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Mains earthing / grounding

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flatulent

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human resistor

The basic problem that is being solved is that humans can touch two things at the same time and therefore be part of a series circuit passing current. Most of the thing people can touch are earth grounded (such as water pipes) and one side of the power line is connected to earth ground. This allows the possibility of a series circuit formed by the hot side of the line going thorugh some capacitance to a piece of metal that humans can touch while they are also touching some piece of metal that is connected to earth ground. The solution is to make all electrical devices either covered in insulatiion or to have the outer shell be metal and connected to earth ground through a dedicated wire.
 

chinoRuiz

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Yes, you are right, but my question wasn't well expressed.

I would like to know how to join the local zero potential inside one equipment with the mains earth.
I've seen in several audio equipments a diode bridge for this purpose.
In my device, I have a linear power supply with a transformer. I think that I shold join the ground (0 v) of this power supply directly to the mains earth. It have a RS232 interface, and I think that its chassis, the pin 5 (DB-9), the local ground (0 v) and the mains earth must be joined on a star topology.

Do you think it's correct ?
 

mrmookiebud

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If I may ask - Why would you want to do that? Just curious. The transformer gives you isolation from the mains.
 

chinoRuiz

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I don't know many things about this subject, so I'm searching information only.
I think that in equipments with linear power supplies these things are different to switched power supplies ones.

In MY case, earth MUST be connected to the chassis of the device, this is clear. Its serial port connector (DB9) is metalic, so its chassis is connected to earth. If I put a serial cable to a PC, the chassis of it, and the pin 5 are joined in its PCB (it's proved), so through the cable, in MY device, the pin 5 is connected to earth. I think it's better that this connection is done into MY device, not through 15 meters of unpredictable cable.

I can't explain this, but if two devices must share a reference it's better that this is acomplished with the earth line (the other option could be through a thin wire into the RS232 cable).

One more time this is withcraft to me.
What are your opinios ????
 

klug

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Hi ChinoRuiz!

You have rised very interesting and very important question in electronic design. How to connect grounds of different parts of system and chassis and external earth?

First you have realize that your ground is not absolutly massive and fully conductive line. Your ground is not absolutly 0 Ohm conductor - your ground has distributed resistance.

If you accept that your ground has distributed resistance then there follows that any current flowing on your ground will produce different voltage potential in different poins of ground. So your ground is not uniform.

As constructor you have mission to avoid ground currents through sensible points of your circuits. Also you need to avoid currents through your shields. To solve this problem you need to avoid ground loops.

Look at Figure 1 of your link page. If you disconnect "E" wire from amplifier or tuner then you will not have different potentials on this devices and you will avoid dirty current through shield of your cable. So you will avoid hum in your sound. Why it is? Your circuit has different potentials on different points. You have 2 devices and each has different potentials on different points. If you connect grounds of these devices by one wire you will equate potentials of connected points without big current. But if you connect additionally other two ground points of devices than you will connect two low impedance voltage sources with different potentials - so you will obtain big ground current.

You can take rule to connect grounds of devices only in one point - and you will avoid ground loops. But what if you need to connect circuit to shield at another point as it shown in Figure 3 of your link page?

If you connect chassis on first point in connector ground, and secondary you need to connect ground to chassis in other point you will have current loop. As I have said above in second point you will connect two low impedance voltage sources, but you can see the difference of potentials in these points will be not very big - much less than 1 Volt. If you will make connection through diode bridge then diodes in this bridge will be closed, and current will not flow - so you avoid current loop in ordinary situation. But if you will have big pulse potential then these diodes will pass it through. High frequency noise will come to chassis through capacitor.

In your case you make correct decision to connect chassis and signal ground and ground of serial connector in one point. If you will need on future to connect devices to external ground you will make connection to the same point.

Best wishes & 73! Klug.
 

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