Objectives of Earthing
The field of Earthing or Grounding tends to conjure up different perceptions to different people. Hence a summary of the objectives of earthing is always useful.
Objectives of Earthing: as related to Power Supplies.
All power circuits comprise of a SOURCE and LOAD:
Typically the Neutral (or -negative pole of a DC supply) is bonded to earth. There are 2 objectives in doing so:
EARTH FAULT PROTECTION: The earthed neutral ensures that an abnormally high current will flow during an earth fault. This current will cause protection devices such as fuses and circuit breakers to operate quickly.
OVER VOLTAGE PROTECTION: The power supply earthing plays a critical role in reducing overvoltages (with respect to earth) that would otherwise be caused during conditions of Earth Faults and Transient Over-voltages.
For the sake of brevity we will not explain the mechanisms of how system earthing reduces overvoltages.
The items that draw power can be considered as equipment. All metallic items such as steel equipment enclosures, racks, etc. which could become "live" (by live power conductor accidentally touching the metallic item) must be earthed. The objective of this requirement is singularly and simply SAFETY. However, there are 3 mechanisms which come into play:
Potential Equalization via "EARTH BONDING": Assuming that "earth" is at zero volts bonding enclosures to "earth", should render the enclosures safe. Here "earth" must be related to the environment within which equipment is placed. "Earth" could therefore mean mother earth, steel structure of factory or the water pipes in your kitchen. The definition of earth should be considered as: The metallic conductor that is normally at zero volts & likely to complete a circuit which the human body (and animals) makes when in contact with live conductors. Hence earth could constitute the kitchen tap, concrete floor or hand rail.
Potential Equalization via CROSS BONDING: Assume a plastic environment exists (except for the metal equipment enclosures). Assume two items of equipment are within touching distance. Then assume that the potential of one item is caused to rise above earth (by say an internal earth fault) whilst the second item remains at earth potential. There is an obvious shock hazard to anybody touching both items of equipment simultaneously. Earthing conductors will minimize this hazard and short direct bonding between the items will eliminate the hazard.
The earthing of equipment allows protective devices to isolate the power from faulty equipment. This minimizes the duration of exposure to the hazards of shock and heat (caused by high fault currents).
Earthing only plays a protective role during abnormal conditions caused by earth faults and overvoltages. Although there are exceptions, we can say that Earthing does not enhance or degrade the functional aspects of electrical systems during NORMAL conditions.
Earthing (or Grounding) as related to Electronic Circuits
If perfectly normal conditions could be guaranteed Earthing would not be required for most electronic systems. (The exceptions would be systems that rely on earth paths such as LF radio and certain telephone systems).
Also similar to power systems, we must consider the "circuits" interpretation of earth, e.g.: your car radio considers the car chassis as earth; marine electronics "view" the ship's steel structure as earth. These and other systems, like portable calculators, are "not interested" in mother earth.
Let’s assume we are dealing with an electronic control system. The control system can be designed to be totally isolated from earth. This requires that the DC power supply must be totally isolated from earth i.e. NO CONNECTION to chassis.
However, all signals must be referenced to a common point. This is usually one leg of the DC power supply or the center of a symmetrical supply.
Theoretically such systems which FLOAT should be able to perform with great reliability. In fact they do give the best reliability of all systems, e.g.: pocket calculators. In these cases the systems are physically small however, and self-reliant with regards to power. For systems which connect to mains power and signal cabling, the word "isolation" becomes relative, i.e.: absolute isolation is now not possible due to capacitive and inductive coupling mechanisms.
The degree of isolation can be expressed in different terms:
Q = "Does a small battery operated device give reliable service because it’s isolated from earth?"
The answer is ‘no’. Its reliability stems from its physical size and not because it floats. Its small size ensures that:
The zero reference remains at zero volts throughout the little system.
The small dimensions ensures that there will not be any significant coupling to earth (via stray capacitance).
The small dimensions ensures that there will not be any significant "aerial affect" which cause pick-up of external EMI fields.
Q = "What would happen if we took a wire and bonded the zero reference (say negative pole of battery) to earth?" (Assume we are still dealing with a portable system).
A = There would be no change. If, instead of earth, you took the wire to a 1000V AC generator, the portable device would still operate reliably.
Large isolated systems pose the following problems:
The systems need AC mains or UPS power. These supplies are solidly earthed by law. Some coupling will therefore exist between the electronic system and earth. This can only be minimized by super isolation transformers which are seldom installed by system manufacturers.
As the signal cables become longer the signal conductors become capacitively coupled to earth.
The combined result of these two factors is that the system is only isolated in DC terms. The stray couplings mentioned in a) and b) have an impedance that is frequency dependant, i.e. from AC to the higher frequencies the coupling becomes stronger and stronger.
The system therefore presents itself to EMI as a return path to earth. As all EMI fields are referenced to earth at their source, the system will carry EMI current.
As the signal cables become longer, so their "aerial effect" becomes more significant. This is because they will pick-up or resonate at ever decreasing frequency as their lengths are extended. The danger of this is twofold:
- Field strengths of EMI fields tend to be stronger at lower frequencies.
- The bandwidth of the system and the frequency of the EMI field tend to coincide at the lower frequencies.
Depending on the physical orientation of the cables, an "open transmission line" effect becomes more appropriate a description. However the mechanisms are similar.
As signal cables become longer, static DC charges can build up to significant levels.
Q = "Will it help to earth the zero reference of the electronic system?"
The answer is Yes and No. ‘Yes’ because:
The problems a) and b) (whereby the floating system provides a unpredictable path for EMI currents) is reduced by providing a lower impedance (hence lower voltages are developed) path to earth.
Problem c) is reduced by converting a relatively large voltage picked up into a relatively small current.
Problem d) is similarly reduced.
Problem e) is totally eliminated in most cases.
However there are negative effects i.e. ‘No’ because:
The grounding of the signal zero does have a potentially negative effect of changing a harmless common-mode voltage into a series-mode voltage. This can cause problems for high input impedance, voltage amplifier circuits. However these are less common in industrial instrumentation where most signals are current driven and received.
Should any signals become earthed at their remote ends (field instrument) then the loop formed becomes very susceptible to low frequency magnetic fields.
The above pose major problems, especially to Analog systems.
Fortunately the decision of whether the signal reference is grounded or left floating is seldom left up to the end-user. The manufacturers predetermine this during the design stage.
Because of the above Yes / No's, compact / extended, unknown site earths, etc., manufacturers often have to compromise on the best of the two evils. You will find, for example, that the analog system reference is grounded via a HIGH PASS filter (we endorse this practice). Conversely, digital system manufacturers sometimes ground the logic reference via a LOW PASS filter. This is usually only good for compact systems where the major threat is from mains bourne transients.
The point is: once you find that the reference is solidly grounded, then you must realize that the quality of the earths that you provide becomes more critical.
To summarize: the large floating systems (compared to small systems) tend to be more susceptible to EMI, which is reduced by earthing the electronic reference.
The measure is but one in a package of EMI measures that must be taken via a Systems Approach.
The above is rather philosophical and probably raises more questions than it answers, e.g.:
"How do you affect the bond between the reference and earth?"
"What is earth?"
"Should it be done at one point only?"
"Should cable screens be earthed?"
"Which end? Both ends?"
"What is a clean earth?" etc.
The above questions can only be addressed after you establish bandwidths, potential EMI sources, electrical lengths, etc. This will take a text book to cover.
Q: Why is Earthing Important?
It is common knowledge that Earthing, and especially Bonding, plays a most dominating role when it comes to personnel Safety and prevention of Fire and Explosions.
Yet this is only half the story.
What is seldom appreciated is that EARTHING plays a fundamental role in preventing over-voltage conditions. Hence it impacts on the short and long term LIFE of electrical equipment, such as the motors, lights fittings, etc. and electronic systems. At the low cost of implementation there is no measure that is more cost-effective.
From the point of view of Preventative Maintenance, it is an absolute gift.
From the point of view of Risk Control, it is an absolute must.
The objectives of earthing are therefore multi-faceted and herein lies its IMPORTANCE, namely:
Earthing is the one common denominator which controls over-current and over-voltages during abnormal conditions.
The technical explanations of the above are best understood by attending a 30 minute presentation at our offices in Randburg. We make use of dynamic graphics (as used in animated cartoons) to demonstrate the simple principles involved. Using this computer technology as a training tool, non-electrical engineers have had no difficulty in grasping the mechanisms, which are considered as confusing to many electrical people. The short presentation is an extract of our Computer-Based Training course.
Q: Why is Earthing often sub-standard?
If earthing is so important, why is it so often found to be of such a LOW STANDARD?
Firstly, you must appreciate that Earthing plays no role during normal conditions. Its functions only come into play during electrical faults or lightning conditions. For most of the time it is totally dormant. Under normal conditions, earthing systems should not carry any electrical current at all.
Hence, we have a problem in that there is no natural or automatic monitoring taking place. Therefore, deficiencies are never detected during the life of the plant. (Until something nasty happens, and deficiencies are noted after damage has been done.)
Secondly, Earthing has no direct bearing (in the SHORT TERM) on the PERFORMANCE of productive equipment such as motors, etc. For this reason Capital Expenditure (at Project stage) on Earthing is normally limited to what is required by LAW only. The law in turn, is only concerned about aspects of Safety and not in the other long term benefits of earthing.
The practical enforcement via Inspection and testing in this field is almost zero for many man-power related and technical reasons.
Thirdly, the low standards are caused by lack of understanding of the technical issues at all levels from consulting Engineers and Authorities to end-users.
The lack of motivation at contractor and electrician level makes matters worse because it is to these people that the task of earthing is most often delegated to. Any person who has attended our course will testify to this statement.
Q: What are the objectives of Earthing?
I am not an engineer, so give me a basic explanation of just 2 of the objectives of Earthing.
Earthing forms an essential component of the subject we electrical people call Overcurrent Protection.
Firstly, lets get a feel for the power of your electrical system:
Take the Authorized Demand figure on your electrical account. (Say its 2000kVA, this is about 2 megawatt or 3000 Horse Power). Multiply this figure by 2 and now imagine this power (twice the amount of power consumed by your whole plant) is concentrated at one point. This is what happens when a fault occurs on your distribution systems. The potential for damage is enormous and can be likened to a large dam bursting. (We call this "fault-levels")
This is where earthing comes in:
The earthing system has to somehow CHANNEL this potential energy back to where it came from. Here is objective Nº 1: “Provide a PATH via which large amounts of destructive energy can return to their source, in relative SAFETY, i.e.: safer than say, the aluminium ceiling (which is also a conductor) above your office chair.
Enhanced Over-Current Protection
Once this power flows, we obviously must “seal it off” - with the shortest possible delay.
This is the job of FUSES, CIRCUIT BREAKERS and such protective devices. These devices are rather primitive in that they understand only ONE "language" namely: “current”. In practical terms, such devices will only do their job of rapid disconnection if they “see” lots of current (at least 10 times their rated capacity).
Hence the second OBJECTIVE of earthing is quite IRONICAL and confusing: to encourage the MAXIMUM available current to flow during fault conditions, in order to ensure that the PROTECTIVE devices isolate the power with minimum of delay. Thus, the best fuse in the world can be rendered totally useless by ineffective earthing.
To summarize then:
Earthing must provide a safe return to the source of power.
Earthing must provide an easy return to the source of power – to encourage the maximum current to flow, to ensure rapid sealing-off of the destructive energy.
Q: What about electronics systems?
I have an electronic system which occasionally gives trouble. The supplier says that it is my power supply and mumbles something about earthing. He recommends I purchase surge protection.
What should I do?
Firstly, let us presume the cause is either power and/or lightning related.
Your first option is to call in the supplier’s mate who sells surge protection gadgets. If it is a larger system, then consider the following:
Many gadgets will need to be purchased. One gadget will not do the trick, unless you have been lucky enough to identify the exact source of your problem.
The gadgets work on the principle of short circuiting the excess voltage to earth.
This has many implications:
1). The earth itself must be of low impedance. This is easier said than done.
2). The earth leakage unit will probably trip and cause total power loss.
3). If you bought a cheap gadget it will self-destruct and cause a worse problem.
The gadgets are reactive devices. This means they can only treat the symptom and do nothing to eliminate the CAUSE or origin of the surge.
The surges therefore will continue to stress your equipment. Admittedly the stress level may now be below the damage level, but the repeated stress over time will lead to equipment failure.
We could write a book on all sorts of other negative scenario’s our clients have endured before they called us in.
If the equipment involved is a single PC or a household alarm system, then fit the gadgets and hope for the best.
The other option is to seek the advice of matra. The chances are that we will be able to identify the source by inspection or testing. Our next step will be to assess how to cure the cause at the source (and not the symptom). The cure will probably not require gadgets.
Our recommendations rarely require the fitting of gadgets.
The end result will be a report which will outline the cause and provide a list of fully illustrated measures that you should implement.
This investigation will cost you money but it will result in a more cost-effective overall solution. Our fees are always quoted in advance and are Fixed, Firm and All-Inclusive.
Q: What about the Law and good practice?
I am still NOT CONVINCED, why can I not rely on the LAW and GOOD practices to provide the necessary protection?
This is best answered by statement of a FACT:
Notwithstanding all the Laws, etc. ELECTRICITY IS THE BIGGEST SINGLE CAUSE OF FIRE (and is becoming increasingly so with each year).