Instrument, protective and Clean Earth? 3

[SL1000]

Can anyone explain the difference between the above?

In the main the diffrence between the clean earth and instrument earth.

Thx

 

[dogleg43]

Because grounding is such a misunderstood topic, your question will undoubtedly open up a can of worms.

There are two books that do a good job of explaining this: “Control System Power & Grounding Better Practice (publ. by Reed Business Information) and a book on grounding by Soares (sorry I don’t have any other info with me)

I’ll do my best to explain it and let others correct or add to my explanation:

Instrument grounds (for transmitters & other low voltage devices) are usually ran to an isolated ground bus and then an insulated ground wire is taken back to the AC source ground. It is common to the Protective and Earth grounds.

Think of protective grounds as the wire pulled to the ground terminal on receptacles and motors, etc. They normally terminate on the non-isolated ground bus in a lighting panel or MCC. It is common to the Instrument and Earth grounds.

Earth grounds (and here’s where I might be a little unclear) tie the building steel, tanks, and other conductive framework, etc. together. It is also common to the Instrument and Protective grounds.

Finally according to the NEC, at some point all of these grounds must be tied together. Generally, it is OK to drive a separate ground rod, but that ground rod MUST be tied to the other main grounding system. Generally, it is NOT permissible to have a totally separate ground unless you are using a generator. I’ve used the word “generally” because odd configurations are out there and may be an exception.

 

[BrianG]

As far as I know, at least in Europe, there is little or no difference between "Instrument" and "Clean" earth and the terms are often used interchangeably. The main purpose of course is that devices such as transducers and other low current components in a control system have a clean common reference point for measurements without earth leakage currents or induced voltages getting in to skew the readings. (For this reason in some systems they even go to the trouble of using screened cables in the earth circuit)

Protective earth is heavy duty stuff which is designed to blow fuses or trip breakers in the event of a fault which would cause the casing / body of equipment or appliances to become "live". Large fault currents must be handled by such wiring, and the point of connection to the building ground or earth rod must be carefully controlled so that these fault currents don't get into signalling circuits by mistake and cause lots of damage.

 

[SL1000]

Thanks for that info guy

so do I take it if clean and instrument earth are interchangeable that IS circuits could use them for the point of termination of the screen or barrier earth?

 

[sreid]

I highly recommend the book "Noise Reduction Techniques in Electronic Systems."

http://www.amazon.com/gp/product/04...104-6543385-3656727?s=books&v=glance&n=283155

 

[blackwed]

SL1000, If "instrument ground" is the same as "supply common" in Figure 4.5 of ISA RP12.06, I believe you are correct. At least if you were in the USA.

The thread has been a big help to me. It got me to doing some research and I've been saved installing a few miles of blue wire. Stars to all.

MTL Instruments' TP1121-1 covers your situation (and many others).

DB

deja moo: I've heard that BS before.

 

[stevenal]

"Clean" in relation to the subject of grounding was invented those who advocate violating safe grounding practices. Best to avoid the term. IEEE agrees, see IEEE 1100 sec 2.3.

 

[dpc]

As stevenal says, whenever someone starts talking about "clean" ground, it's best to walk away quietly.

Your implementation is going to be heavily dependent on local codes and customs.

The NEC does recognize the use of isolated grounding for certain situations, but this isolated ground is still connected to the overall system grounding system at some point.

Just keep in mind that a lightning bolt that travels 30,000 feet *through the air* is not going to be too impressed by any ground isolation/insulation techniques. So Keep it Safe.

 

[Skogsgurra]

I have to add some negative points of view. There are very few "clean" grounds nowadays. Frequency inverters and their high-frequency stray currents (via motor capacitance, cable capacitance and filter capacitors) have been polluting the good old quiet Protective Ground/Earth for a couple of decades and the pollution grows exponentially.

The "inverter infarct" that we see coming has already made fiber optics absolutely necessary in many installations - weak signals don't survive in a cable that has its screen connected to an HF polluted PE.

It is a sad state of affairs. The only way out seems to be a low-impedance ground plane across the whole installation. Not easy to implement. The next best thing is a mesh of heavy gauge copper wires bonded to each other and to building steel, cabinets, control desks, tubing, machinery and tanks. It has been done in pulp and paper mills and it does work.

Gunnar Englund

http://www.gke.org

 

[SL1000]

Excellent discussion.
skogsgurra you make a good point about ground pollution and screens which brings me on to my next point.

I have seen varios screen connection systems for an IS installation all with their own system certificates. But wondered if anyone could explain the reasoning and benefits behind them. i.e.

1. I have seen the screen terminated into signal common at both ends, field and panel (earth free).
2. I have seen the screen terminated in the field to local earth with the panel/isolator end terminated to signal common.
3. Screen cut back at field end and terminated to IS earth at isolator/panel end.
4. Screen at field in sig common terminal and to the IS earth at isolator/panel end.

Obviously these were isolator and not barrier installations. and I believe the differences are probably to accomodate differtent practical installation requirements. Other than that what are the main influences from an EMC point of view? (and safety of course although I believe all are capable of IS certification) correct me if I'm wrong and I'll try top give more info if neccessary.

Thanks in advance...

 

[Skogsgurra]

There are some technical reasons for the four different techniques mentioned by SL1000. And there are misconceptions and there are myths.

As I see it, it started with audio amplifiers and microphones more than sixty years ago. Microphone signals were (and are) low level and the amplifiers had lots of amplification. So any "hum" picked up used to be very disturbing. Protective earth was not very common in those days and the result was hum pick up in the microphone cable. Screening helped to some extent, but if the screen was connected to earth at the microphone end, stray currents from amplifier through screen to earth induced hum in the cable. Isolating the far end of the screen helped a lot. That is how the first rule: "Connect screen in one end only" came about. And also how the dreaded "hum loop", which many people still refer to as if it was an eternal truth - valid in all installations. Which it is not.

Second scenery: Digital communication like RS232C and other crude techniques made its debut. These signals had immunity levels around a couple of volts and the "hum" was not really a problem any more. But transients and HF pollution were. Transients are HF, so the technique where the screen was grounded wherever possible started to be used. Many ground connections allowed transients and HF to find its way to ground without going through the input section of the DCT/DTE. So, "ground whenever you can" started to be a sucessful strategy.

The earlier communication lines were mostly intra-building and seldom more than twenty or thirty metres. Short-haul modems (remember the KM-1?) were used for longer distances.

When field buses made their entry, longer distances could be covered. Often between buildings. It then happened that the supply system for one building was not the same as for the other building. A quite common situation, actually. The result was that current was flowing in the screen if it was connected to the different grounds in the buildings. And if there was an earth fault in one building, the potential rise forced large currents through the screen and it very often scorched the screen and cable so that equipment failure resulted. That is when users started thinking.

The thinking resulted in different ways of solving the problem. There are simple means like grounding via HF capacitors, using carrier and isolating transformers, using opto-couplers, systems with enhanced common-mode range (like the RS485) and also bonding of the different buildings to a common potential. Every manufacturer and every communication standard now have found their preferred way of handling the problem with connecting screens and these installation are often very reliable. It is in special situations and when equipment from suppliers with different philosophies shall be connected together that problems start to pop up.
Those that haven't thought the problem through will then grab one of the earlier myths and try to apply the "avoid the dreaded hum loop" principle or the "ground everywhere" method. Sometimes both at the same time.

The best way of getting out of the mess is to analyse the situation and use the standard solutions that are available. Be it isolation amplifiers, fibre-optics, extended common-mode range, carrier frequeny and transformers, differential amplifiers or equipotential bonding throughout the plant.

But do not go with myths - and do not believe that old truths are always the only truth.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

It is worth mentioning that the techniques employed depend heavily on the source of the interfering signal. Capacitively coupled - or E-field - noise is usually attenuated effectively by electrostatic screening and by use of ground planes. Magnetically coupled - or B-field - noise essentially doesn't care a jot for electrostatic screening and requires different techniques such as twisted pairs and possibly mu-metal screening. The 'dreaded hum loop' is susceptible to inductive coupling and is very much real if there is a source of B-field noise. Large air-cored inductors and reactors are wonderful sources of B-field noise and the cheapest countermeasure is to put distance between the 'victim' equipment and the source.


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Your body may be a temple. Mine is an amusement park...

 

[VEBill]

"...the techniques employed depend heavily on the source of the interfering signal."

And then there's lightning to consider...

 

[ScottyUK]

Hmm, or should that be 'hum'?

I saw the inductive loop problem first hand by inadvertently forming a closed loop between a Tektronix AM503 current probe and the DSO we were using: the loop was formed by the earth conductors of two mains leads and the screen of the BNC lead between the instruments. The interfering signal was from the load coil of an induction heater which we were working on. The waveform of the CSI thyristor currents was peculiar to say the least: the nice trapezoid waveform in one limb of the bridge had a gentle 'dip' on the expected plateau, and the waveform in the other had a matching 'hump'. Took a while to work out what was going on! No lightning though... do your instruments get struck by it a lot?


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Your body may be a temple. Mine is an amusement park...

 

[SL1000]

so good stuff above but I've not absorbed it all yet..

1/ So what does tying the signal common (or return) to the screen actually acheive. And how does this help?

2/ Also am I right in assuming that in the talk of circulating currents (in the screen or cable armouring and earth) is due to differences in potential of the equipment at different locations i.e. you have a pd to drive current around the circuit. Hence you only earth carefully at one point ? But I have also heard that substantial current circulates around plant structures anyway. Is the earth at one point to protect against this also. Are these two separarte arguments catered for by earthing at one point??

Cheers hope this makes sense