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What's the problem in grounding systems used in buildings ?
Érintésvédelem Szabványossági Felülvizsgálat
What's the problem in grounding systems used in buildings ?
NOTE: This documents tries to give you general information about electrical wiring inside building. This documents is only written to give a general understanding of some of the most common electrical wiring systems. This document is not ment to be an accurate description of all wiring systems. Don't do any electrical work based on information provided here. Leave the work to professionals who know all this and the local regulations.
In the construction of most commercial buildings, one ground is usually run throughout the building to keep the impedance as low as possible. Low impedance in the ground is needed to makse sure that the the fuse blows when something gets short circuited to ground wire (for example cable insution breaks inside the equipment and touches the grounded metal case). The grounding system is primarily designed for electrical safety in mind. The Protective Earth connection should be able to carry a heavy current to protect the user from live-to-chassis faults by ensuring that the fuse or will operate so the requirement is that the Protective Earth conductors can carry a 25A fault current for at least 1 minute. The grounding system in the building electricity distribution has only effect on the electromagnetic environment inside the building which you must understand.
Unfortunately all building have big electrical equipment such as air conditioning units, refrigerators, washers/dryers and other high current devices connected to the building ground (the same ground you use for your AV system). Computers and other equipments which use stiched power supplies generate harmonics to the electrical power which usually end up being noise in the gounding system. Light dimmers are also a storng source of interference and some of it typically ends to the ground wires also. Thus the chances of getting a clean ground in a typical audio visual installation is slim, especially in large commercial buildings, hospitals or convention centers.
Grounding practices in buildings
Electric power distribution throughout Finland and many parts of Europe is made by 230/400Vac, 3 phase, four wire, Multiple Earth Neutral (MEN). One, two or three phases are brought into the customer's premises depending on the maximum demand. This applies to both residential and business premises.
For those not aware of the MEN system, the neutral bar is connected to an earthing stake driven into the ground as near as possible to the customer's switchboard. All earth from power points, etc, is connected to the neutral bar. In the UK the same practice is called Protective Multiple Earthing (PME). With P.M.E. the neutral and earth conductors of the supply are combined. The supply company connects the neutral solidly to earth frequently throughout the distribution network. At the customer's connection point the company supplies an 'earth' (which is actually connected to the neutral) to which all the installation earths and equipotential bonding are connected.
Another approach to bring grounding to the building is to bring it through armouring of the supply cable. If the electricity company cannot easily supply or guarantee an adequate earth conductor (for example supply comes on a pair of overhead wires), the ser is generally responsible for the adequacy of the earth electrode. The method of earthing can normally be found out by tracing the wiring from the meter/consumer unit. It is usually fairly obvious.
How Good Should the Earth Be ?
This is a difficult question to answer; in general the impedance of the earth connection must be low enough to ensure that sufficient current can flow through the protective device so that it disconnects the supply quickly (<0.4 second) and that voltage on the earth connection does not rise more than 50V. Measuring the resistance of an earth electrode is not easy and should be left to professionals.
Safety if the neutral in the feeding is cut
How safe or unsafe MEN or PME is depends on the rules which cover its application, and the record of the supply utility in avoiding neutrals going open circuit.
The key word in the titles is MULTIPLE. The exact situation will vary dramatically depending on where the break in the neutral occurs. If it is just outside the substation, then the neutral conductor will be replaced by all of the multiple earths in parallel, and providing the load is balanced over the three phases, the voltage disturbance will not be too serious.
When the supply company neutral goes open, the neutral return is via the earth stake. Soil types here range from sand to loam to clay to rock, so the ground stake can range from a good to a very poor earth. The voltage from each active to the "neutral" will depend on the loads on each phase and the ground resistance.
The worse the balancing of your load over the three phases, the worse the voltage disturbance will be. If we assume that the earthing spike has a resistance of 100 ohm, it is pretty clear that your equipment is not going to work, but your neutral and earthed metal work is going to rise to something near to phase voltage. This sound horrific, but is actually not dangerous provided that all earthed metal work is nicely bonded, and there are no unbonded earthed objects around that are better earths than your earth spike.
How the ground connections are made in the main distribution board
The three phase power power comes from power company using just four wires L1, L2, L3 and PEN (protective earth and neutral). L1, L2 and L3 are just connected to the power bars in the main distribution panel. The PEN wire is connected to the PE (protective earth) bar which is connected to the central grounding bar. Neutral bar is connected to the PE bar in the main distribution panel and nowhere else in the building which has 5 wire 3 phase wiring.
The central grounding bar works as the central point for whole building grounding system and every grounded system in the building is connected to it. The central grounding bar is just a metal bar which connected the grounding wires from the mains power, telephone equipments, antenna wiring, lightning protectors, metal , water pipes, building steel structures and building grounding electrodes together.
As mentioned elsewhere, a fault current flowing in the earth wiring will cause the voltage on that wiring to rise relative to true earth potential. This could cause a shock to someone touching, for instance, the case of a faulty washing machine and a water tap at the same time. In order to minimise this risk, an 'equipotential zone' is created by connecting the services to the main earthing point. Such services are metal pipes (gas, water, etc.), central heating, metallic trunking, exposed parts of building structure, lighting conductor and any other metallic service.
The equipotential bonding reduces the voltage difference which could exist between the metalwork of these services if an earth fault occurred to any one of them.
How the ground is wired to electrical outlets
The best way would be to run all the grounds separately back to a single block of copper at the central grounding bar. Unfortunately this is often difficult to achieve in practice. The practical way is to arrange your grounds as a strict 'tree' structure, with equipment only connecting to the leaves of the tree.
For ground wire routing the electrical installation regulations worldwide generally state that the ground wires should be routed on the same route than the mains curren carrying wires going to the same outlet or distribution panel. This is the most often used practice. Usually the safety ground is a separate yellow/green wire in the cable and sometimes it is a separate wire in the same cable bundle (for example for 3 phase distribution you might see sometimes a 4 or 5 wire cable and a separate safety ground cable bundled on the side of the cable). In some countries (for example in USA) in some case the metallic piping used to protect the mains carrying wire inside the walls can be used as safety ground conductor (not usually very good or reliable in practice I think, but is allowed in some cases).
How power is delivered to the house
Typical one phase feeding to building
One phase distribution is typically used in small residental building. The power company feed live wire and neutral+gound wire to your house. The power in the power company system is typically three phase power and the power company then feed one phase to your house (you neighbour can have their power from other phase for even distribution of load).
Single phase power in North America
ANSI C84.1 "Electric Power Systems and Equipment - Voltage Ratings (60 Hz) sets the preferred nominal voltage at 120V and allows a range of 114 - 126V (240V nominal, range 228 - 252V). Equivalent Canadian spec is CAN3-C235.
Voltage at a 120 volt nominal single phase receptacle should be 110 to 125V under normal conditions.
However, the California Public Utilities Commission has specified that the service voltage shall be kept in the range 114-120V, with some exceptions. This was done because some studies showed a reduction in energy consumption at the lower voltages.
Information on NEMA plug configurations is available in NEMA Configuration Chart, Form No. H4513. For availability check http://www.hubbell-wiring.com/.
Single phase power in Europe
The nominal European voltage is now 230V 50 Hz (formerly 240V in UK, 220V in the rest of Europe) but this does not mean there has been a real change in the supply.
Instead, the new "harmonised voltage limits" in Europe are now:
his is really a fudge and means there is no real change of supply voltage, only a change in the "label", with no incentive for electricity supply companies to actually change the supply voltage.
- 230V -10% +6% (i.e. 207.0 - 243.8V) in most of Europe (the former 220V nominal countries)
- 230V -6% +10% (i.e. 216.2 - 253.0V) in UK (former 240V nominal)
To cope with both sets of limits an equipment will therefore need to cover 230V +/-10% i.e. 207-253V. This will actually become the official limit for the whole of the EU in 2003.
Single phase power in rest of the world
A listing of nominal voltage/frequency and plug/socket types used in many countries is given at http://kropla.com/electric2.htm.
Three phase distribution
The 3-wire system that the user sees is typically derived from three phase distribution, which uses a 5-wire system. In the 5-wire system, there are 3 hot wires, 1 neutral wire, and 1 grounding wire. The common 3-wire receptacle uses only one of the 3 hot wires. This 5 wire wiring system is basically good and it is used in most buildings and places where ground loops are expected to be a problem.
This three phase power system is called THREE-PHASE STAR; FOUR-WIRE; EARTHED NEUTRAL system. This is the most common way used in European wiring systems (and used almost everywhere in Finland), but note that three-phase distribution circuits come in several flavours. There is a distinct difference between those in the US and those in Europe. They are classified as follows:
- TN: star point earthed. Protective Earth and Neutral share the "ground" conductor (PEN) and are separated at the fuse panel. This circuit is also referred to as TN-C (C for common PE and N). In UK this is called TN-C-S (i.e. combined in supply and separate in the installation), and it is also referred to as Protective Muliple Earthing (PME - as the PEN supply conductor is grounded at regular intervals along the supply).
- TN-S: As above, but PE and N are brought separatley all the way from the earthed transformer and never allowed to get into contact with each other elsewhere. The idea is that PE shall never carry any current (it shall consequently not carry any potential and is supposed to be very "clean". All return currents go through the N conductor all the way to the transformer star point. This system has become very popular in new installations in Europe and has been a standard in hospitals for a long time.
- IT: The transformer is not erthed at all. The star point floats. Mostly used in heavy and process industry where continued operation - even if there is an earth fault - is required. The more common (european) voltages in these systems are 500 V and 690 V. In this case housing of the objects are connected to local grouns.
- TT: Transformer and objects have separate grounds. Common in US.
Three phase in Europe
In Europe most use 230/400V where the 230V can be found between any of the 3 phases and neutral and the 400V can be found between two of the three phases. Phase difference between phases is 120 degrees. Three phase power is normally available in at least Finland, Sweden and Germany being used for ovens, electric stoves, large motors and dryers. Three phase power is also typically available in places where large sound and light systems are used (around stages etc.).
Typically there are 4 wires routed to every house for 3 phase feed. Those are typically them are labeled R, S, and T, the fourth being ground. The phase shift between R and S are 120 deg., the same phase shift exists between S and T and between T and R. The voltage difference between the live phases is 400 V, the voltage difference between any live phase and ground is around 235 V. The usual household power outlet connection uses one phase and ground. Three phase is usally used only on some permanetly wired high power loads (typically ovens and electric stoves in normal household). A typical rating for mains fuse in typical household in Finland which has three phase power is 3x25A (25A per phase).
If three phase connector is avaible some heavy equipments (in places where heavy machinery is used), then the most common one available is 3x16A connection.
Other possibilities for power distribution
Some smaller electrical installations (small houses) only use one phase feed. In those cases the power company only brings one of the three phases to the house. In those cases the wiring from power company is implemented using two wires: "neutral+ground" and "phase". The frequency in USA is 60 Hz and nominal voltage in USA is defined in the following manner:
So if the outlest are very lightly loaded, you will get nearly 120V and if wiring hevily loaded, the voltage drops to around 110V. In Europe the frequency is 50 Hz and voltage on the outlet is nowadays is 230V (the real voltage typically is between 220V and 240V).
- 120 volts is the voltage at the transformer
- 115 volts is the voltage at the panel (voltage drop losses in the cable from the transformer to the panel)
- 110 volts is the voltage at the receptacle (voltage drop losses in the cable from the panel to the receptacle)
In USA the domestical service has typically 3 wires: 2 hots and a neutral. The voltage between the 2 hots is 240 and the voltage from either hot to the neutral is 120 (half). Normal electrical outlets are connected between the neutral and one hot wire. Some heavy loads (like air conditioners) are connected between those two hot wires and receive the full 240V load.
House wiring details
What does a typical power outlet look like ?
A typical office wall outlet has three electrical connections, which are the "hot", "neutral", and "grounding" wires.
All office equipment requires only the hot and neutral wires to function. The third or grounding wire is connected to exposed metal parts on the equipment. Within the building, the grounding connections of all electrical receptacles are wired to one another and are connected to the water piping. This ensures that all electrical equipment with exposed metal parts has these parts electrically connected to each other and to exposed metal fixtures in the building such as water fixtures.
The hot and neutral wires are interchangeable as far as the equipment is concerned (be warned that there are some exceptions in some countries). Both are power carrying wires. One of the power carrying wires is grounded for reasons of safety. In many parts of Europe (nordic counties, Germany etc), the normal 3-wire receptacle is symmetrical so that the neutral and hot wire connections can be swapped by simply rotating the plug.
Earthing of Electrical Installation
Each circuit requires an earth conductor to accompany (but kept separate from) the line and neutral conductors throughout the distribution. All metal boxes should be connected to the earth.
What are the wire colors used in wiring
House wiring colors used in USA
Green body color Grounding Conductor
White body color Grounded Conductor (Neutral)
ANY other Body Color Figure that it's HOT
Wiring colors used in equipment cables
GREEN with YELLOW stripes Ground
Typical colors used in house wiring in Europe
Information of this is from regulations in use in Finland.
GREEN with YELLOW stripes Ground
BROWN or BLACK Live
Grounding (Green or green/yellow) means that it's there to tie all of the stray metal parts together so that (hopefully) none of them can get to where they'll make a hazard. A far better term for this wire is that it is the "Bonding" conductor. Grounding wire should NEVER be asked to carry current.
Do not thrust the color coding unless you know under which standard the wiring is done. There some some other color codes also in use. Inside of any electronic equipment, it is dangerous to trust any color codes unless you know which "Standard" that unit was built under.
House wiring problems
Problematic old wiring
The most problematic are those builing which are wired using 4 wire 3 phase wiring, where neutral and ground share the same conductor at some places of wiring. This is a bad thing because in this situation there will be always current flowing in the same wire which should distribute the same ground potential to different places. If your building has four wire 3 phase wiring you can expect quite noticable ground potential differences of the power taken from different distribution panels.
The practice where safery ground is connected using the same conductor as neutral is called PEN (TN-C) and practice where there is separate ground wire in whole system is called PE (TN-S).
Two wire 1 phase grounded outlet
And worst of all is a 2 wire 1 phase jack wiring where neutral and ground share a common wire. This practice is very often used in older buildings in Finland and causes terrible ground loop problems even between nearby power outlets. If your are planning to install any dedicated equipments (computer connected to LAN, interconnected audio or video equipment etc.) to building which has this kind wiring system is advicable to get a lincensed electrician to rewire the room with proper outlets. This wiring has also some other problems and that's why it is not allowed anymore in new installations in Finland.
Circuit breaker boxes: The main breaker box to the building is the single location where the neutral and the ground wires come together. The electrical service will be grounded at this point. IN ALL DOWNSTREAM BREAKER BOXES BOTH THE NEUTRAL AND GROUND WIRES MUST BE KEPT APART FROM ONE ANOTHER. Otherwise you will have neutral currents flowing on the ground wire. This is extremely important and is a major safety and signal issue.
These simple rules apply to ALL cabling including CATV, Video, AC and signal. One exception is the ethernet. Ground the computer LAN one end (preferably to the same point as your audio system) and make sure that the thin ethernet connector metal parts do not touch any parts of computer case (there are nice plastic isolation cases available for them). I would recommend to use 10 Base-T ethernet which used twisted pair wiring because it does not need any grounding and does not cause ground loops in any case.
What are isolated ground receptables ?
NOTE: The following description describes isolated ground as defined in USA wiring system. In other countries "isolated ground" can mean different things (for example real ground but not connected to the power ground bar).
Many new buildings in USA are equipped with "Isolated ground" receptacles. These are normally recognizable because they are bright orange and have a triangle marked on the face. Basically, these receptacles have a separate "green wire" equipment ground, and the wire goes back directly to the circuit breaker panel, without being connected to anything else. Isolated ground receptacles are installed in the hope that electrical noise generated in the building, or by other pieces of equipment, will not disturb the operation of delicate computer equipment plugged into them.
As far as what the NEC allows, an isolated ground is a grounding connection which is grounded only at the separately derived system from which the circuit is supplied. It is permitted to pass through panelboards, junction boxes, etc. without being bonded to the equipment grounding conductor which serves those devices, thus minimizing electromagnetic interference. It must be used in conjunction with an isolated grounding receptacle to be effective. More details of isolated ground can be found at NEC 250-74 Exc #4.
My understanding on using a term "isolated ground" is mostly for marketing purposes. I feel that their purpose is a marketing response to inadequate design by some electronic equipment manufacturers and to inadequate grounding practices by some electricians. The ground terminal is isolated from the mounting yoke; in conventional receptacles the yoke and the ground terminal are connected. The "idea" is that the electrician connects a special "clean" ground to the ground terminal, while the yoke and all other non-energized metallic parts are connected to the "dirty" equipment ground.
How to avoid ground loop problems
Most electronic equipment is sensitive to ground loops and ground-induced noise. A proper earth ground at the building services entrance is the first step to avoiding such problems. In many cases, a proper earth ground is provided by a connection to the steel rebar in the building's foundation.
All outside service grounds must be solidly connected to this ground