In aircraft power supply, why is the neutral not connected to chassis GND unlike household utility power?

Question

Usually the chassis of any household equipment is connected to earth ground for safety.

Part of the culprit is neutral being connected to Earth ground. (Reasons are mentioned in this post.)

My question is for an aircraft power supply (115V/400Hz AC), why is the neutral not connected to chassis GND? (In fact, the standard specifies strict isolation between the power lines and chassis GND.)

• What is the real consideration for it being different from utility power supply?
• Is it for safety reasons?

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Answer 1

This is typical in aircraft installations and for a few reasons. Firstly, if the supply is truly floating then any single fault that connects either live or neutral to chassis won’t cause a problem. Secondly, aircraft structures aren’t designed to carry any significant current (unlike automotive electrics) so a formal return conductor is always provided. Isolating this from the chassis ensures that there’s no current through the airframe. This helps to isolate different circuits so that a fault in one circuit won’t cause a failure in another.

Answer 2

As far as I get from your question and the other answers, in aircraft, neutral is not connected to chassis ground. This is called IT (at least in French), i.e. isolated earth (=terre in French).

So both neutral and live are floating compared to chassis ground. This means that a single fault (i.e. touching either neutral or live) is rather safe (you might get a small shock due to capacitive coupling, but probably nothing really dangerous). The only case you are at risk is if there is a first fault making contact between the chassis ground and either live or neutral, and then you touch something connected to the other one. So you need 2 faults to be at risk.

What's more, in such systems, there is usually a system monitoring the resistance between chassis and ground, and between chassis and live. This way, if you get a single fault (whether a short to chassis ground or just some leakage), you are notified at once, but without tripping any circuit breaker. As soon as you can, you find out the error and repair it.

Note that by measuring the resistance, and not just tripping, you can also detect very early components that might start failing. This technique is also quite used for sub-sea robots, where a decreasing resistance is considered as a not critical fault (dive might continue) but has to be solved as soon as the robot is back on ship. One classical case is a motor where the isolation of the electromagnets starts to be damaged: better replace the motor now (even if it works fine) than having a motor that stops working while diving.

If on a plane you used the "normal" wiring as at home, at the first fault, some equipment would be powered off. The wire of one unimportant button at the control panel of the pilot went loose an touches the frame? Too bad, all the control panel for the pilot goes off. Now you have only the copilot to handle the plane, without any redundancy left. Too bad. Doing IT (i.e. isolating chassis from neutral) makes that everything still works fine, you just have to do maintenance once you land.

Hospitals also work on the same principle: you can't have a single instrument failing taking power down for a whole section of the hospital.

So why not do the same at home?

• because it is far more expensive
• and mainly, because if there is a fault, and you don't check the monitor, or don't repair it quickly, the second fault is really dangerous. (And honestly, which proportion of population would call an electrician to come same day for a fault that don't prevent anything from working? 10%? And many would never solve the fault at all, until someone dies).

So basically, IT (i.e. isolation ground from earth/chassis-ground) is an excellent solution if you can't afford to have power cut of unannounced because of a "minor" fault. But it requires constant monitoring, and quick intervention to solve the fault as soon as it occurs. Usually this means having your one electricians on site (or at least on call) 24/24.

Answer 3

Usually the chassis of any household equipment is connected to earth ground for safety.

The illustration you provided is slightly misleading. Typically, when a live-to-chassis fault occurs and the chassis is grounded, the resulting short circuit current will trip a circuit breaker or blow a fuse in an electrical panel. If the current is insufficient to blow a fuse or trip a circuit breaker, then there is sufficient resistance in the circuit that goes through the chassis to keep the current below the trip level. If there is such sufficient resistance, then the chassis is potentially unsafe to touch.

When a the circuit breaker trips, or the fuse blows, the fault must be cleared (repaired) in order to for the circuit breaker / fuse not to trip again when it is reset.

My question is for an aircraft power supply (115V/400Hz AC), why is the neutral not connected to chassis GND? (In fact, the standard specifies strict isolation between the power lines and chassis GND.)

No design for safety is perfect. But the risks in an aircraft are significantly different from the risks in a house or in a car.

First, consider the risks associated with isolating the chassis from the electrical system. Such a system is safe if there is only one isolation failure. The danger of shock requires a circuit. For someone to receive a shock through the chassis, there must be a fault between the electrical system and the chassis, and a "fault" between some piece of equipment and a passenger.

Aircraft have periodic and continuous monitoring of important safety parameters. I'm not an expert in the area, but I assume there are either periodic or continuous tests of the resistance between the chassis and the electrical system. Thus a single electrical-system-to-chassis fault will be discovered and corrected, hopefully before a second, shock-producing, fault occurs. The quality control standards for electrical work in aircraft are higher than for general (i.e. residential) electrical work. Together these make isolated chassis systems in aircraft quite safe from shock.

In contrast, in a residence, an electrical safety issue might not get fixed unless it causes a noticeable (and annoying) problem with service. When residential electrical systems rely on isolation, a single fault might go undiscovered, and/or uncorrected until someone receives a shock. Thus, isolated power is less safe in a residential setting than in a aircraft.

But avoiding dangerous electrical shocks and fires is not the only safety concern in aircraft. As mentioned above, part of the safety mechanism associated with the grounding of chassis is the expected overcurrent associated with a fault causing a circuit breaker to trip (or fuse to blow). In an aircraft setting, one must consider the safety issue of a possible loss of power to an electrical circuit during flight due to a single fault. In some cases the consequences will be rather benign, like passenger's being unable to use their laptops. In other cases, loss of a circuit might be much more significant to flight safety.

No safety design is perfect, but given the factors above, using an isolated electrical system, rather than bonding the chassis to one of the electrical system conductors, seems a sound safety design choice for aircraft.