How do you earth planes in flight?

[Wigglyamps]

Hi all,

Slight bit of thread creep, having recently qualified as a domestic electrician where the emphasis is on ensuring you have earth paths down to the mass of earth, I was wondering how you earth an airframe, especially in flight, is there a similar concept in the wiring of having a phase, neutral and earth wire??

thanks for any replies

Wigglyamps

[Fargoo]

The neutral of the generator on a typical metal airliner is connected to the airframe as is the neutral lead of the battery.

The airframe is used as the earth return for both dc and ac systems. The aircraft only needs to be grounded when it's on the ground to avoid static build up.

In flight static build up is discharged through a series of static wicks on the trailing edges of the flying controls , boat fairings, fin/stab tips etc...

On the ground the tyres allow static discharge but during refuelling and maintenance operations a seperate earth is attached.

[Touloser]

Althought the term earth is used in aviation, clearly there is no actual connection to a mass of 'earth'. The airframe itself is used as the common current return point. The same sort of earthing system as used in your car.

[Pugilistic Animus]

one of the biggest and most annoying cause of problems [so I am told] with respect to autopilot malfunctions are 'bonding,' 'earthing', 'grounding faults',....Electrickery:)

[GarageYears]

Oh my goodness.... having recently qualified as a domestic electrician where the emphasis is on ensuring you have earth paths down to the mass of earthYou do understand why this is so?

The Earth connection used for buildings is nothing to do with the flow of power (current). The Earth connection is for SAFETY from lightning...

A building Earth rod is used to ensure that a lightning strike on a house that finds its way into the electrical wiring has a path to ground.

It is most CERTAINLY not to prevent shocks due to equipment faults, etc. Shocks are prevented by tying the earth wires to the neutral back in the fuse box. The earth wire acts as the last line of defense should you decide to drop your toaster in the sink - since the manufacturer will have connected the nice metal case to the earth pin of the plug - this forces the current flowing down the "hot" wire to flow back down the safety earth, and blows the fuse. Normally the working current returns down the neutral, but the fault current flow will be many times the working current.

In a vehicle - car, airplane, etc - there is no path for lightning to ground, hence they are not EARTHED. The internal electrical systems have a "return" or mis-named "ground" but this is to provide a return path for current to flow (and hence do work). Current MUST flow in a circle... from source back to the source (hence the name "circuit").

(Even so lightning can obviously still hit an aircraft and cause nasty things to happen, but nowhere near as bad as if the aircraft were joined to ground).

- GY

[Checkboard]

The Earth connection used for buildings is nothing to do with the flow of power (current). The Earth connection is for SAFETY from lightning...
...
It is most CERTAINLY not to prevent shocks due to equipment faults, etc.

Hmmm. :hmm:
HowStuffWorks "What is the difference between two- and three-pronged plugs?" (http://electronics.howstuffworks.com/question110.htm)
The ground slot and the neutral slot of an outlet are identical. That is, if you go back to the fuse box, you will find that the neutral and ground wires from all of the outlets go to the same place. They all connect to ground (see How Power Distribution Grids Work for details on grounding). Since they both go to the same place, why do you need both?

If you look around your house, what you will find is that just about every appliance with a metal case has a three-prong outlet. This may also include some things, like your computer, that have a metal-encased power supply inside even if the device itself comes in a plastic case. The idea behind grounding is to protect the people who use metal-encased appliances from electric shock. The casing is connected directly to the ground prong.

Let's say that a wire comes loose inside an ungrounded metal case, and the loose wire touches the metal case. If the loose wire is hot, then the metal case is now hot, and anyone who touches it will get a potentially fatal shock. With the case grounded, the electricity from the hot wire flows straight to ground, and this trips the fuse in the fuse box. Now the appliance won't work, but it won't kill you either.
(Red emphasis mine)

[GarageYears]

HORRIBLY WRONG.

Trust me this is horribly wrong.

Try measuring the resistance of 1m of soil. Just plug your multimeter probes in and set the meter to Ohms....???

The fuse in a circuit is in the HOT side. The RETURN for the current is in the NEUTRAL wire, and secondarily in case of a fault in the SAFETY EARTH. The term EARTH is used with respect to the function related to LIGHTNING. The current in a fault situation will flow from the HOT wire, to anything connected that will let the current RETURN to the source, which in this case is the mains supply to the house. The current does not want to go into the soil...

See this (contradicting my "never trust the internet"): Breakers and Ground Wires (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/bregnd.html#c5)

National Electrical Code Article 250.4

250.4 General Requirements for Grounding and Bonding
(A)Grounded Systems
(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path.
EARTH IS NOT the return path for current flowing in the national grid - the source transformer is, via the neutral line.

- GY

[DFC]

Wigglyamps,

Since you will understand the theory here is a brief synopsis which while not 100% technocally correct will give you an idea of the difference;

Domestic - We will start on the distribution network at the transformer - star 3 phase system with centre earth. Each phase will have the same potential to earth and at this point the centre is connected to earth.

From here two "wires" go to your house - Phase (live) and another "wire" which is connected to the centre point (Neutral).

Therefore at the transformer, the resistance of the neutral wire to earth is very low but as one moves farther from the transformer, the resistance of the neutral wire causes the resistance back to earth via the centre point to increase.

Let's now think of your house. It will most likely these days have a earth conductor which utilises an earth rod close to the property. The earth wires are of minimum sizes and this combined with the shorter distance top earth via the earth wire compared to the neutral.

Therefore given the choice, current will flow via the earth wire and not via the neutral. This is used to trip the ELCB.

Connecting metal enclosures to earth ensures that any fault current caused is suficient to trip a CB within the desired maximum time.

Bonding all pipework etc and earthing them ensures that you don't end up with possible voltage differences.

However,

If you stand in the bath and take hold of a bare phase wire - you will be electrocuted (briefly).

If you stand on rubber and take a phase in one hand and neutral in the other - you will be electrocuted.

If you stand on rubber and take a phase in one hand and earth wire in the other you will similarly be electrocuted.

Messing with electricity is bad news.

As for the aircraft.

DC system is just like your car. the metal fuselage provides the return and the live voltages are either 12V (old system, small aircraft) or 28V.

Aircraft with AC systems may use voltages as high as 110V (400Hz). But they always use a two wire system.

Therefore unllike the above list, unless you hold both wires of the aircraft AC system you can not electrocute yourself and then it is a 110 V shock compared to 230V.

Of course it is the current that kills you - and while your domestic full short could reach a very brief peak of 100A or perhaps a bit more (62A Fuse), aircraft electrical systems have much lower current potential.

The most dangerous part of the aircraft electrical system is the battery connector.

Having said all that aircraft are bonded to prevent different potentials being built up in various parts - similar to the bonding of metal pipes in the domestic situation.

What is mostly forgotten is that messing round the battery connector has the most danger in an aircraft - short out those terminals with a metal watch strap and 600 to 1000 amps with no protection will do a nice welding job on it.

[brooksjg]

Before anyone states 'The Truth' about Earthing / Grounding, please be precise about which system(s) and regulations you refer to!

It is simply NOT the case that the majority of UK mains-supply 'Earth connections' are to 'The Earth' as such, via (a) grounding rod(s). Most are in fact clamped to Neutral at the electrical intake. But you need to understand all the implications of exactly which type of supply arrangement is involved before saying anything about safety, or what else should, or should not, be the case. In UK, there are several different supply configurations. In the US and elsewhere, YMMV. And, if you get it wrong, also your potential lifespan!

[west lakes]

OK here goes

Wigglyamps, an aircraft wiring system is basically simililar to a car,live to the equipment, neutral (also called earth) return usually by the frame.

(as DFC said)

Garage Years
Your quote is only valid in the country to which it applies - USA

Wigglyamps is UK based and the regulations/systems & legislation are totally different. With building wiring coming under the Institution of Electrical Engineers 17th edition wiring regulations. Public supply networks under the Electricity,Safety, Quality & Continuity Regulations 2002.

Similarly Lightning protection systems are totally sepereate and are never connected to any point of a wiring system
DFC

Since you will understand the theory here is a brief synopsis which while not 100% technocally correct will give you an idea of the difference;

Domestic - We will start on the distribution network at the transformer - star 3 phase system with centre earth. Each phase will have the same potential to earth and at this point the centre is connected to earth.

From here two "wires" go to your house - Phase (live) and another "wire" which is connected to the centre point (Neutral).

Therefore at the transformer, the resistance of the neutral wire to earth is very low but as one moves farther from the transformer, the resistance of the neutral wire causes the resistance back to earth via the centre point to increase.

Let's now think of your house. It will most likely these days have a earth conductor which utilises an earth rod close to the property. The earth wires are of minimum sizes and this combined with the shorter distance top earth via the earth wire compared to the neutral.

Therefore given the choice, current will flow via the earth wire and not via the neutral. This is used to trip the ELCB.

Connecting metal enclosures to earth ensures that any fault current caused is suficient to trip a CB within the desired maximum time.

Bonding all pipework etc and earthing them ensures that you don't end up with possible voltage differences.

At the transformer the neutral is connected to the centre point, this is also connected to earth.
(at this point the resistance to the general mass of earth is below 1ohm in urban networks and 20ohm in rural networks)

In older systems a seperate earth is part of ALL cables and is often connected to the house earthing bonding/wires
In modern systems the neutral and earth run combined upto the customers property where they are then split and presented as seperate terminals

The majority of houses in the UK have a seperate earth terminal connected to the supply network. A seperate earth rod is only seen in rural areas and even then not very often.

An ELCB well a modern Residual Current type operates by monitoring the difference between the current in the live & neutral, if they differ by more than the setting of the device e.g. 30mA it will operate

You are correct with the bonding of all metal work, the theory being to ensure during a fault that everything should be at the same potential

The other reason in the UK to bond the centre point of a 3 phase transformer to earth is to maintain the neutral at electrically zero potential. Loss of this bond, can due to unbalanced loads, allow the neutral to electrically float leading to excess volts being delivered to customer's premises - with serious results.

[Brian Abraham]

A building Earth rod is used to ensure that a lightning strike on a house that finds its way into the electrical wiring has a path to ground.
Not in Australia. Ever seen a domestic wiring system that's been the subject of a lightning strike? Blows your appliances to bits, and the power points out of the walls, and fries the cabling insulation.

Wiring Rules of Australia
0.5.41 Earthed - connected to the general mass of the earth in accordance with the appropriate requirements of this standard in such a manner as will ensure the electrical isolation of any defective equipment through the operation of protective equipment. (an earth rod adjacent to the building)

0.5.42 Earthed Situation - a situation wherein there is a reasonable chance of a person touching exposed metal and, at the same time, coming into contact with earth or with any metal or conducting medium which may be in electrical contact with the earth or through which a circuit may be completed to earth.

0.5.64 Neutral (Neutral conductor or Midwire) - the conductor of a three wire or multi wire system which is maintained at an intermediate and approximately uniform potential in respect of the active or outer conductors, or the conductor of a two wire system which is earthed at the origin.

5.8.1.4 Lightning protection system - Lightning protection systems of buildings may be connected by means of an equipotential bonding conductor to the earthing system of the installation provided that the arrangement of the protection system is in accordance with AS 1768. (Not a feature of domestic buildings, only seen on large buildings generally)

Rural properties quite often have only a single wire (the active - due cost of infrastructure to remote sites) supplying the power to the building and the return is via the earth.

[GarageYears]

Equipment earthing conductor. This provides an electrical connection between non-current-carrying metallic parts of equipment and the earth. The reason for doing this according to the US U.S. National Electrical Code (http://en.wikipedia.org/wiki/National_Electrical_Code) (NEC), is to limit the voltage imposed by lightning, line surges, and contact with higher voltage lines. Note that equipment earthing does not provide protection from equipment ground faults, unless it is a grounded system (see below) and the voltage is over one thousand volts (typically). This is because the earth is generally a very poor conductor—it takes a large voltage to push enough current through it back to the electrical system's source to operate a circuit breaker or fuse. The equipment earthing conductor is usually also used as the equipment bonding conductor (see below).

Equipment bonding conductor. The purpose of the equipment bonding conductor is to provide a low impedance path between non-current-carrying metallic parts of equipment and one of the conductors of that electrical system's source so that should these parts become energized for any reason, such as a frayed or damaged conductor, a short circuit will occur and thus cause an overcurrent protection device such as a circuit breaker or fuse to activate and disconnect the faulted circuit. Note that the earth itself has no role in this fault-clearing process since current must return to its source, not the earth as is sometimes believed. By bonding (interconnecting) all exposed non-current carrying metal objects together, they should remain near the same potential thus reducing the chance of a shock. This is especially important in bathrooms where one may be in contact with several different metallic systems such as supply and drain pipes and appliance frames. The equipment bonding conductor is usually also used as the equipment earthing conductor (see above).

Grounding electrode conductor. is a conductor which connects one leg of an electrical system to one or more earth electrodes. This is called "system grounding" and most but not all systems are required to be grounded. The U.S. NEC and the UK's BS 7671 (http://en.wikipedia.org/wiki/BS_7671) list systems that are required to be grounded. The grounding electrode conductor is usually but not always connected to the leg of the electrical system that is the "neutral wire (http://en.wikipedia.org/wiki/Neutral_wire)". The grounding electrode conductor is also usually bonded to pipework and structural steel in larger structures. According to the NEC, the purpose of earthing an electrical system in this manner is to limit the voltage to earth imposed by lightning events and contact with higher voltage lines, and also to stabilize the voltage to earth during normal operation. In the past, water supply (http://en.wikipedia.org/wiki/Water_supply) pipes were often used as ground electrodes, but this was banned in some countries when plastic pipe such as PVC (http://en.wikipedia.org/wiki/PVC) became popular. This type of ground applies to radio antennas and to lightning protection systems.

OK, I will restate - the EARTH has NOTHING to do with protection. This is a HORRIBLE myth and one that probably will never be understood by the majority. The earth is not a magical sink for current....

The reason that neutral is bonded to earth is to prevent lightning strikes ON THE POWER lines themselves conducting down the wires INTO buildings and frying people, and to provide a reasonable reference for neutral.

PHYSICS says a current will flow only when the circuit is COMPLETE. In the case of mains power that is from the "hot" conductor out of the transformer, through the wires that feed the house, into the fuse box, down the internal circuit hot wires, back along the neutral wires to the fuse box, and BACK up to the transformer neutral. The current does NOT flow to earth.

The same is true ALL OVER THE WORLD. Physics is global.

If you want to prove this - try this experiment: Breakers and Ground Wires (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/bregnd.html#c5) If you don't understand this experiment p/m me and I will explain.

Apologies for being pain in the rear over this, but this is SO OFTEN misunderstood it drives me nuts!

- GY

[west lakes]

GY

All of that is understood, the main difficulty is the "common" usage of the term earth that encompases all of your definitions.
As I stated it is UK practice to provide an "earth" terminal that in most cases is a hard connection to the transformer neutral connection.

[Fargoo]

Aircraft with AC systems may use voltages as high as 110V (400Hz). But they always use a two wire system.

Therefore unllike the above list, unless you hold both wires of the aircraft AC system you can not electrocute yourself and then it is a 110 V shock compared to 230V.

Not true, for starters its 115v 400hz and secondly if part of your body is touching bare metal attached to the airframe and you happen to touch a live AC wire or terminal you will get an electric shock.

[Fargoo]

OK, I will restate - the EARTH has NOTHING to do with protection. This is a HORRIBLE myth and one that probably will never be understood by the majority. The earth is not a magical sink for current....

As west lakes above notes our Earth (ie yellow and green) in domestic wiring in the uk is used for fault protection.

[west lakes]

To put it in technical terms
In the UK the earth loop impedence measured between the live and earth terminals at a customer supply position is not allowed to exceed .8 ohm.

If you work the circuit out that is the live conductor,the transformer winding and "earth" return path to the transformer. That value has been calculated to ensure that the. normally, 100A supply fuse will operate for a fault at the supply position.

[GarageYears]

As west lakes above notes our Earth (ie yellow and green) in domestic wiring in the uk is used for fault protection.

But it is not the same as SOIL earth. The wording "Earth" has historic reasons mostly... the UK uses a PME system (Protective Multiple Earths) with only 2 conductors entering each house. These are live and neutral. As noted by other posters the neutral is connected to Earth (soil) at the transformer and likely several other soil earth bonds along the way. In the UK each property does not have a local earth stake that ties the neutral to soil, but here in the US we do. However the UK Earth wire (yellow/green) does the same job as the green wire here in the US - it ensures there is a second uninterrupted return path for any current that finds it way onto any metal parts of an appliance back to the neutral terminal in the fuse/breaker box, to ensure the breaker trips.

FYI - as an ex-pat living in the US, I still prefer the UK system with local fuses in each supply plug. It is infinitely more preferable to have the local fuse blow, than the whole circuit... especially if that happens to be in the lighting circuit, after dark, and you can't remember where the flashlight (torch) is!

- GY

[west lakes]

GY

Nearly, but PME has only been in use since the late 60's/early 70's there is still a lot of 3 wire system in use with a seperate "earth" back to the transformer.

(and some weird legacy bits in rural areas!!)

[Basil]

. . and, in the UK, a neutral to earth short will trip the RCCB - well in my house it does.
Bl00dy nuisance that :*

[GarageYears]

. . and, in the UK, a neutral to earth short will trip the RCCB - well in my house it does.
Bl00dy nuisance that

And it should! If you have a neutral<>earth short, then some of the current that should be in the neutral line will flow in the earth, which the RCCB will interpret as a fault (since the current flow in the live<>neutral will no longer balance). It's just doing its job. :D

-GY

[Basil]

GarageYears,
Your 'Breakers & Ground Wires' is very interesting reading, esp:
Grounding to a Water Pipe

A bare 12 gauge copper wire was inserted into the hot wire side and the voltage was confirmed by meter to be 120 volts. It was touched directly to a cold water pipe and did not trip the breaker! This is a copper pipe and extends without interruption directly out into the earth.

The DC resistance from both the ground and neutral electrical terminals to that copper pipe was measured and found to be essentially zero. The digital ohmmeter measured about 1 ohm or less to the pipe. If the earth were acting as a simple ohmic conductor back to the ground at the service box, it would have conducted 120 amperes and would have immediately tripped the breaker.

So, to the tiny meter voltage and current it exhibits < 1 Ohm but rather more resistance to the phase line?

[Pugilistic Animus]

Westlakes definitely know what he is talking about:ok:



he's an electrickerical injunir:}

Edit; did you know that delta/wye connected transformers are being phased out:}

:ouch:

[GarageYears]

So, to the tiny meter voltage and current it exhibits < 1 Ohm but rather more resistance to the phase line?

Principally the issue is impedance versus resistance - a meter measures resistance using a DC voltage as a source (typically a 9V battery in a digital meter like a Fluke), but the mains voltage is AC, so the current is switching back and forth at 60Hz (or 50Hz in the UK). The impedance is a measure of opposition to the AC current flow. Since we only need a marginal increase in "opposition" to prevent our breaker tripping, it is not hard to see that a small increase in the impedance at 60Hz to around 4-5Ohms or greater would prevent the breaker tripping (assuming say a 30Amp breaker).

- GY

[Dan Winterland]

Aircraft tyres tend to be conductive to ensure any static potential is discharged on touchdown. An old British type I used to fly actually had a copper mesh moulded into the nosewheels and you could see bit's of copper glinting in the sunlight sometimes. These days they put something in the rubber which makes the earth.

Helicopters can generate massive static potentials. If you're in the unfortunate position of having to be winched in a rescue, you may notice the winchman has an earthing lead danging from his harness. He will touch that on the surface before touching either the sea/ground or you. It's very important to let that happen - otherwise there may be a big flash and a yelp! If you're in water, it's you who will take the shock so don't be too enthusiastic to be rescued! With other rescue systems where they use either a strop or a basket, the crew will dip it in the water before it comes near you.

[Basil]

I know this is a bit of thread drift but:
If 'Earth' was strapped to Neutral, say, just before the domestic meter, would a Neutral short to Earth still trip the RCCB? i.e. like Mig15's system.

Don't worry, I have no intention of carrying out above modification :)

[Checkboard]

If you're in the unfortunate position of having to be winched in a rescue, you may notice the winchman has an earthing lead danging from his harness. He will touch that on the surface before touching either the sea/ground or you.

A bit like this video :}

YouTube - High Voltage Cable Inspection (http://www.youtube.com/watch?v=9tzga6qAaBA&feature=related)

[GarageYears]

If 'Earth' was strapped to Neutral, say, just before the domestic meter, would a Neutral short to Earth still trip the RCCB? i.e. like Mig15's system.Yes - that would cause the RCCB to trip. The way the RCCB works is to compare the current flowing in both the live and neutral wires (remember: mains is AC, or alternating current, so it flows in both directions, in both wires). When the circuit is working correctly current should ONLY flow in the live and neutral wires and be exactly in-balance. Any current flow outside of those wires will cause the RCCB to trip, since the current flow will no longer be equal, which is exactly the point of the RCCB! A short between the Earth and Neutral will allow the current to flow in both Earth and Neutral lines (split as a function of resistance), and will pop the RCCB. The RCCB has no idea where that current has gone, but it will see the imbalance - for all the RCCB 'knows' that missing current could be flowing through YOU!

Typically the trip current imbalance will be 5mA, so any mismatch greater than this will pop the breaker. For humans as little as 50mA @ 120V can be fatal, depending exactly where the current path flows. The worst possible path is across the chest for obvious reasons, so grabbing the live and neutral lines with left and right hands is a VERY bad idea. Many electricians I know will purposefully slip their left hand into their back pocket or belt strap to ensure they don't complete a chest path if they are forced to work on a live circuit.... not something recommended ever but occasionally unavoidable.

- GY

[stickjocky]

who, alot of complicated ideas flying about here, so i thought i would just simplify things;

a conventional earth is not a stick into the ground, consider it a 0v reference point with a very low impedance (IE lower impedance than through your body) therefore an attractive route to compete a circuit for any stray current (IE from a loose wire or whatever) if it were to touch a metal case, and somone was in contact with that case- should the case be earthed the current would go through the earth line (AC AND DC ALIKE) instead of through you.

In an AC it is for exactly the same reason, remembering that for a current to flow there has to be a potential difference (if you and the aircraft are all at 10,000v and you touch a 10,000v wire it doesn't matter because there is no difference - hence how birds sit on power lines). Therefore an ACs earth can merely be a mass connection of everything metal that stops a difference from forming between different parts - IE connection to the neutral of the battery too. A loose wire would therefore short through the low impedance mass of metal and not through the higher impedance human flesh. On the ground, the AC ground can be connected to the real world ground so there is no difference between the AC and the rest of the world - remembering it is a difference that causes a current. you do not want this difference to equalize through the fuel hose - a grounding reel is much more preferable.

source: electronic engineering degree

[stickjocky]

Many electricians I know will purposefully slip their left hand into their back pocket or belt strap to ensure they don't complete a chest path if they are forced to work on a live circuit

I would like to bet the risk of injury from falling of a ladder because they have their hand in their pocket is greater than that of completing a chest path! (at least for a competent electrician)

[A321COBI]

Go with the second answer