Hello, I'm trying to design a kwh meter using the ADE7878A and i had some questions about something I found in the datasheet. In the picture below you see one of the typical setups for the metering IC. In this example they show the mains neutral connection being connected to the ADE7878A through a filter and directly to the ground of the circuit. My question is about the second part because I saw online that you shouldn't connect a ground plane to neutral but here they are doing that. I'm kind of new in designing something which uses mains connections so any clarification on this would be really helpful.



link to datasheet ADE7878A:
https://www.analog.com/en/products/ade7878a.html?doc=ADE7854A_7858A_786A8_7878A.pdf
link to datasheet Evaluation kit:
https://www.analog.com/en/resources...on-boards-kits/eval-ade7878a.html#eb-overview

 

The circuit 0V is mains neutral, therefore the entire circuit is regarded as "mains live" for safety purposes. If you want to connect the data outputs to equipment that is earthed or can be touched, then the data link must be isolated.
The ground plane for the pcb can be mains neutral, provided that it cannot be touched and is isolated from the case and any other metal parts the user may touch.
The device is designed for electricity meters which are read by radio or wi-fi link so they can be totally enclosed, and therefore can use a simple non-isolated power supply.

 

" that you shouldn't connect a ground plane to neutral "

??? That is the basis of electrical supply, in the UK for e.g. the star point of a 3ph transformer is connected to earth and therefore becomes the safety neutral.

 

" that you shouldn't connect a ground plane to neutral "

??? That is the basis of electrical supply, in the UK for e.g. the star point of a 3ph transformer is connected to earth and therefore becomes the safety neutral.

So there is not a separate neutral and safety ground as in the U.S.?

 

So there is not a separate neutral and safety ground as in the U.S.?

The UK method is the same as N.A. , The difference is in the UK it is the grounded star point of a 3ph transformer becomes the neutral.
One phase is used for domestic supply.
In N.A. it is the grounded centre tap of a single phase transformer secondary which is defined as neutral.

 

I saw online that you shouldn't connect a ground plane to neutral but here they are doing that.

The ground plane on a PCB can be any voltage you want. I have made product where the PCB lives on the hot wire and not on ground or neutral. I don't like it, but it is done. You must have isolation to humans and the outside world.
You need to measure L to N so your ground plane is at N.

 

The ground plane on a PCB can be any voltage you want. I have made product where the PCB lives on the hot wire and not on ground or neutral. I don't like it, but it is done. You must have isolation to humans and the outside world.
You need to measure L to N so your ground plane is at N.

Mostly.
Most of it is TNCS - So the supply transformer start point is earthed at the start point, the earth and neutral from the supply transformer are combined in one conductor (that's the 'C'). At the point they enter the premises they are separated (that's the 'S') and the live and neutral supply go through a residual current circuit breaker and the protective earth is bonded to exposed metalwork, water pipes etc.
So, yes, the neutral is connected to earth, but only where it enters the building. Elsewhere it must be kept separate, and neutral conductors treated the same as live conductors.
So back to the TS's circuit: the neutral ground pane is connected to earth, but it can't be exposed so that anyone can touch it.
The rest of it (where the supply transformer is too far from the premises) is TT, so the supply transformer is earthed at the transformer, but where it enters the building it becomes the neutral, and the protective earth is formed by an earth rod in the ground which is connected to water pipes and exposed metalwork. In that case there is no guarantee that earth and neutral are at the same voltage, nor even that neutral is at a safe voltage.

 

Exactly as Ian mentioned…..
If you are using the Eval Board documentation, you’ll see on page 29 the isolation circuitry to connect the board to the outside world. You will also see that there are two different ground symbols, one for the “hot” ground and another for the “safe” ground on each side of isolation barrier.

 

Symbols not always abided by correctly !!

• Universal Symbol (IEC 60417):
  The most common symbol for earth ground (three decreasing horizontal lines) is widely recognized but not always consistently applied.
  
  
• IEC 60417 Standards:
  IEC 60417 defines specific symbols for different grounding types:
  • Earth Ground (5017): The general symbol for earth ground.
    
    
  • Protective Earth (5019): Specifically for safety grounding.
    
    
  • Chassis Ground (5020): For grounding the chassis of equipment.
    
    
  • Noiseless/Clean Earth (5018): For preventing interference.

 

I was not saying that the symbols are/ aren’t followed correctly in the data sheet.
I am only saying that the data sheet shows two distinct and isolated “grounds”.

 

That is where the confusion can come in, in N.A. the term ground is practically all encompassing. Whereas my training in the UK distinguished it by using the term 'Earth' (conductor), !

 

That is where the confusion can come in, in N.A. the term ground is practically all encompassing. Whereas my training in the UK distinguished it by using the term 'Earth' (conductor), !

And now we've thought up and acronym for it instead. It's now a CPC.

 

If you were on the Moon, are your feet on the ground? (or Earth or earth)

 

You would then bond the neutral to moon.!

 

You would then bond the neutral to moon.!

Imagine the linguistic permutations:
Virtual moon
Moon currents
Moon fault interrupter
Digital Moon
Analog Moon
Mooned chassis
Moon wire
Moon plane
…..

 

Give Dr Bruce a bit more material.!!

 

Joking aside, this is what the documentation should be showing:

 

Joking aside, this is what the documentation should be showing:
View attachment 352170

No, you cannot connect mains live, and protective earth to the same pair of inputs. It won't pass the 2.5kV flash test. The original is correct in showing signal ground.

 

ooopps...