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XLR shell earthing


chris_bollard
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Over the years that I've made cables for myself & others there was not one cable with an XlR that had pin 1 jumped to the shell. OTOH the mix input mini xlr (TA3M) on Sound Devices mixers require that pin 1 be connected to the shell of the input cable TA3F to activate the mixin connection.

Eric

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Same point I was making. In addition, I have on more than one occasion encountered equipment that was not properly grounded, which really makes life exciting. (On one show, I found that the video assist video cable feeding us was something like 40 volts above ground!)

I have also encountered problems in damp conditions, where small amounts of leakage current from AC distros, lamps, etc. get conducted through damp pavement or landscape. Back in the day, a lot of gaffers would switch to DC in these situations, but HMI's rule that out now (as does the lack of DC winding on most gennies).

Having had good and bad experiences both ways, I finally threw in the towel years ago and made up cables in both confiurations. Has saved my ass on more than a few occasions.

Note that even with a fully floating and balanced input, noise currents induced on the shield will still be an issue. Just not as bad as with an unbalanced circuit.

Again, I highly recommend Bill Whitlock's "Grounding & Shielding" workshop. One of the most concise seminars I've ever seen put together on the subject, aimed at the real-world situations we work in. Should be a required course for everyone working in audio, IMHO.

--S

Never encountered hazardous voltages, but a lot of times I've encountered edison ground contacting the shell... either because of a grounded box and plate in a permanent installation, or because the shell was touching a stand or rack or conduit that had chassis-grounded equipment (including lights) mounted on it.

If the audio input is pure balanced/floating (i.e., transformer with no center tap) it's not a problem.

But if the input is grounded to the audio ground in any way, and the audio ground is connected to edison ground even through something as innocent as a video feed from elsewhere... bingo: ground loop.

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According to AES standards found in technical bulletin http://www.aes.org/publications/standards/search.cfm?docID=44

Shell should never be connected to either pin 1 or shield for cable mounted connectors, in the event that inadvertent shell contact with another grounded surface can cause an alternate path to ground (hum).

Jeff, as far as induced hum goes from high voltage sources, connecting shell to ground would have no impact. Best way to avoid induction is to cross high voltage cables at right angles and use star quad canare cable.

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According to AES standards found in technical bulletin http://www.aes.org/p...ch.cfm?docID=44

Shell should never be connected to either pin 1 or shield for cable mounted connectors, in the event that inadvertent shell contact with another grounded surface can cause an alternate path to ground (hum).

Jeff, as far as induced hum goes from high voltage sources, connecting shell to ground would have no impact. Best way to avoid induction is to cross high voltage cables at right angles and use star quad canare cable.

I was not even concerned about induced hum, how little or how much you might encounter depending on the wiring, what I was concerend about has been perfectly described: "Shell should never be connected to either pin 1 or shield for cable mounted connectors, in the event that inadvertent shell contact with another grounded surface can cause an alternate path to ground" resulting in the above recommendation by AES.

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About 30 yrs ago I was roundly chastised by a very experienced audio truck engineer for having cables with ground tabs connected to pin 1 at both ends--he was from the "female only" camp and required us to make sure any cable we used in conjunction with his truck (basically all of the cables) were configured this way. This same config was policy at the large video production company I worked for (and its 3 remote trucks, stages, edit suites, MCR and location gear), as well as LA-based companies I worked for as well. But that doesn't mean I don't see the wisdom of the other methods, so I have cables done all 3 ways (since there are plenty of engineers in all 3 camps and I often have to do things the way the engineer from a given venue wants).

philp

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  • 6 years later...

This is an old thread but . . .  To prevent the person holding a given mic from being earthed and potentially shocked, do not earth any mic's metallic case.  This means connecting the earth-tab to pin one on the input device side only, not the mic/instrament side.  The mic cable's braid will shield the signal conductors along the length of the cable.  Any EMI striking the cable will be absorbed by the braid and then drained to the chassis ground of the input device.  Keep your musicians away from being earthed or they may burst into flames.  

 

 The differential noise cancellation scheme, which XLR connectors and two conductor mic cables support, will strip away induced EMI at the input device side.  However, even better is to divert any EMI away from the signal conductors in the first place; hence the shield.   The braided shield should have only one drain location and as it turns out, the only drain location is going to be at the input device.  

 

Steve Shockway

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Hi Steve,

thanks for the input. Out of interest, does your advice also apply for battery powered recorders isolated from main?

I could imagine in such a case neither would the input device act as drain not is there any danger of frying the person holding the mic, but my understanding of the electrics involves is very limited.

personally I like the idea of not grounding the shield at all thus reducing the risk of any power transmission at all.

 

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On 2/28/2019 at 5:00 PM, Steve Shockway said:

This is an old thread but . . .  To prevent the person holding a given mic from being earthed and potentially shocked, do not earth any mic's metallic case.  This means connecting the earth-tab to pin one on the input device side only, not the mic/instrament side.  The mic cable's braid will shield the signal conductors along the length of the cable.  Any EMI striking the cable will be absorbed by the braid and then drained to the chassis ground of the input device.  Keep your musicians away from being earthed or they may burst into flames.  

 

 The differential noise cancellation scheme, which XLR connectors and two conductor mic cables support, will strip away induced EMI at the input device side.  However, even better is to divert any EMI away from the signal conductors in the first place; hence the shield.   The braided shield should have only one drain location and as it turns out, the only drain location is going to be at the input device.  

 

Steve Shockway

 

I've heard this rationale before, and while it's an interesting theory, I don't think it has value in the real world...

First, for the talent to be shocked by holding a microphone with accidental voltage potential, they would have to be grounded to complete the circuit, such as standing barefoot on a wet surface, or possibly by touching a metal part of a set that was inadvertently grounded. Even in that case, most microphones have pin-1 grounded to the body of the mic, and those that don't, ground the body through a low value resistor. So, when the XLR is plugged into the mic, an annoying shock is just as likely with or without pin-1 grounded to the XLR shell.

 

On 10/12/2012 at 12:48 PM, Philip Perkins said:

About 30 yrs ago I was roundly chastised by a very experienced audio truck engineer for having cables with ground tabs connected to pin 1 at both ends--he was from the "female only" camp and required us to make sure any cable we used in conjunction with his truck (basically all of the cables) were configured this way. This same config was policy at the large video production company I worked for (and its 3 remote trucks, stages, edit suites, MCR and location gear), as well as LA-based companies I worked for as well. But that doesn't mean I don't see the wisdom of the other methods, so I have cables done all 3 ways (since there are plenty of engineers in all 3 camps and I often have to do things the way the engineer from a given venue wants).

 

philp

 

This mantra around production trucks (which I recall being to ground the male end only) is sometimes known as "_uck the Truck". But there is a big difference: Sensible grounding schemes are easy in permanent installations like a studio or production truck, as those XLR cables connect one device to another, and it never changes. But XLR cables used in production sound can be used very differently from day to day, and even differently in the same day from location to location. We use them for extensions, for line devices, for microphones, and reverse the direction with gender bender adapters. This is why, when making 3-pin audio XLR cables that we use in location sound, the odds are much more in our favor by always grounding pin-1 to the shell.

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On 3/1/2019 at 12:00 AM, Steve Shockway said:

This is an old thread but . . .  

 

Yes it is. And you signed up here yesterday to post this unprompted reply 7 years later. I‘m just really curious why? 

Please, don’t take this in any way as criticism. I am just so very curious how these sometimes seemingly odd occurrences come about...

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On 10/12/2012 at 11:15 AM, BudRaymond said:

According to AES standards found in technical bulletin http://www.aes.org/publications/standards/search.cfm?docID=44

Shell should never be connected to either pin 1 or shield for cable mounted connectors, in the event that inadvertent shell contact with another grounded surface can cause an alternate path to ground (hum).

Jeff, as far as induced hum goes from high voltage sources, connecting shell to ground would have no impact. Best way to avoid induction is to cross high voltage cables at right angles and use star quad canare cable.

 

Hi Bud. I downloaded the AES paper your link references, and it seems to be in conflict with your except about grounding pin 1 to shell. Can you post the excerpt in detail? After all, it's only been 7 years.

 

Glen

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A 45-year experienced audio engineer that I know very well (wink) has never grounded the shell on any of his thousands of feet of XLR cable.  But lately,  sometimes, he puts a little capacitor between the shell and Pin1, sometimes at the male end, most often at the female end, and sometimes at both ends.  He has not heard or measured any difference between any of those cables.  Oh, and he talks about the audible difference between Canare quad mic cable and Gotham GAC3.  But he can't hear the difference there under any circumstances.  As well, he really thinks that all his stereo speaker cables should be up off the ground on maple bridges and that a $3000 AC cable sounds much better than the Monoprice $2.99 cable even when it is plugged into the 60-year-old solid copper wire in the wall of his house.

 

In other words, do what you will.  If you don't like the result, try something different.  Just don't tell me (or that grumpy old soundman) that there is only one way to do it.

 

D.

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My comment about that old truck engineer from many years ago wasn't advocating any particular grounding scheme, just illustrating that there are many different closely held opinions on this subject, and that in those sorts of cases the truck engineer makes the rules for that job.  I did many other jobs with many other trucks with different (or no) ideas on the subject, and learned the hard way to ask in advance how they wanted to deal with the issue so I could avoid any embarrassing "public soldering" if possible. 

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On 3/2/2019 at 12:03 PM, Philip Perkins said:

My comment about that old truck engineer from many years ago wasn't advocating any particular grounding scheme, just illustrating that there are many different closely held opinions on this subject, and that in those sorts of cases the truck engineer makes the rules for that job.  I did many other jobs with many other trucks with different (or no) ideas on the subject, and learned the hard way to ask in advance how they wanted to deal with the issue so I could avoid any embarrassing "public soldering" if possible. 

 

Plenty of reasons to avoid public soldering, especially when show time is approaching, and you have some insane amount of cables to be fixed!

 

Having worked with plenty of trucks over the years, my default is always to go with un-grounded shells, which usually will cause less grief in the long run, for all the reasons I've outlined previously. As Phil points out, when doing remotes, what the truck engineer has to say it is absolutely the final word.

 

When dealing with one's own equipment, it might be another matter, but always CHECK IT FIRST! I've have had some rather unpleasant surprises over the years.

 

Also, there is still a certain amount of equipment in use out there where pin 1 has a different ground path to the electronics than chassis ground, and the results from grounding pin 1 to the chassis at the input or output connections will result in less than optimal results.

 

Stage boxes can be a particular source of grief. Some have a common ground at the box itself, others carry pin 1 back to the truck/console. Because of this, we still have two separate groups of XLR cables and snakes, some of which are shell un-grounded, others shell grounded.

 

(Also, do not confuse issues of RF and induced AC current into the signal conductors with ground issues!)

 

And if you've never taken Bill Whitlock's seminar on grounding and signal flow, sign up for it when it comes around the next time. There is no better authority on the subject!

 

 

 

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  • 2 years later...

Digging this one up again (taking cover)  :)

 

- I found another interesting take on this from a electrical engineering forum:

 

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.”

 

Link to that complete discussion:

https://electronics.stackexchange.com/questions/574776/why-should-the-shell-of-an-xlr-connector-not-be-connected-to-pin-1

 

 

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