[chbot] Earth issues, noise currents, and hair-loss

Tue Nov 22 04:43:09 GMT 2016

I would hope that the building you work in has better earthing than you 
suggest.!!

There is a minimum resistance that any outlet should meet and this is 
shown in table1 page 12
https://www.energysafety.govt.nz/documents/legislation-policy/electricity-act-regulations-codes/standards-and-codes-of-practice/nz-electrical-codes-of-practise/NZECP%2025%201995%20New%20Zealand%20Electrical%20Code%20of%20Practice%20for%20Earthing%20and%20Equipotential%20Bonding.pdf

While 7.8 ohms might seem high, this is the resistance all the way back 
to the electode and not the common point at the switchboard.

One thing that is troubling the electricity industry is current in the 
earth system.
The neutral is derived from earth at the pole, or transformer, and is 
sent along with the phase/s to each house.
At each house an earth is a stake /s are driven into the ground and 
joined to the neutral at the switchboard.

If the houses form an arc from the transformer/supply point, then at 
some point, the resistance to a non sinusoidal waveform might be lower 
resistance going via the earth back to the transformer/supply point.
In this situation you have current flowing in the earth system which can 
cause electrolysis in metal buildings and other nasty stuff.

In the old days the electrician was very neat and formed the spare earth 
wire into a very neat coil ... ie an air-cored choke.
So anything that might have gone to earth is now sent back to the 
neutral wire and propogated into the system.

So yes it is an interesting subject once you add harmnincs and other 
nasties into a 50hz sinewave system.

Mark

Mark Atherton wrote:
> Safety comes first, so ground things.
>
> Component protection is important during manufacture, so ground things.
>
> Operation, may be an issue with lots of different grounds, and may 
> rear it's ugly head in the form of ground loops, hum etc.
>
> ====
>
> In my experience, ground loops are usually caused by earth leakage 
> current.
>
> In a typical building, each power outlet (or maybe a small cluster) 
> has it's own earth connection back to a central point (fuse/breaker box).
>
> So, lets connect different loads, each with different ground leakage 
> current to each socket.
>
> ... and as if by magic, we now can measure (hopefully small) noise 
> potentials between sockets in the same room.
>
> Into socket A, let us plug a low level audio signal generator, with 
> chassis grounded. Also into this socket, lets plug a laptop.
>
> Into socket B, let us plug a very sensitive oscilloscope, with chassis 
> grounded. Also into this socket, lets plug an LED lamp.
>
> Assume that socket A & B have separate ground wires back to the fuse box.
>
> And lets connect the audio between the sig gen and the 'scope.
>
> ... and as if by magic, hum, and all manner of other rubbish appears 
> on the scope along with the expected wanted signal.
>
> As well as the equipment being connected by the audio cable earth, 
> they also share a separate ground, each with their own noise sources.
>
> Noise leakage current from the laptop PSU is making socket A ground 
> noisy. Noise leakage current from the laptop LED lamp making socket B 
> ground noisy,
>
> For audio gear, one easy fix is to elect a single power socket, and 
> run a multi-box from that socket, and only use it for audio.
>
> Hopefully you get the idea.
>
> This is quite a broad subject, and one that can be discussed for hours 
> on end.
>
> Have spent many years chasing ground current issues, in many types, 
> sizes, and forms and the whole subject is still fascinating.
>
> Try laying out a PCB with digital stuff at one end, RF in the middle 
> (next to a switched mode PSU), and analogue stuff, with >100dB dynamic 
> range at the other end. It can all be done, but please leave a few 
> days/weeks to allow circulating currents be understood by the poor-old 
> product designer.
>
> -Mark
>
>
>
>
>
> On 22/11/2016 2:45 PM, Marshland Engineering wrote:
>> Ok not Kaiakora type but electrical ones.
>>
>>
>> I have a digitally controlled soldering iron, PSU and Scope. For the 
>> past XX
>> years, I never connected the scope earth to the plug earth thinking 
>> that the
>> probe ground would therefore float with reference to my power supply 
>> earth.
>>
>> Any way, when soldering components on my test PCB, a few LED's 
>> started to
>> glow. I measured the voltages on the test equipment and I had 90 
>> volts between
>> my soldering iron and scope probe earth. A few more measurements and 
>> I found
>> scope probe earth was the problem.
>>
>> What is normal? Earth the scope, disconnect all cables to the board 
>> before
>> soldering, run an isolating transformer?
>>
>> Often USB powered boards get supply from the PC and some laptops only 
>> have 2
>> pin plugs so all sorts of earth issues can occur?
>>
>> Any suggestions ?
>>
>> Just ordered a battery pocket scope from Aliexpress. That will solve one
>> issue.
>>
>> Cheers Wallace.
>>
>>  
>>
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>
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