Clean, protective and instrument earth

[SL1000]

Can anyone explain the difference between the above?

In the main the diffrence between and requirement for both a clean earth and an instrument earth.

Thx

 

[bmsg]

It is a practice to have an instrument earth (grid)for instrumentation SIGNALS completely different and isolated from the electrical system earth grid. The reason is that the electrical earth grid typically carries currents of the order of kilo-amperes to ground, especially when there is an earth fault.
The instrumentation system typically has signals of 4 to 20 mA DC (or, 1-5V DC, etc.) and cannot take kilo-amperes of current. That is why the instrument earth (for the instrumentation SIGNALS) is separate and isolated from the electrical earth.

A clean earth is the combination of a secure connection to the earth and a very low earth resistance (using a battery of earth pits, if required) - so that the resistance to earth is very low (typically of the order of 1 ohm). This is required so that the potential of the instrumentation system with respect to the earth is very small, for any current to earth.

 

[SL1000]

Thanks..

So as somebody else has said the term clean earth and instrument earth are interchangeable?

If so are these the points generally used for barrier earths and screen termination?

 

[Skogsgurra]

I think that you should keep this discussion in one forum. Having it in more places is confusing and a bad thing. You have at least 8 answers in that other forum.

Gunnar Englund

http://www.gke.org

 

[SL1000]

Yes good point

sorry about that I saw related stuff in the other electrical / electronic group too.

Not sure how to put a link in from here..

 

[ilan7]

SL1000
We are using JUST for INSTRUMENTATION:
1. Protective ground (connecting instruments base to the ground Electrode.
2. Signal Ground - Connecting all the instruments ground
3. Shielded Ground - Connecting only the analog wires screens.
Each ground has it own grounding electrode. The electrical equipment has a different grounding system.