is. But, like you yourself said, most residences around the world do not have proper grounding. There are two concerns:

1. The user behaves as a ground condutor. This means that \$V_{negative}\$ applied to the user. Normally, since distribution transformers are grounded, that voltage will be minimal, but still might cause minor electrical shocks, not enough to harm, but it might cause some issues on the some users. You can test this yourself. Plug your converter on a earth-less circuit and touch it. You will not be harmed, but it might cause disconfort. Just put in some flip-flops (pun intended), and you will be fine. Bear in mind that this always happens, even with earth condutor. The difference is that the earth condutor has a lower impedance than humans, so there are lower currents flowing into the user;
2. The second issue is more dangerous. During thunderstorms, if your converter remains pluged into an residence without earth, if an atmospheric discharge happens close to your residence, you converter might fry. But, since atmospheric discharges are random in nature, it might not happen.

Answer 4

Without Earth ground, does that only mean that the DC output of the power supply will be floating? Or do some filters or other things require the Earth ground to be effective for the power supply to work properly?

On almost all DC DC SMPS power supplies the output is floating and not connected to ground. Usually an isolation or step down transformer is used to separate out the AC and DC sides.

If you are selling this power supply with a product and you are getting testing done at an ETL (electronic testing lab) then it will probably be tested under the 62368-1 standard. (I am not familiar with that standard, I file under IEC 61010). If a product passes this standard than it is considered safe for users. If you are testing at an ETL I'd recommend a regulatory compliance consultant as it can be difficult to navigate any ETL testing or IEC requirements.

Is there any safety concerns using such type A cables (with only two prongs) on a grounded AC-DC power supply?

Maybe, an ungrounded supply won't be able to pass a fault current back to the electrical outlet. Usually this doesn't matter because double insulated supplies (most converters like the one above) have two sets of insulation to guard the user in the event of a fault.