What Is the Ground (Earth) Wire For?
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
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. At the appliance, this is connected using a screw terminal to the outer casing of the appliance. It 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
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.
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. Basically grounding the neutral pulls down the voltage of the line so that neutral is close to 0 volts and the voltage on either of the hot lines doesn't greatly exceed 120 volts.
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 grounding conductors 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, a conductor runs outside the premises to a grounding rod (earth rod), 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. 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 or the neutral conductor breaks outside the home, 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.
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.
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.
TT, TNS and TNCS Grounding/Earthing 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.
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"
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.
Electric Wiring Color Codes
This Wikipedia article gives lots of info about wiring and color codes used in various countries around the world.
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