Author Topic: A question on clean earth and protective earth  (Read 1803 times)

Offline nrml

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A question on clean earth and protective earth
« on: May 04, 2016, 01:00:42 PM »
My VFD manual specifies that there must be separate protective earth and clean earth connections (for 0 volts reference). In actual practice does this mean I will need to run an additional earth cable to a ground spike or water pipe in addition to the earth connection from the mains socket. How close or far apart should the earth ends be to avoid interference and ground loops?

Offline PekkaNF

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Re: A question on clean earth and protective earth
« Reply #1 on: May 04, 2016, 01:55:11 PM »
Basically you want one busbar where all ground points are combined. Purpose of the different ground wires is different use.

N (Neutral) is return for the current and therefore has some current flowing on it. Wire resitance will cause different potential at the end of this wire. Bad if you need a "reference"

PE (Protective earth) or G (Ground) is for personal safety. VFD filter dumps noice to this line, it's pretty dirty. Also because it is "earth" it has tendency have ground loops trough metal structures. Also regulation dictates pretty thick wire.

https://en.wikipedia.org/wiki/Earthing_system

In industrial installations there is one more earth wire for "clean" earth, sometimes marked TE, sometimes called "instrument" earth. Idea is to keep that "clean" and provide a refference.

If you don't have a extensive plant just connect both earth wires into one point, that should do it.

Pekka

Offline nrml

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Re: A question on clean earth and protective earth
« Reply #2 on: May 04, 2016, 04:01:03 PM »
Thank you for the explanation. It is just a single VFD bolted to the back of a lathe in a garage/home workshop. 

Offline John Rudd

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Re: A question on clean earth and protective earth
« Reply #3 on: May 04, 2016, 04:23:52 PM »
Basically you want one busbar where all ground points are combined.

https://en.wikipedia.org/wiki/Earthing_system

In industrial installations there is one more earth wire for "clean" earth, sometimes marked TE, sometimes called "instrument" earth. Idea is to keep that "clean" and provide a refference.

Pekka
Inevitably, the earths are combined at the Star point, so they all end up the same whether clean or dirty earth....
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Offline PekkaNF

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Re: A question on clean earth and protective earth
« Reply #4 on: May 05, 2016, 03:33:30 AM »
Basically you want one busbar where all ground points are combined.

https://en.wikipedia.org/wiki/Earthing_system

In industrial installations there is one more earth wire for "clean" earth, sometimes marked TE, sometimes called "instrument" earth. Idea is to keep that "clean" and provide a refference.

Pekka
Inevitably, the earths are combined at the Star point, so they all end up the same whether clean or dirty earth....

What is your point?

True, all earths are ideally connected into one point.

Problem is that most of the "earth" is very poluted and it's voltage referenced to ground is whatever, when it comes to instrumentation. Who cares if it is 0,5V different on 50 m from buss bar, but on +/- 10v reference signal it is a big difference. No matter how thick wires you use, it is there and worse near drives.

Now, if you take even a (relatively) thin wire from this ground star point and take it as a signal reference "ground" you have relatively clean ground, if you don't loop it or connect to dirty ground anywhere else. This ground is good for control/measurement signal reference (ground).

We had FOUR separate bus bars (DC-minus, N, PE and TE) on out control cabinets and some one of them were linked right there together or conected to customer grounding system and it worked when done right. Lot of problems were encountered when someone with a bright idea "any gound is a same ground" jumped them on distribution enclosures....

On small scale it is basically the same, but scale is different and problem is smaller.

Usually there is a sample wiring diagram on VFD manual and it pays to follow it. Stand alone problem is pretty straight forward specially with VFD's internal speed signals, just mount a potentiometer and don't connect on that circuit any extternal power supplies or such.

Pekka