Why should the "shell" of an XLR connector not be connected to pin 1?

Question

I don't quite understand the grounding strategy of XLR connectors. The Wikipedia article on XLR 3-pin says the following:

AES standards mentioned above recommend that shells of cable-mounted connectors should never be connected to pin 1 or the shield, because inadvertent contact of the shell with another grounded surface while in use can create unwanted current paths for fault current, potentially causing hum and other noise.

How is the "shell" different than the "shield"? What do they mean by "fault" current, what is the scenario they are talking about here?

I thought that the two signal wires run inside of a conductive tube which shields the wires from external sources of noise and that tube should be grounded so that any electromagnetic energy entering the shielding is drained away to ground.

Answer 1

There are many reasons, such as safety and audio quality when avoiding ground loops, and avoiding electromagnetic interference.

And you can look at the issue from the cable point of view, or from the device point of view.

Basically, having a standardized safe way of how cables are wired together, it leaves the device side socket to decide how to connect to the metal parts of the connector on the cable.

First of all, the pinout for XLR connector is such that pin 1 is common audio ground reference pin, pins 2&3 are the balanced audio pins, and depending on how you want to count it, pin 4 is the connector shell.

On a device, the metal device chassis typically connects to XLR socket pin 4, so if the device is earthed via mains plug or other means, then also the connector metal shell becomes earthed via the device metal chassis. It makes sense on the device end to connect the connector metal shell to metal chassis, so it does not float unconnected so it acts as a grounded shield over the device connector where wires are not twisted.

The power supply of a device may be floating, i.e. not mains earth ground referenced, so the pin 1 common ground is for audio circuits only, it may not even be connected to the metal chassis or mains earth ground, depending on power supply design. Obviously, metal chassis being earth grounded or not, the metal chassis may be capacitively coupled to audio ground to prevent floating metal parts which is again bad for electromagnetic interference.

When the shell is grounded by the metal chassis, it allows the actual audio common ground to be connected inside the device as is best fit for a ground reference, to minimize ground loops such as in single point star fashion, instead of letting ground currents flow via device metal chassis.

Then the safety aspect. Cable has insulation, but the XLR connector is metallic, and typically you touch the metal parts when handling the connector.

If you have a cable that connects the shells of both connectors together, that is a risk of electric shock, because if you hold one end of the cable from the metal shell when connecting it, and the other end is connected to malfunctioning equipment or something causes the wire to become contact with hazardous voltages, that's dangerous. It is simply better to leave the metal shell to be not connected to the cable at all, so you would have to be touching the pins in the connector for it to be dangerous.

And that is why the pin 1, the audio common ground reference is used for the shielding of the two balanced audio wires, and the connector shell is on purpose disconnected from the cable, and the connected device defines how the shell is connected.

Sometimes, also the connector shell is made of plastic if it is irrelevant for shielding the connections inside the connector.

So that's how XLR cables for analog audio, for AES/EBU, for DMX-512, and even DIN cables for analog audio and MIDI work.

Answer 2

I think the general reason for this confusion is that bad circuits and habits have lead to mains hum in audio due to 50 Hz ground current entering the audio signal. A quick way to remove the nasty hum often was to cut the continuos ground loop at one point, eliminating the circulating ground current.

IMHO, this has led to a witch hunt for grounded shields (which are a good thing) and have even led to absurd things like cable shields that are left entirely floating out of fear for ground loops.

Long story short: Yes, do connect cable shields to chassis GND and treat pin 1 also like chassis, when you have balanced signals. Design audio circuits properly, so that ground currents do not enter the signal. Then you will have no trouble with ground loops, better noise immunity and less EMC problems.

Answer 3

An XLR chassis connector is nearly always connected to the chassis, never floating. If you use a cable with both shells tied to the shield, you are connecting the chassis together. From a ground loop prevention standpoint, you may not want this.