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What Is the Ground (Earth) Wire For?

Updated on September 20, 2017
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Eugene is a qualified control/instrumentation engineer Bsc (Eng) and has worked as a developer of electronics & software for SCADA systems.

What is a Ground Wire?

The hot and neutral cores in a power cable are used to supply current to the appliances in your home. So what about the green wire, why is it needed? This is known as the ground and is an additional wire which is included for the safety of you and your home.

Note: This article was written for a US audience so I refer to the protective conductor as "ground". However it is also called "earth" in other countries. Another difference is that the term "hot" is used, which is also known as "live". The third difference is that a second hot line is often provided to homes resulting in a 240 volt supply (between the two hots) in addition to the 120 volt supply

Source

What Are the Wires in a Power Cable?

Hot

This wire is at a potential of about 110 to 120 volts relative to ground. Current flows out through this wire to an appliance. Hot is also referred to as "live" in other countries.

Neutral

The neutral wire is at a voltage close to or equal to ground. Current which flows to an appliance via the hot wire returns via the neutral core in a cable. (See note below)

Ground

This is a protective conductor, included to prevent shock and/or fire. Ground is also known as "earth" in some countries.

The supply from the transformer feeding your home is split phase and in the U.S., 2 hots in addition to a neutral are provided. Lower power appliances are connected between either of the hots and neutral and this gives a 120 volt supply. The voltage between the two hots is 240 volt for supplying higher power appliances.

Note: The electricity supply in our homes is AC. So while we tend to think of current flowing out through the hot wire to an appliance and returning via the neutral wire, current actually flows both ways. So for one half of an AC cycle, current flows out through hot and returns via the neutral wire. During the second half cycle, the process is reversed and current flows to the appliance via neutral and returns via hot.

Ground - The Protective Conductor

The flex or fixed wiring supplying metal cased appliances includes a ground conductor (colored green in US or green/yellow in EU) in addition to hot and neutral. Inside the appliance, the ground core of the cable is connected to the outer casing of the appliance. The connection may be made either using a screw terminal or a ring crimp and self tapping screw/bolt. Ground acts as a "bypass" for currents in the event of a fault. Fixed (e.g. storage heater, kitchen range) and portable i.e corded appliances with extraneous metal which can be touched in normal use must be grounded

The fault could be due to:

  • Conductors (e.g. wires, terminals, components) at hot or near full mains potential breaking, bending or detaching and touching the casing of the appliance
  • Breakdown of insulation. For instance insulation on cores of the power flex could become damaged inside an appliance or insulating spacers could become dislodged. Also metal parts such as screws or nuts which have come undone could bridge the gap between hot and the metal casing
  • Making contact with a power cable when drilling through a wall

Inside a microwave oven, the ground core of the power flex is connected to the casing using a ring crimp and screw. Note the symbol for ground
Inside a microwave oven, the ground core of the power flex is connected to the casing using a ring crimp and screw. Note the symbol for ground | Source

What Happens During a Fault if an Appliance isn't Grounded?

If a fault occurs, the external metal of an appliance will become live and the voltage with respect to earth will be anything up to 120 volts, depending on which part of the internal circuit touches the casing. If the metal isn't grounded and someone touches the appliance, current will travel through their body to ground. If they are lucky and have rubber soled shoes and are standing on a dry floor, they may just experience a tingling sensation. However if conditions are damp, they have wet hands and standing outdoors, they are more likely to experience a severe shock. If one hand touches the appliance and the other touches a grounded object, e.g pipework, poles, radiators or whatever, current will travel across their heart, a more dangerous scenario. If the person is unlucky or has a heart condition, this can kill.

Why Does Current Flow to Ground?

The reason why current flows to ground is because the neutral point in the supply transformer is connected via a ground conductor to a ground electrode. This raises the potential of the hot conductor to about 120 volts with respect to the ground surface. During a fault, or if someone touches a live conductor, current flows through the ground back to the transformer. Isolating safety transformers, which are sometimes used for powering tools on construction sites, isolate the neutral from ground so that current cannot flow (or at least very little) if a fault occurs. These transformers are also used convert voltage to 110 volts in countries where 230 volts is the standard supply voltage. This reduces the current to a safer level if someone experiences a shock.

Why is the Supply Transformer Grounded?

Grounding the neutral of the supply transformer is a safety measure taken to eliminate dangerous rises in potential (greater than the hot voltage) on the hot or neutral conductors entering a home. This could occur for instance if a very high voltage power line (possibly hundreds of kilovolts) breaks and lands on a "low" voltage (120 volt) line. Another scenario is the insulation between the primary and secondary of the transformer being breached. This could allow the primary voltage (>10kv) to appear on the secondary. Yet another possibility is a lightning strike on the lines. Static charge can also cause a buildup of voltage on lines. Basically grounding the neutral pulls down the voltage of the line so that neutral is close to the potential of the earth which we are standing on and the voltage on either of the hot lines doesn't greatly exceed 120 volts.

The path of fault current when someone touches a non-grounded faulty appliance (2nd hot in U.S. wiring systems not shown)
The path of fault current when someone touches a non-grounded faulty appliance (2nd hot in U.S. wiring systems not shown)

How Does Grounding Solve the Problem?

Grounding provides a bypass, shunt or shortcut through which electricity can flow, instead of passing to earth through the person who touches the appliance. Wires called equipment grounding conductors (EGC) in the US (or protective earth (PE) in the UK) are run from the electrical panel (sometimes called a breaker box), through the fixed wiring to all socket outlets, fixed appliances such as ranges or water heaters, light switches and ceiling roses in your home. In the case of a portable appliance, this grounding path continues from the pin in the plug through the flex, to the metal body of the appliance. At the electrical panel, all of these conductors are joined at the main grounding terminal (main earthing terminal in the UK). A grounding electrode conductor (GEC) runs outside the premises to a grounding electrode (earthing electrode in the UK), embedded in the soil.

When a fault occurs, current flows via the grounding conductor back to the electrical panel. If a TNC or TNCS earthing system is in use, all neutrals are joined to ground at the panel (or the neutral and ground may be joined at the output of the supply meter see earthing systems schematic below), and so the hot to ground fault at the appliance effectively becomes a hot to neutral fault, practically a short circuit. A large over-current flows, and this trips the MCB (miniature circuit breaker) and possibly also the GFCI (whichever acts first) for the circuit, cutting power and making everything safe.
Grounding however, also has another important function. Even if current is insufficient to trip a breaker (in a TT grounding system), the neutral conductor breaks outside the home, or stray currents in the neutral cause a dangerous rise in potential, it reduces the touch voltage between the casing of the appliance and the area on the ground on which the person is standing to a safe level.


The low impedance ground conductor shunts current away from the person touching the appliance, reducing the touch voltage to a safe level. (In reality the conductor passes via the plug and flex of the appliance and electrical panel to the ground rod)
The low impedance ground conductor shunts current away from the person touching the appliance, reducing the touch voltage to a safe level. (In reality the conductor passes via the plug and flex of the appliance and electrical panel to the ground rod)

Double Insulated and Non Grounded Appliances

Appliances such as hair driers, TVs, hand held kitchen appliances, etc. generally have plastic casings. If a fault occurs inside the appliance, e.g. a wire or component touches the inside of the casing, there is no danger since the plastic body is an insulator. These appliances don't have a ground wire in the flex. Some appliances such as power tools are not grounded and instead are "doubly insulated". This means that although the external casing of the tool or appliance may be metal, sufficient separation and isolation of the external metal from internal high voltages is effected to prevent electric shock. These devices don't have a ground wire in the cord either. Double insulated appliances can be extremely dangerous if they get wet. This is because the casing is not grounded and can become live if water breaches the separation between live parts and casing. Also the MCB is unlikely to trip and the GFI may not operate either.

Double insulated symbol
Double insulated symbol | Source

GFCI

A safety device called a GFCI or Ground Fault Circuit Interrupter (also known as a GFI or RCD - Residual Current Device) is likely to be fitted in most modern installations. This device monitors the current flowing out through the hot conductor and back via neutral. Normally these currents are equal. If current leaks to ground, not all the current returns through the GFCI. Electronics in the device detects this imbalance, and it trips out, shutting off the power. The trip current for a GFCI is normally 30mA but can be higher or lower depending on conditions. A GFCI handles situations such as someone touching a live conductor, e.g. a damaged power cord with exposed cores, or the connector of a kettle left in a pool of water on a sink. (It may also even trip if damp bread gets stuck in a toaster and touches the element!)
A GFCI also responds to faults as described above where hot makes contact with the grounded body of an appliance. The device cuts the power, if the MCB doesn't "get there first".

Another function of the GFCI is to prevent fire. Consider the situation where a damaged and exposed conductor makes contact with damp timber or grounded material, e.g. conduit or piping. This could produce sparks and start a fire if there is any flammable material nearby, e.g. sawdust, wood shavings or insulation. The current may not be sufficient to trip a breaker, however the small leakage current to ground is more likely to be detected by the GFCI, making it trip and shut off the power.

GFCIs can be installed at the electrical panel, they are available in the form of a GFCI socket outlet, and you can also buy a GFCI adapter which plugs into a socket. An appliance is then plugged into the adapter. This is a worthwhile safety accessory for an extension lead if you use power tools in the garden.

Three Types of Grounding Systems

TNCS or PME (Protective Multiple Earthing)

This system uses a combined ground/neutral back to the supply transformer. This is then split into separate ground and neutral conductors after the meter. A hot to ground fault effectively becomes a hot to neutral fault, and since the impedance back to the transformer is low, the large, short circuit current ensures that an MCB for the circuit will trip. The problem with this type of system is that full mains potential could appear on extraneous metalwork of an appliance if the neutral breaks outside the premises. This is why the ground electrode is so important. The bulk of the earth between the ground electrode at the premises and the point where the supply transformer is grounded acts like a potential divider. If someone touches a grounded appliance, the touch voltage between their hand and feet is equal to the voltage between the point at which the electrode enters the ground and their feet. Since this distance is likely to be a fraction of the distance to the supply transformer, the voltage is reduced proportionately. The electricity supply company may install multiple earthing or ground points from the neutral line between transformer and premises to reduce the consequences and hazard of a broken neutral (especially if they are widely separated)

TNS

The TNS system is often used when a ground can be provided by the armor of the supply cable. If the armor becomes corroded causing a bad ground, this system can be converted to TNCS.


TT

The TT system is used when power comes in overhead. The system uses the bulk of the earth as the return path for fault currents. It doesn't have the risk of a broken neutral. If a home is distant from the supply transformer, the fault current during a hot to ground fault may be insufficient to trip a breaker because the resistance of the earth is too great. Since the development of GFCIs which can detect small leakage currents to ground, this is less of an issue. TT systems may be converted to TNCS systems where the ground and neutral are neutralized or joined together at the exit point of the meter.

Un-Grounded and Grounded Socket Outlets - NEC Regulations

In the U.S., both ungrounded and grounded receptacles are used. Ungrounded outlets are prohibited in new buildings but in the situation where an equipment ground conductor is not present, NEC code exceptions allow these to be replaced by either another non grounded receptacle, a GFCI receptacle or a grounding type receptacle fed by a GFCI as long as the receptacle is marked "No equipment ground" and "GFCI protected".
2 pin in-grounded receptacles can be upgraded to 3 pin grounded receptacles with the addition of new ground wiring.

Bonding

Metal services such as water and heating pipes and hot water heaters are grounded with a heavy gage wire routed back to the electrical panel. This ensures that if a hot wire makes contact with these services, a large current will flow and trip the breaker. The heavy gage wire is rated so that it can carry the current which may flow if a hot from a high current circuit makes contact with the service. Also the heavy gage keeps the resistance of the cable low. This ensures that as current flows through this resistance, the resulting voltage rise is kept below safe limits. This is vitally important in bathrooms where everything is damp and we may be in our bare feet and making relatively good electrical contact. Everything such as radiators, water pipes, wall heaters and the drain in the bath/shower are connected together by a bonding conductor. This "equipotential bonding" keeps everything at the same voltage and there is no difference in voltage between for instance a shower head and the drain.

Source

Electric Wiring Color Codes

This Wikipedia article gives lots of info about wiring and color codes used in various countries around the world.

http://en.wikipedia.org/wiki/Electrical_wiring

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    • profile image

      Tyrone 6 weeks ago

      I have to say this is the best article on grounding I have found on the net. Congratulations.

    • eugbug profile image
      Author

      Eugene Brennan 6 weeks ago from Ireland

      It depends on the grounding system Tyrone. If TT grounding/earthing is used, the bulk of the ground may have a large resistance and so current would be lower than that in the neutral in a fault situation (possibly insufficient to trip an MCB, but when the installation is protected by a GFCI/RCD, this will trip). In a TN-S system, a large short circuit current will flow back via the separate ground to the supply transformer. In a TN-C system which has a common combined neutral/ground back to the transformer and appliance cases connected to neutral, a larger than normal current will flow on this conductor during a fault. A broken neutral conductor can cause a dangerous potential on the metalwork of appliances. A TN-CS system provides separate ground and neutral to appliances. If there's a fault, a large current will flow through the equipment ground (protective earth) back to the panel. This will be larger than the current flowing in the neutral core of the cable feeding the appliance, but will flow in the neutral conductor back to the transformer.

      So it's really only TT systems where the resistance is high. The actual ground wire from the appliance back to the panel would have a low resistance.

    • profile image

      Tyrone 6 weeks ago

      I'm thinking there wouldn't be much current travelling along the ground path because of the much higher resistance compared to the neutral wire?

    • eugbug profile image
      Author

      Eugene Brennan 6 weeks ago from Ireland

      A circuit is normally completed via neutral, but if a hot to ground (live to earth) fault occurs, there's also a parallel path for current to flow through via ground back to the transformer (and also the neutral conductor, if ground is connected to neutral at the panel).

    • profile image

      Tyrone 6 weeks ago

      In the event of a fault it seems the circuit is not being completed via ground but via the neutral wire. Therefore the grounding on both the transformer and residence are stand alone and are only there to bring the neutral down to 0 volts?

    • eugbug profile image
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      Eugene Brennan 8 weeks ago from Ireland

      Hi Izhaan - This link shows the colours of earth wires in electrical installations:

      https://en.wikipedia.org/wiki/Electrical_wiring#Co...

      Earthing/grounding conductors may have a coloured sheath or be bare without a sheath in fixed wiring cables. The earth conductor in a flexible power cord supplying an appliance normally has an insulating sheath. In socket and lighting outlets and electrical panels, bare earth conductors are provided with a coloured sleeve to insulate them from inadvertent contact with live/hot parts and also to identify the conductors.

      If you are considering making any electrical modifications, I highly recommend you consult a qualified electrician. A simple mistake can prove fatal!

    • profile image

      Izhaan 8 weeks ago

      Plz tell me

      How can i identify earth wire in household circuit.?

    • eugbug profile image
      Author

      Eugene Brennan 5 months ago from Ireland

      ....the hair drier's lead/cord/flex that is, not the dog's. (Maybe you should replace the dog also.. LOL!)

    • eugbug profile image
      Author

      Eugene Brennan 5 months ago from Ireland

      Hi Denise,

      Possibly there is invisible damage, i.e. broken wires, in the flex from the dog stretching it. Sometimes if the stranded copper cores in a flex are broken, the wires can be pulled until they snap and the lead then shortened. However the safest thing would be to completely replace the lead.

    • profile image

      Denise field 5 months ago

      Thank you so much Eugene. Youve really put my mind at ease. Unfortunately the lead has been shortened so no exposed wires!! But! It worked for about 1 minute then stopped. There was no smell indicating the motor burnet out no no bang no spark just stopped maybe the plug is old what do you think obviously without seeing it. Just glad to know I'm not going to be shocked etc.

    • eugbug profile image
      Author

      Eugene Brennan 5 months ago from Ireland

      Hi Denise,

      If the casing of the hair drier is plastic, it doesn't need to be earthed. I presume you mean the copper of the wires was visible? If there are still sections of the flex, after the plug was replaced, where the copper wires are exposed , the whole lead needs to be replaced (or possibly shortened if the bad bit is near the plug end).

    • profile image

      Denise field 5 months ago

      Okay here goes my hairdryer lead (flex) has been chewed in half so I gave it to my neighbour (yes I'm rubbish!!)

      He put on a new plug, the other one was fixed on the lead. But said its not earthed it only has 2 wires which were visible cutesy of the dog!! These visible wires are copper im not going to get electrocuted am I?

    • eugbug profile image
      Author

      Eugene Brennan 6 months ago from Ireland

      Hi Ned,

      The earth wire doesn't carry a dangerous voltage (at least it shouldn't assuming the earth rod is working properly and the touch voltage is below safe levels during a fault situation).

      The covering on an earth wire is used for identification but can also give the inner core some protection from damage. It also acts as insulation if for instance the wire feeds into a socket outlet/light fitting. This prevents live parts/wires inadvertently touching the earth conductor when for instance socket plates are removed/replaced. Usually however live wires are insulated and live parts are shrouded, so this doesn't often happen.

      I don't think it's a major issue if the cable insulation is split, in any case the pipes it connects to are uninsulated. I would be more worried that the inner core has been damaged and strands broken from the cable being repeatedly bent during installation/maintenance work. It may have just been chaffed when work was done, but it's no harm getting the cable checked by a qualified electrician.

    • profile image

      Ned Tallyho 6 months ago

      The green/yellow (EU) cable connecting copper pipes near my gas combi boiler is bent over to fit in but the bare wire

      can be seen where the cable has split due to being bent. Is this dangerous and should I get it changed?

    • eugbug profile image
      Author

      Eugene Brennan 2 years ago from Ireland

      The older style NEMA 1 -15 two pole socket was 15 amp rated and un-grounded. It is not permitted in new construction but there are probably lots of these still out there. If an appliance has a metal casing (e.g. a metal kettle), it needs to be grounded. The exception is double insulated appliances which have metal on their exterior but are not grounded. This is because the metal is separated from the inner workings to the extent that it is highly unlikely to become "live". If a plug isn't moulded onto the flex, you can always check whether the appliance is grounded or not by opening it up to see if a green wire is connected to the ground pin. However this isn't a total guarantee, and I have come across appliances which were marked as double insulated and I think should have been grounded, but weren't during manufacture. Many appliances are un-grounded nowadays simply because of their plastic construction.

      Anyway, while a 5 -15 adaptor with the pin removed would fit into a 1 - 15 socket, you could end up forgetting, and plugging an appliance requiring a ground into it (or someone else not realising the change could do so!). Also 5 - 15 plugs or adaptors with the ground pin removed are unpolarized. So hot and neutral going to an appliance could be reversed if the adapter is inserted upside down. This would result in the switch in the appliance not shutting off power. So if anyone does any maintenance on an appliance with the plug still inserted into the wall, (and assumes the switch in the appliance has cut the power) , they could be electrocuted. It can be very dangerous if lamps are wired incorrectly with the hot and neutral reversed because not only does the switch in the cord not cut the power, the outer screwed shell in the ES lamp holder also become live and could possibly be inadvertently touched when screwing in a bulb.

      Ideally you should get your sockets upgraded to 3 pin grounded types. I know this can be a real pain because new wiring with a ground conductor would have to be run back to the fuse box, which isn't always possible and can result in unsightly surface wiring if hiding the wiring isn't possible. The next best alternative is to have the socket replaced by a 3 pin type protected by an integral GFCI.

      If anyone's interested, this is the link to the Wikipedia page with details of connectors:

      https://en.wikipedia.org/wiki/NEMA_connector

      NEC code 406.4(D)(2) covers replacement of 2 pin socket outlets

    • MG Seltzer profile image

      MG Seltzer 2 years ago from South Portland, Maine

      This topic was just on my mind as our GCFI outlet in the bathroom isn't working, which means that the one near the kitchen door has kicked itself off. I had just been thinking, "I really need to understand how the current flows through the system." Also, years ago, a constractor told me the grounding plugs on adaptors were "useless," and could be pulled out with pliers to let a three-prong plug fit a standard outlet. And I remember thinking, "Now is this true?" I am bookmarking this Hub because I see lots of good detail that I think will answer my questions. Voted thumbs up, of course.

    • eugbug profile image
      Author

      Eugene Brennan 4 years ago from Ireland

      Thanks for the comments! Yes bad or non existent grounds or missing earth electrodes can cause all sorts of shocking experiences! Also relying on water pipes for earthing is a bad idea as water supply authorities can replace sections of metal pipe with plastic.

    • tirelesstraveler profile image

      Judy Specht 4 years ago from California

      Then there is the electrical systems in old houses that can give you a thrill. Dad was an electrical engineer and husband was an electronics tech in the Navy. This is a well written hub. Nice work.