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Topic Title: Instrinsically Safe Earth / Clean Earth / Main Earthing Terminal
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Created On: 02 February 2013 09:09 PM
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 02 February 2013 09:09 PM
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Apostolos1983

Posts: 21
Joined: 03 December 2012

Hi to all of you.
I hope you are well.

Lets say that I have an MCC where the last compartment is the well known I.C.A. section (Instrument Control & Automation).
In the I.C.A. section I have Intrinsically Safe Bariers (Zener or Galavanic) that feed many IS instruments such as Pressure Transmitters, Level Float Switches, Flowmeters and Ultrasonic Level Sensors. There is also the PLC, the Telemetry Outstation, the HMI etc

My MCC has a busbar as PE busbar from where earth is distributed to motors, exposed conductive parts, extraneous conductive parts. Practically this is the Main Earthing Terminal of my installation

Questions:
1. The exposed and extraneous conductive parts (e.g. motors, cable armour, metallic ducts, cable trays etc) are earthed to a different earthing terminal from the one for my Instruments? I suppose yes. Then what earthing terminal is used for my instruments??? Is it connected to the MET of the MCC?

2. What is Clean Earth? Is there a "dirty" one? Is this the earthing system used for the instruments and signals?

3. Intrinsicall Safe Earth is used exclussively to earth Intrinsically Safe instruments? How do we create Intrinsically Safe Earth? Is it different that Clean earth?
 02 February 2013 09:43 PM
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davidwalker2

Posts: 194
Joined: 29 April 2009

I may be able to throw a dim light on "clean earth". In broadcasting, technical equipment uses a clean earth which all signal circuits use as earth reference. This does not carry current and is separate from the power earth, which is the safety earth that power circuits are connected to.

All earths have impedance, so any current in the earth will create a voltage, albeit small. If the signal input to very sensitive instruments is "unbalanced", that is a single signal wire having a voltage referenced to earth (as most consumer amplifiers, televisions etc and many instruments are) any disturbance on the earth is seen as signal. As signals can be of the order of millivolts, even a few microvolts of earth noise is undesirable.

So the power earth, carrying switching spikes, inverter noise etc was kept separate from the clean earth. The clean earth will be connected to the safety earth at a single very low impedance point.

David
 03 February 2013 08:04 AM
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maltrefor

Posts: 121
Joined: 01 November 2009

http://www.theiet.org/forums/f...atid=221&threadid=6636

http://www.sparkyfacts.co.uk/Electricians_CompEx_Course_type_of_protection_i_Intrinsic_Safety.html

Edited: 03 February 2013 at 08:10 AM by maltrefor
 03 February 2013 09:32 AM
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Apostolos1983

Posts: 21
Joined: 03 December 2012

So, there are three types of earth. Ok?
Lets say:
Power Earth = The eath provided by the PE busbar inside my MCC that is used to the power circuits and for equipotential bonding of exposed and extraneous conductive parts

Clean Earth = The earth provided by a separate busbar inside the I.C.A. (instrumentation control and automation) section for the instuments and the automation signals

Intrinsically Safe Earth = The earth provided for the Intrinsically Safe Instruments.

Do I get it right?
Is there also a busbar or a terminal for IS earth?
How do we create a Clean and an IS earth?? In all probability the terminals or the busbars for these earths should not be electrically conected to the main PE busbar of the MCC (MET).
 03 February 2013 10:04 AM
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MickeyB

Posts: 181
Joined: 18 January 2003

Having undertaken quite a bit of electrical work in the media sector over the years I can echo the points raised by David.
In BBC land it was a 'Technical Earth'..... which basically was a parallel earth system that was for technical equipment use only.

The 'technical' earth bar was insulated from the general earth (dirty safety earth) and was designed to provide the lowest impedence path/ reference for the sinsitive equipment that would use it in the zone of use.....Each zone had it's own reference technical earth bar.

The high frequency current of the equipment had an impact on the copper cable types..... we ended up with multi stranded earth cable rated at full size of the circuit in lieu of 7 stranded...... in the real world it probably made very little difference..... just like the use of 4mm minimum cable in lieu of the 10mm the BBC wanted....... but hey ho, that's what the license fee pays for.....

My knowledge of intrinsically safe installations is limited to waste water installations where we had 'blue' cable to identify the IS installation.... the basic idea was that the IS installation should not be affected by an electrical fault or noise outside the zone of use...... from the general power side and therefore a dedictaed IS earth was required with an impedence below 1 ohm.

The 'IS' earth network went back to the MET in the switchroom via a dedicated earth cable, we basically had 2 earth bars (MCC IS & MCC dirty) that were linked/ could be 'split' for isolation if required because of problems. The use of a 'signal reference grid' is the more correct method of earthig for a new build installation, although this is expensive, although the parallel earth back to the MET path is also acceptable.

In reply to your questions:

Questions:
1. The exposed and extraneous conductive parts (e.g. motors, cable armour, metallic ducts, cable trays etc) are earthed to a different earthing terminal from the one for my Instruments? I suppose yes. Then what earthing terminal is used for my instruments??? Is it connected to the MET of the MCC?

Reply: Extraneous/ exposed conductive parts should be bonded together to create the equipotential zone within the zone of use and connected to a local earh bar/ MCC that is connected back to the MET via the incoming dedicated earth cable. The IS earth reference should come from a local earth bar/ MCC that is connected to the equipotential zone/ local earth bar and therefore back to the MET. The intention being to create a 'MESH' of earth paths that is so low in impedence that any faults on the network are not 'seen' by the IS equipment.

Do you have a seperate dedicated earth path via local insulated earth bars or shared with the sub main/ MCC back to the source MET?

2. What is Clean Earth? Is there a "dirty" one? Is this the earthing system used for the instruments and signals?
Reply:
The clean earth is your IS reference, the dirty earth is the safety earth for power circuits.

3. Intrinsicall Safe Earth is used exclussively to earth Intrinsically Safe instruments? How do we create Intrinsically Safe Earth? Is it different that Clean earth?
Reply: The IS earth is insulated from the dirty earth by virtue of you using an insulated earth bar with a cable connected to a source earth bar...... the 'clean' earth could be a different, but in your case it sounds like the IS earth is the 'clean' earth.
 03 February 2013 05:49 PM
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gkenyon

Posts: 4478
Joined: 06 May 2002

Interesting discussion.

To build on the points raised so far, how can we ensure that "Clean Earth" does not get contaminated with "Protective Conductor Currents"? (and vice-versa)?

In many cases, you cannot, and in fact many items of electronic equipment make a direct connection between the two, e.g. PCs, or guitar amplifiers to name just two.

AV systems are interesting in their continued use (and there are often good reasons for this) of grounding the signal path, often in multiple places, and of course this can cause one type of "earth loop" (*). Many items of Information Technology equipment, on the other hand, use differential, "isolated", signal paths these days ("isolated" is in quotes, because there is often a controlled discharge path made available, but this means of discharge does not affect the effective isolation).

(*) There are other types of "earth loop" interference mechanisms.


So, a "clean earth" will often be the same as the "dirty earth", or "protective earth", and therefore we need to apply some good practice in ensuring that we don't have problems - and this is where BS EN50310, and BS IEC61000-5-2, can be useful.


The differentiator with the Intrinsically Safe system, is that we don't want our "Non-IS" protective conductor currents to flow across the IS earth system, for obvious reasons.

-------------------------
Eur Ing Graham Kenyon CEng MIET TechIOSH
 03 February 2013 08:05 PM
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jcm256

Posts: 1861
Joined: 01 April 2006

The differentiator with the Intrinsically Safe system, is that we don't want our "Non-IS" protective conductor currents to flow across the IS earth system, for obvious reasons.

Yes correct, reading from CompE x sitting in front of me.
Definition
A type of protection that limits the energy available in the hazardous area to a level below that which could ignite a flammable atmosphere.
Two basic types of barrier are in use. The shunt diode barrier and the Galvanically isolated barrier.
I understand both because have diagram in front of me. While galvanically isolated barrier that everyone electrical already has worked with and understands ( transference of energy without directly wired interconnection.
It says here the shunt diode barrier (zener Barrier) you need at least 2 diodes, shunts current in the safe area before entering hazardous area.
A lot more, but two more points>
Earth conductor minimum size is to be 4mm
Impedance to earth , current practice suggests that a value of 0.1 ohm for the earth impedance is desirable. However, the standard permits an impedance of up to 1.0 ohm.
Statistics

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