Instrument and Electrical Earth Bars

Dear all,

Instrument earth must design to minimize the potentials among instrument earth, safety earth (= electrical earth, main earth, power earth ) and conductive part of building for instance re-bar of the reinforced concrete, by bonding each other.

If some reason, potential of safety earth raises, instrument and building potential follow the safety earth's potential. Thus there is no potential difference.

This "no potential difference" is what we want to achieve by earthing design.

Safety earth and instrument earth are better installed side by side and bond with short wire to keep the potential difference as little as possible.

Regards

Usually there are practical restrictions at site to put the different earth systems side by side and I doubt it if there is such a statuory requirement.

Common successful practice is to install seperate earth pit/rod for each system and later join these (pit/rod) by a bonding jumper, I am not suggesting jointing of earth continuety conductors of the different systems together at any stage.

In case the different earthing systems are displaced considerabley apart, OP can take care of worries by installing parallel earthing pit(s) for any of the dedicated earth.

Dear all,

The reason to install an instrument earth (or signal earth) is to minimize the potential between instrument's signal reference and its nearby conductive body.

The nearby conductive body is, probably, building structure. So, instrument earth must be connected to the building.

Safe earth is already connected to building, so instrument earth to connect to safe earth is a natural idea.

To have instrument earth pit and safe earth pit is not necessary if two earth system is combined inside the building.

I often see such specifications to have to have purpose wise earth pits, such as safe earth, instrument earth, intrinsic safe earth, shield earth, lightning protection earth. Then I discuss with the client to revise the specification to combine all earth system into one. If the project is eXtension of an existing facility, to combine all earth system is difficult, it is actually case by case.

Different earth pits are bonded outside the building, so, why not change to bond the all inside the building.

By integrating all earth system into one, and connect to building will make a best connection to the earth soil. Building foundation is much better earth electrode than any earthing rods.

Regards

Akihito Shigeno: My electrical system is an EHV grid station with system voltage 1000 000 Volts (1000 kV), in the event of an electrical fault voltage decay in electrical earth conductor does not occours instantaneously it gradually decays to zero in a matter of milli seconds.

If one follows your advise the bonding of instrument and electric continuety conductors will cause the instruments to be momentarily subjected to several times the rated value causing the instruments to burn in the event of an earth fault.

Another point is that the power station is several hundred kM away from my grid station, Please tell me how to interconnect the two earthing systems together for your satisfaction?

Dear all,

Even EHV or Lighting strike, there is potential a raise in earth system.

By connecting safe earth bar and Instrument earth bar in the building, you can keep the both potential the same. Thus there is no potential difference among electrical, instrument and building.

If instrument is connected with other instrument inside the building, such instrument is still safe, because there is no potential differences between instruments.

If instrument is wired with other instrument outside the building, the signalling must be independent from the earth potential, using such as 4-20 mA loop (only one point earthed), RS485 (differential signaling), potential isolation (such as photo coupler) or Fiber Optic link etc.

If the destructive potential raise, such as lighting or earth fault circuit is concerned, instrument need isolation (isolation transformer or photo coupler) and surge protection (TVSS).

You can not have same potential between facilities some km away. If signalling is required between such facilities, we must select equipment which will work under potential difference between two locations, using such as power line carrier or FO link. If the classic pilot wire is still used, functional and safety measures, such as isolation transformers or neutralizing transformers have already been included when they were installed.

Regards

So there's foreseen problems as to where the two earth bars are bonded? (ie. in the switchroom, near the electrical equipment / away from the pit)?

Your point is right the Instrument and electrical earthing link is bonded as per my previous projects at a single point. this point is near the two earth pits of the power earthing and safe earthing pits.

During any fault occurance since the shortest path is taken by the fault current to flow, the fault current cann't sustain a long distance when it has already come across an earth pit. The System should be designed in such a manner that the fault current flows to the shortest point and the fault current is minimised.

Hi!

Earlier earthing system for both electrical and instrumentation used to be from the same earthing network. However, with the extensive computerisation of Instrumenation Engineering and its metamorphosis from pneumatic to electronic system a huge susceptibility of Instrument system to voltage spikes has crept in as you rightly pointed. Now-a-days our Instrument colleagues are seeking and ensuring insulated earthing for their systems which is completely isolated from electrical earthing system.

Even if we have well designed and well maintained earthing system (which we anyway have) still it is preferred to have separate earthing for instrument systems. We practice it this way only. The thread you have referred to seems to be outdated and obsolete.

BBRaina

Dear all,

Both safety earth and instrument earth need to be connected to building structure, to total isolating of instrument earth is not possible.

Even isolated grounding in NEC requires one point of isolated earth to be bonded to electrical earth. Other than this point isolated ground is isolated form other earth system.

Regards

You are absolutely correct.The isoltated ground (Orange receptacles) are still required to be grounded at a single point.The purpose is to eliminate the possiblilty of loose conduit connections inducing noise on a data line.This gives an assured clean ground for the computer power supply.

A dedicated computer or data processing building can require a copper grid to be imbedded into the concrete, and all cross-points cad(Thermite) welded for integrity, and this in turn bonded to the electrcial system ground.

HTRN

Here in the USA, the National ELectrical Code prohibits multiple grounds unless they are bonded together.The purpose is to give ground faults a single return path.Multiple paths can induce dangerous voltages across very low reistances.

Consider a 10KA surge, typical of a lightning strike neraby, or a dead-short to ground throught a 1 ohm resistance.The voltage developed is 10KV.This is enough to kill personell or destoy equipment, or start fires.

All grounds must be bonded together to reduce and hopefully eliminate multiple ground paths.I have heard arguments for and against separate equipment grounds, and if they are bonded to all other grounds, that is ok.But an isolated ground rod is asking for more trouble than it will prevent.

HTRN

Licensed Unlimited Electrical Contractor

Bonding together of earthing system implies to bonding of earting rods/pits of different systems and not the bonding together of equipment, the two are different.

Take into account two scenarios (1) the electrical system 10 kV earth conductor is first connected to an instrument(s) and later reaches to the earth rod/pit (2) the electrical system is directly connected with earth rod/pit and later that pit is connected with instrument earth rod/pit.

In the first case in the event of fault the instrument(s) are subjected to voltage much higher than its bearing capacity. In the second case the 10 kV fault goes to ground and travels shortest distance to the transformer neutral or to generator neutral thus the instrument(s) are not sbjected to high voltage although the two earthing pits are jumpered together for an equipotential bonding.

Since at times the above crucial difference is negleted I agree with BB Raina that there is a rising trend to use Isolation transformers to keep the instrument system totally seperate from the electrical system in all respects, here instrument active circuitory are sometimes not earthed at all, commonly reffered to as floating earth.

One practical example is industrial UPS check for yourself Isolation Transformer has been buit in by the manufacturer.