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In home utility power supply, AC neutral and earth GND are connected together at distribution transformer output. enter image description here

Is there a reason why aircraft power supply is not following the same practice? Why does the aerospace standards specify a strict isolation between neutral and chassis GND?

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    $\begingroup$ Note: it is the same on boats, ships, mobile homes, etc. Just lack of ground. Forget the old "there is no absolute potential, but only relative potential". So careful before to plug some devices (which doesn't expect such setup, like many surge protections, and measurement devices). $\endgroup$ Nov 9, 2023 at 12:51

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Electrical engineering question, but here is the idea.

In short

The usual earthing/grounding scheme used in large aircraft is TN-C. T indicates the source has its neutral connected to earth (ground). This scheme is the same than for homes in the US. Earth is used as a protective connection against accidental contact between a phase and an exposed metallic part (e.g. casing).

Isolating neutral from earth is an IT earthing specificity. IT is being used in new and more electric aircraft. I believe the documentation you've seen is related to this IT scheme.

Design differences:

  • In TN-C, a fault leads by design to a short-circuit between phase and earth, resulting in a current growing to a huge value in the time required for detection and source disconnection. The rate of growth is proportional to the voltage.

  • In IT earthing a fault is not a short-circuit, and current is limited to a very small value, there is no need to disconnect the source as long as there is a single fault (at the second fault, the source is disconnected).

IT is safer and tolerant to a single fault. More electric aircraft (MEA) which rely on more powerful electric sources, and higher voltages, will use IT earthing instead of TN-C.


What is the point of earthing?

Earthing is a way to protect people against an accidental connection between a phase and exposed conductive parts which are not intended to carry a current, what electricians call a fault. The protection consists in preventing a significant voltage to exist between the exposed parts and the ground.

If this was not prevented, a person in contact with the ground, and at the same time with the exposed part, would create a path for a current between the two contact points. Due to the low body resistivity, this current would usually exceed the lethal combination of 100 mA during 3 seconds. To prevent this, phase faults must be detected, this requires a connection to earth, either at the source point, at the consumption point, or at both.

(Earthing doesn't help protect against touching two phases, or a phase and the neutral.)

Disconnecting or not disconnecting?

Usually the remedy for a phase fault is to disconnect the source before the current raises to a lethal level, this is what is done for home consumers.

But disconnecting the source is something industrialists want to avoid, it means stopping equipments which may be long and costly to restart (e.g. a blast-furnace). Alternate protective measures have been devised, offering protection without disconnection as long as there is only one phase fault. Such tolerant protections cannot be used for home consumers, because they require the low-voltage transformer connecting the grid to the consumer to be dedicated, with its neutral isolated from earth.

Common earthing schemes

All earthing schemes are described in IEC 60364-1, and are known by an abbreviation:

  • First letter → mode of connection of the source neutral.

  • Second letter → mode of connection of conductive exposed parts (chassis).

  • Optional third letter → possible combination of protective conductor with neutral.

Common schemes are (pictures from Schneider Electric):

  • TT: Source neutral and chassis connected to earth.

    enter image description here

    If any current leaks to earth through the chassis and/or the body, then this quantity of current flowing through earth is missing in the neutral. If this difference exceeds 30 mA, it triggers a breaker to disconnect the source. Use in most countries in Europe.

  • TN-C: Source neutral connected to earth, chassis connected to neutral, protective earth combined with neutral into a single wire (PEN).

    enter image description here

    When a phase leaks into a chassis and earth, a short circuit is created with the neutral, it is detected and remedied by a breaker. Used in the US (and UK in part) for homes.

  • IT: Source neutral isolated, chassis connected to earth. Used around the world by industrials. The plant has its own transformer, its neutral is nearly-isolated from earth. There is naturally a high-value parasitic impedance between earth and neutral, and often a true fixed impedance (1.5 kΩ) is added to prevent the random change in the potential of the neutral.

    enter image description here

    When a phase is in contact with a chassis, a current flows through earth, the impedance, and the neutral. However this current is limited by the high-impedance on the neutral to a safe value. It nevertheless creates a detectable voltage at the terminals of the impedance, which is used as an indicator of the fault, but the source is not disconnected.

    A second phase in contact with a chassis is still not dangerous for a person, but it creates a short-circuit between phases, this is dangerous for the equipments, the source is disconnected.

Aviation earthing (grounding) schemes

There is no earth connection in an aircraft, but the aircraft conductive structure plays the same role. Earth (ground) continuity is ensured by the metallic elements of the airframe (frames, struts, crossbeams, and metallic skin) and a specific electrical network made of cables (ESN) for composite skins like in the A350 (an electrical continuity must also exist in the composite skin to evacuate static electricity and maintain the electromagnetic shielding against lightning).

The most common scheme in aerospace is TN-C. The documentation you read, which insists on isolating neutral and chassis, is not for TN-C.

There are plans for using an IT scheme. One reason is the use of composites which removes the advantage of having an inexpensive and strong PEN, TN-C can be as well replaced by the more tolerant IT.

IT scheme and Airbus

There is a thesis from Cédric Baumann, in French, about using the IT scheme in aeronautics. It led to a patent now owned by Airbus.

The advantages mentioned in the patent include using the IT scheme for the high-power technical loads (270 VDC/230 VAC) of a more electrical aircraft, the commercial loads (28 VDC/115 VAC) kept for compatibility still using TN-C. This duality allows to introduce some separation in the system. In addition, in TN-C scheme with a powerful source, in case of fault the current between the chassis and earth may reach large values in a short time (short circuit), a problem not found in IT with the high-impedance coupling.

I believe the documentation you have read is related to this IT scheme, not to the usual scheme.

More details about this in Grounding topologies for resilient, integrated composite electrical power systems for future aircraft applications.

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    $\begingroup$ It should be mentioned that ships are generally IT for the fault tolerance reasons as well; a single fault is no problem for reliability. $\endgroup$
    – vidarlo
    Nov 11, 2023 at 21:39

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