Electrical Earthing

[alan340]

Hello,

Where are all the electrical components earthed to on an aircraft? I am thinking about when the aircraft is in the air and not on the ground.

Thank you,
Alan

[vapilot2004]

Hello Alan,

While a bond to the aluminum has been practiced in the past, for heavy loads the earth , ground or return circuit is carried on its' own wire.

With DC power, the earth or ground circuit is the negative return, while the 3 phase AC power really does not have a return, but may be grounded for safety. Most 3 phase motors do not have a ground return.

In several common circuits, the safety ground , RF (radio frequency) ground or point of zero potential is often the skin and structure of the aircraft.

Welcome to the forum !

vap

[Saintsman]

Actually, its no difference to a car. Everything is earthed to the airframe (chasis).

[alan340]

Thank you both for your help.

Alan

[PhilM]

Hello Alan,
With DC power, the earth or ground circuit is the negative return, while the 3 phase AC power really does not have a return, but may be grounded for safety. Most 3 phase motors do not have a ground return.


Sticking my neck out a bit here as I am not certain, but usually the Star point from a 3phase AC generator is bonded to the airframe. That way you halve the wiring (same as using DC earthing to the airframe).

For example a 28v AC Nav light on the wingtip will have 28v AC on one terminal of the S.B.U and the other will be an earth to the airframe.

[ARINC]

Hello Alan,
While a bond to the aluminum has been practiced in the past, for heavy loads the earth , ground or return circuit is carried on its' own wire.

vap



All aircraft electrical circuits use the Airframe as the "Earth" There is no seperate earth wiring !

Component chasis are bonded to the airframe to complete the fault path. Should a short to the chasis occour and to provide uniformity of bonding. (The latter An all together different subject.)

Yes 3 Phase star wound gennies are neutral earth bonded

[ICT_SLB]

All the above replies are true but a bit simplistic for larger air transport or biz jets. While ultimately all grounds end up connected to airframe, for ground loop & interference reasons each bus & signal type will be wired to a seperate grounding point in each area. These are normally divided by electrical bus type (AC or DC, Essential, Battery, Service etc) and whether power, signal or chassis. For aluminum airframes each ground wire is kept as short as possible from unit connector to ground but on composite aircraft like the Starship they were all run back to common grounding plates adjacent to the circuit breaker panels - a lot of extra weight & voltage loss.

BTW the electrical potentials on aircraft are kept as low as possible above ambient by means of the static wicks that are designed to leak away charge and thus reduce the possibility of lightning strikes.

[HotDog]

BTW the electrical potentials on aircraft are kept as low as possible above ambient by means of the static wicks that are designed to leak away charge and thus reduce the possibility of lightning strikes.

Yes static wicks are designed to leak away static buildup in the airframe but they do not reduce the possibility of lightning strikes. Their main purpose is to reduce interference to Nav and Com equipment. The associated discharges take place at frequencies of 10KHz to 350MHz range, well within the range of all the avionics equipment used in the aircraft.The wicks should be checked on a walkaround as they are very susceptible to damage from lightning strikes. If you notice that any are missing, you would be strongly advised to get your engineers to conduct a post lightning strike inspection in case other damage has been sustained.

[L Peacock]

0V DC uses the airframe. AC uses the airframe for 115V phase to neutral. 200 v is available phase to phase.

All major components are electrically bonded for lightning current dissipation.

[On-MarkBob]

The reason for using high voltage AC is to cut down on the weight of cable reqired. Low voltage DC requires heavy cable to supply the same current. You cannot share cables between DC and AC, thus if you use the airframe as a return path or 'Ground' for DC you cannot use it for AC as well! AC systems are kept seperate since they require smaller gauge wiring. DC often uses the airframe since it makes a wonderfully big 'wire' return path and thus cuts down the risk of an overload and electrical fire.

[mono]

Not too sure where you got your info from On-MarkBob,

There are plenty of a/c which use the airframe to earth both a.c and d.c. Including the B767 that you operate. The important thing is that the earths are segregated to prevent cross circuit interference. This is not just true of a.c to d.c interference but also different phases of a.c too.

In the nose of the 75 and 76 are a battery, battery charger and static inverter. The earths of which are all within about 3 ft of each other! (all the earths are to the airframe)

[PhilM]

On-MarkBob, as mono has said, aircraft can, and indeed do have both AC neutral and DC ground bonded to the airframe.

Slightly unrelated, but a Sky dish has an LNB on it, which requires power, yet there are only two conductors going to the dish, and they carry the signal ontop of the 18v for the LNB :) They don't affect each other at all.

[L Peacock]

Mono

I concur. On-MarkBob is incorrect. I accept that ground returns are segrated but there is no reason not to use the airframe for both DC and AC returns.

[H Peacock]

Some older military aircraft actually ground one of the 3 phases of 115 vac. Having looked at the circuit diagrams it appears that the 'B' phase output of the inverter (28vdc to 115vac) is grounded, while the 'A' and 'C' phases are sent to the relevant busbars. The only ac loads are very low current flight instruments. There is no 'star' or neutral output from the inverter, just the 3 phases. All of the 3-phase loads are therefore fed from the 'A' and 'C' busbars and a connection to ground provides the 'B' phase.