Where is an airplane's electrical ground point?

I recently had a conversation on Reddit where someone claimed that the ground reference on airplanes was their fuel tanks. This seemed a bit off to me but I know nothing of aircraft in general. I assume that there is Ground Fault protection on the circuits accessible to passengers but where is that wire grounded to?

• I don't know about the actual grounding of planes. But what is critically important is that there is no potential between the fuel and the tanker when refueling - otherwise you get the potential for a big bang (pun intended) So the fuel tanks as a reference makes sense Commented Feb 23, 2022 at 19:47
• conventional wisdom says that they're grounded to the airframe. Commented Feb 25, 2022 at 3:19
• This would be a great question for aviation.stackexchange.com. I'd love to hear an A&P (aircraft mechanic)'s perspective on this vs. an EE's perspective. Commented Feb 25, 2022 at 7:45
• Probably far off the mark, but the railroads had this idea of isolate both sides from the chassis because they really didn't want to deal with it. If you design for no ground you don't have to ask where the ground is. Commented Feb 26, 2022 at 7:55

Grounds are used as a reference point from which voltages in a system are measured. Voltage is a potential difference, not an absolute quantity, so you need some common point to measure from. That's what a ground does - it provides a reference point that we can call "0V", and we can measure all other voltages in the system from that point. 5V really means "a potential difference of 5 volts between here and the 0V reference point".

Two grounds in two independent systems do not have to be at the same electrical potential as each other. If you take two portable generators and measure the voltage between their negative terminals, you'll almost certainly find a non-zero potential difference.

We call the "0V" reference "ground" because on land we literally drive a metal stake into the ground to provide that reference point. However, a ground does not have to be physically connected to the ground, nor does it have to be connected to anything outside the system. Consider your phone: it has a ground plane, but it's not attached to anything. Everything else in your phone references that ground plane. If you take a multimeter and measure the potential difference between the ground planes of two disassembled phones that are not plugged into anything, you'll almost certainly measure a voltage.

On an aircraft there is no actual physical ground to hammer a stake into, so the chassis becomes the general point of reference. However, this isn't the full story. There are usually multiple ground references in a complex system like an aircraft, ship, or locomotive, and these may or may not be collectively tied to the chassis, which is referred to as "chassis ground".

For large vehicles like these, it is unwise to assume that the entire chassis will be at the same electrical potential across its entire length. Indeed, there may be dozens of volts of potential difference from tip to tip. As such, each electrically independent system generally utilises its own local ground reference, and communication between those systems is typically achieved using differential signalling that rejects any DC offset. The independent grounds are physically connected to the chassis for safety, to prevent them from "floating" and creating a high voltage shock hazard, but from an electrical design perspective the different systems' ground references are not assumed to be at equal potential.

As for the interaction between the electrical potential of the chassis and the ground potential of Earth, when the plane lands, any static charge on the hull is dissipated by the use of static dischargers, which you may have seen on the wings of commercial aircraft. This prevents a spark from forming when ground crew touch the fuselage.

• Do you meant that negative power supply wire is also connected to each instrument? Or do they take use of the chassis as a conductor? Commented Feb 23, 2022 at 20:23
• @Ralph The chassis indeed often is the ultimate return path from a power delivery perspective. However, from a design perspective, the systems don't assume equipotential of grounds, even though they are almost certainly all tied to chassis ground. Return currents in a chassis are a complex beast, even before you consider the interaction with external electromagnetic fields, so there are usually significant potential differences across the chassis, meaning physically distant "ground" terminations don't come out at the same voltage. Commented Feb 23, 2022 at 20:36
• @Ralph As an additional point of clarification, there may be cases where return currents are disallowed from travelling through the vehicle chassis, depending on safety and EMI regulations. I'm not really familiar with aviation electrical regs (beyond the fact that they're formidable, at least) but I know on ships they disallow most power feeds from using the hull as a return path, especially on LNG tankers and other tankers with IGGs fitted. In those cases the hull is treated more like the chassis in an IEC Class-I product. Commented Feb 23, 2022 at 20:55
• Static dischargers are for the benefit of the radio systems. They prevent the plane from becoming too charged in flight because this leads to random discharges from sharp points; these discharges jam the radio receivers. On the ground there's an earth cable connected before any fuelling operation. Commented Feb 24, 2022 at 9:14

Airplane will have no literal "ground" point while airborne, in other words is isolated from ground and any point of the aircraft might be in different potential to physical ground.

Before fueling or unloading a grounding cable will be attached by the ground crew to the airplane. The grounding point is designed so that fuel tanks and fuel inlet have no potential difference to physical ground and the fueling system. This is to prevent sparks from the fuel hose starting a fire and to prevent personal injury from a passenger taking the first step to ground or landing gear and getting shocked.

Sparking would occur due to static charge build up from the contact friction between air molecules and airplane surface. This needs to be taken into account in all air vehicles, which famously includes zeppelins.

I assume that there is Ground Fault protection on the circuits accessible to passengers

Ground fault protection - GFCI in the US, RCD in Europe, etc. - does not necessarily need a ground connection. The "ground" in this context refers not to the "ground wire" but rather to a ground connection of current from a device going through a person (or animal or faulty wiring, but "person" is the biggest safety concern) to "ground", which can be actual ground (person standing on wet ground touches a live wire) or any path for the electricity except the designated circuit wire. The end result of all that is a GFCI can actually provide protection without a functioning ground wire, and in the US is actually a way to provide "ground wire like" protection to circuits that have no ground wire (or other proper ground path).

This does not answer the primary question (grounding the plane for fuel transfer, etc.) but it does mean that passenger or crew accessible power receptacles can have GFCI protection even if the plane does not have a traditional grounding system. I suspect that a plane does have something close to a traditional grounding system, except for the lack of ground rods.

It's just like any other metallic device with electronics inside it, be it a stove, car or aeroplane.

The common circuit potential is the metal chassis, so all electronics inside a stove, car or aeroplane is ultimately "grounded" by bonding it to the metal chassis. Including the fuel containers, so that no charges can accumulate and everything is at equal potential.

IMHO- via personal experience..lol There is a difference between bonding and grounding (the concept)! "Grounding" is sometimes used to have a return path (eliminating a wire, think of older cars) but high power/critical needs are served by a return leg dedicated to that circuit. Negative terminals are not necessarily grounded (telephone systems utilize a -48 V scheme with the positive lug of the batteries bonded to ground). Bonding is a method of ensuring 0 potential between two or more objects (ignoring the very small resistance-hello ground loops...) On aircraft, there is usually (always?) a return wire to ensure you have a direct path and not relying on multiple physical connections. Automotive designs are following the same now. GFCI works by comparing the current going out and returning, if they differ (therefore having a leak via you or the device) the GFCI opens which protects you. Fuel systems on aircraft could use the shell of the tank as reference but more likely they use a float or capacitive probe that has separate connections which may be bonded to the aircraft structure mechanically and/or via strap.

The purpose of static eliminators on the wing are to avoid radio disruption due to the static electricity being generated when the aircraft is flying. Similarly, when fueling, aircraft are grounded to the ground at a common point with a fuel truck and also bonded to the fuel truck to eliminate any electrical potential when fuel is transferred due to static electricity being formed. Moving surfaces such as ailerons, elevators,etc. have typically individual bonding straps to eliminate possible potential via a mechanical only method.

Thats deep enough for now.

(25 year telephone tech + 4 year military aircraft mechanic/5 year A&P and now trying to avoid breaking electrical/mechanical stuff in a factory)