connecting Earth to digital GND

[DimaA]

please look at the attached picture
this is our custom power supply in metal enclosure
i'm performing now tests for IEC60950 safety standards
the "-" of the DC output is connected to system's protective "earth" connection (don't know why) - marked red
can this cause fault in safety tests?
and in general, does this connection permittable?

 

[karesz]

Hi,
Im not a safety/earthig specialist, but in my opinion is "the red connection" WRONG!
I agree, that secondary GND can need a connection to chassie, but I would do it only with a i.e. 1MOhm resistor (ev. parallel with 100nF or higher value)...
In most cases you must have a GND separation between primary & secondary (because your system dont work properly with GND_loop).

Otherwise, I cant see a ca. 2500Vrms safety isolation between the both circuit worlds too!:-(
K.

 

[DimaA]

what are the safety hazards? the secondary voltage is less than 60V, so it's not a "dangerous" voltage.
also, what does it mean "ground separation" between primary and secondary? there is no ground in the primary, because it is AC. or maybe i'm wrong, and the "earth" is the AC's ground?

 

[cameo_2007]

wont this set up create a problem like this:-

say the input mains live wire has come in contact with the chasis due to some reason, and that ac is being taken to the grounds of your dc section throught that red connection indicated in your drawing, and can spoil the components associated to it.. ( provided if you have a poor earthing)

this is just a thought and doubt, i am not sure how exactly it works.

probably some experts can give some inputs on this.

 

[karesz]

....there is no ground in the primary, because it is AC. or maybe i'm wrong, and the "earth" is the AC's ground?

Hi,
Of course is GND (as you mentioned) on the AC-side; =the "Earth" ...
In some contries its equivalent with the netral line/wire!_even if you have 3 wires, their are connected at the retailer/transformator point.:-(
If you have in the mains a strong short, your "GND-line" (and your circuit/chassiy) canbe no some higher potencial as nondangerous_maybe at 100V or so...
These is the reason for separated GND/Earth lines (grounded at all houses/consument & is at the transformator on GND too for negligible "failure potencial on your chassies) and isolating between pri & sec. circuits.
In all case; if Im wrong with my knowlidge, correct me pls!
Greetings!
K.

 

[eetom]

I am going to post assuming that there is a transformer present and that the secondary is isolated from the primary.

I don't know what the NEC requirement is for not letting a system float with respect to earth ground, but lets assume that 48v is above that requirement. Then your connection is correct. It is also correct to unhook the negative side and ground the positive side although nobody in their right mind would do that as everyone expects the negative to be tied to earth ground. Assuming that we are not wanting to protect against transients on the low side, the only reason to do that is for safety (excluding noise considerations).

Now if the two sides are not isolated, then there is no connection required because the secondary is already tied to earth ground because the primary is.

Now if 48v is below the NEC requirement for floating systems then you don't have to do anything, but I would probably ground it anyway so that it will pop the breaker in case the other side does come into contact with earth ground or chassis.

Oh, that is not with a resistor of any kind. That is a direct wire connection to the chassis. If there is a 10M ohm resistor in there, then the breaker will not pop, and you will have the situtation of a hot wire touching the chassis in the first place. The addition of the resistor is a BIG NO. USE A DIRECT CONNECTION WITH A WIRE! The 10M ohm resistor is dangerous.

For ground loops, tie both grounds to the same point on the chassis. Ground loops farther down stream on the 48V supply is a different story.

 

[karesz]

Hi eetom,
Nobody is speaking over 10MOhm!!
Only over 1 MOhm was writen...
Im not a safety specialist, but must say; maybe you can live with these "(un)safety philosophy" in the States or some other contries, but never in Europa! :-(
Our safety standars are really better & harder!
Regards!
K.

 

[ngmedaboard]

Assuming your earth ground is good, I think this is the way you'd want it. If one of your voltages were to short to the chassis, you'd be protected as the chassis is grounded to the Earth and you'd safely trip the fuse or breaker for the wall outlet.

The only time this could be dangerous is if someone is touching a high voltage and the case of this at the same time.

I think your circuit is ideal, but I couldn't speak for any specific safety requirements.

 

[eetom]

@karesz: sorry about the 10Mohm thing, but the principle is the same. In my opinion, it is wrong. The reason is: Lets say you do put the R in the circuit from the 48V to earth ground. OK. The negative terminal is now tied to earth ground through the R. Now lets say the 48V positive accidentily becomes connected to earth ground. Nothing happens. The positive is connected to negative through the R i.e. the breaker does not pop. So now an unaware user comes up and touches the case and some how he also touches the negative 48V; he gets zapped. If the negative terminal had been directly tied to earth ground, then the breaker would have popped and the user would not have gotten shocked.

Yes, the EU is much much better in EMC and safety and other things. No doubt about it.

 

[karesz]

...Yes, the EU is much much better in EMC and safety and other things. No doubt about it.

Hi,
Tnx & yes; although the said resistor/capacitor combination is regulation conform_as I know ...
Best regards!
K.