Dear Friends,

Can any expert of grounding explain in detail that whether the ground/earth for Electrical Power system and Electronic and Control system should be separate or same, and why?

What are the IEC standards related to that issue?

 

I always keep them seperate, if you have a high fault current flow through the power earth, the voltage rise could damage electronic equipment. If the load is reactive higher voltage spikes and harmonics can be even more damaging to electronics.

 

earth is earth. they all connect together at some point.

 

What do you mean by separate, two separate pits for safety and logic grounds? I have read somewhere that these should be separate but also connected to a single point, during whole installation.

Where should I connect my -24V DC?

 

Hi dear,

What is your control system architecture?
It is likely depend on it!

 

It is honeywell EPKS with 03 controllers and 04 PLC's. All field signals are coming in C/R and terminating in marshalling cabinets. There is also a hardwired panel which receive data from PLC's.

Some workstations and printers etc, you know the usuals of C/R.

For field instrument power supplies, all 24 V DC power supplies have a common (-24 VDC) connected together, and this common is connected to instrumet/logic ground.

kkk

 

In Instrumentation, we generally have 2 types of earth pits installed. One pit is called Shield Earth Pit while the other is called Power earth.

While you are designing a big Instrumentation based process control system, we generally have the following equipment.

1. Drives Panels
2. System Cabinets - with PLC/DCS IO cards, RIOs, etc. (Analog IO cards)
3. Power Distribution Panels
4. Digital Panels (Digital IOs)

So we have to separate drive panel grounding as the panel contains heavy noise/harmonics. Secondly, we have to consider ONE redundant shield Earth Pit & ONE redundant Power Earth Pit. This way, we can ensure that even earthing is redundant and this eliminates chances of earth failure.

Power Earth Pit contains earthing from bulk power supplies, Controller Power Supplies, Panel Earthing, Panel Door Earthing, etc.

Shield earth Pit contains earthing from Analog IO shielding cables, Analog signal isolators/safety barriers, etc.

Hope I am right.

Sastry MRKS

 

dear all,
Current day concept is to have one common plant ground for electrical &electronic. What is important is "what is the resistance between the earth soil and this grounding grid". It is always better to make one single ground close to the control room/main DCS. Based on soil's nature to integrate itself with copper rod, you may go for more number of copper grounding rods. Coal powder, sand & salt are used for filling different layers around the copper rods.

Nowadays we have remote I/O racks as a part of DCS in different locations and hence making another grid of electronic earth is of no use. In fact, non-insulated copper electrical grounding cables blends very well with the soil as they are planted in numerous places.

I have seen power plants of 660MW capacities are having single earth bit and running fine in all conditions.

 

Dear PARTHIBAN,
What you describe *should* be the common practice, but there is a certain continent which shall remain nameless and begins with Europe, where there is a large contingent of well-meaning engineers who absolutely insist and demand that there be two earth systems in industrial plants: one for the protection of personnel, usually called protective earth, and one for the instrumentation and control systems, usually called functional earth.

The problem is that most control systems and instrumentation is designed with the concept that there will only be one earth system, as you have described. And, then people try to apply these devices and control systems to plants with multiple earth systems and which exist at different potentials with respect to the soil and to each other. And, they do this as if there were only one earth system, not multiple earth systems.

And this leads to lots of problems. More than you can possibly imagine. Finger pointing; screaming; threats of lawsuits; lawsuits; with-holding of monies; lots and lots and *lots* of wasted troubleshooting and investigations and reports and recriminations. It's just sickening.

Unfortunately, most of the world has adopted or is adopting that continent's earthing philosophy, without adopting proper plant and earth design and construction practices. And without understanding all the implications of not properly designing and constructing and operating and maintaining the earthing systems. Control and instrumentation manufacturers have to start demanding that plant designs properly support the multiple earth systems.

The intent and the purpose of having separate earths are all ideally fine; but the implementations usually are less than ideal and very poorly constructed and maintained.

As you have noted, one single earth grid (pit) will work just fine. In fact, most industrial plants were built that way for decades, and operated just fine for decades. But, there always people who can conjure up all manner of calculations and theories to say that this or that new method is better.

But, if not implemented properly it's actually worse. Much worse.

 

Since we are talking about grounding, we need to ground analog instrument signals because analog is more prone to be affected by noise while digital signal grounding is optional as noise has less effect?

Normally we have a pair cable with an earth wire and connect each wire to individual termination and then to a common instrument earth bar. But when we run out of space we just leave the digital earthless.

Also if you connect Power & Control earth via a common earth if say there is an earth fault won't it be dangerous to the instruments? Normally Power earth cables are usually much thicker due to the high voltage? Also power cables are usually connected to Circuit Breakers while controls are connected to DCS/PLC so I'm guessing having separate earths are not a big problem?

 

Where can I find specifications for the designing of instrument earth pit for small DAS with appox. 200 analog IO’s (intrinsic safe).

Plus is it really mandatory to provide a separate earth pit for this size of DAS.

I think if we go for seprate earth pit for instrument signals we mean to connect shield wires of the pair cables. And DAS panel can be connected to electrical power protective earth. Is it so.

 

Dear Friends,

> Can anybody tell me that do we require IS and NIS earth pit as separate? What are the IEC standards related to that issue? <

I am following the philosophy of providing different earth pits for safety ground and instrument ground..

 

I think it might be economy behind the question "one or two grounding networks?" If we advise two redundant groundings,meaning four instead of one or two,we confuse the questioner.

thanks much
[email protected]

 

Economy <b>is</b> the question. And the reasons for having a second earthing network (grid), isolated from any other earthing network, is a matter of economy, also.

Damage caused by lightning strikes or high-voltage or high-current faults to control systems connected to one single earthing network can be very costly, in terms of lost production, as well as damaged components.

Two, independent earthing systems, one for safety purposes (protective earth) and one for control systems (functional earth) makes a great deal of sense. But, it's extremely difficult to implement a second earthing system in an existing plant.

And, without properly designing a plant and all of the instruments and field devices to properly isolate two earthing systems from each other one is simply defeating the purpose of having two systems, no matter the cost spent to design, construct and install.

My issue with separate protective- and functional earths is that they are not properly isolated from each other in most plants, and worse, they are not typically constructed properly so as to minimize any potential difference between the two. Usually, the functional earth network is very small in comparison to the protective earth network at a plant, and during construction (and subsequent operation) people tend to connect things to "earth" without regard to whether or not it should be connected to protective earth or functional earth.

It's all about economics. The money can be spent now, or it can be spent later. Usually, money spent later is more costly than money spent now. But, it's also about the implementation. The best intentions can't always overcome the reality of the situation.

 

some resources:

1) MTL's tech note TP-1121-1

http://194.203.250.243/Download.nsf...e19b058b45002569f2004e4a25/$FILE/Tp1121-1.pdf

2) The current book on the subject:
Title: Control System Power and Grounding Better Practice
Authored by Roger Hope, Dave Harrold, David Brown
ISBN-13: 9780750678261, ISBN: 0750678267
a trade paperback published by Newnes

A good portion of the book appears as the basis of 3 articles in Design Line (only two of which I can find online:

part 1:
http://www.industrialcontroldesignl...=208803203&queryText=Control+System+Grounding

part 2:
http://www.industrialcontroldesignl...=209100218&queryText=Control+System+Grounding

 

I am following the philosophy of
providing different earth pits for
safety ground and instrument ground..

 

> I am following the philosophy of providing different earth pits for
> safety ground and instrument ground..

what is the logic in maintaining different earth pits when earth is one big mass. can any body explain how two earth pits are isolated.

 

earth grounds are not zero impedance. if you have short term currents in the power system ground of several hundred amperes, you will get significant voltage rise at the ground connection.

you take systems to earth for safety, but all instrument grounds need to come to a common ground, the control room instrument ground, and from there to earth, it's own set of ground rods, etc.

the controls need a common reference point to control the noise problems that occur in plant environments.

 

Dear Friends,

While I am not an "Expert", with over 40 years in Process Controls and Instrumentation there are 2 areas that can be effected grounding. #1 is Earth Grounding and should be continuous throughout and #2 is the Isolated Ground which is separate from EG throughout the network.
This should be to isolate ground-loops from Instrument and Power problems. Both should be grounded at the source and only the source. EG will be connected by ensuring all conduit and fasteners are tight. The IG would be a separate wire through the Control System.
Further reading can be found in the ISA "Green Book" and while I am no-longer in the trades, the IEC should also cover this is Section 250 on Grounding.

Good Luck
Jim

 

Dear Friends,

I am newbie to this field and this is my first plunge to project engineering...need your guidance

I have a system which has it's 24VDC common connected to the Chassis ground, because of this the DC ground cannot be isolated from the power ground. This is how the system is designed, unfortunately!

My question to you all is :

1) How much should the resistance value be ?
2) When I check the resistance, should the system cabinet be powered up or powered down ??

Regards..
-S