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Author Topic: Galvanic isolation of all three pins in XLR vs ground lift  (Read 1195 times)

Miguel Dahl

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Galvanic isolation of all three pins in XLR vs ground lift
« on: August 21, 2018, 02:09:45 pm »

This might be a stupid question, but I'm curious about the difference.

If running a signal line to several active speakers, where one of them is powered from a different outlet with a different ground potential. Is there a difference in using a galvanic isolation between all three XLR-pins and just "lifting" pin 1? on the signal feeding the speaker on a different ground potential relative to the ground potential to where the signal originates from?
« Last Edit: August 21, 2018, 02:23:09 pm by Miguel Dahl »
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Pete Erskine

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #1 on: August 21, 2018, 02:58:58 pm »

This might be a stupid question, but I'm curious about the difference.

If running a signal line to several active speakers, where one of them is powered from a different outlet with a different ground potential. Is there a difference in using a galvanic isolation between all three XLR-pins and just "lifting" pin 1? on the signal feeding the speaker on a different ground potential relative to the ground potential to where the signal originates from?

Depends on the input.  if it is transformer isolated no problem.  It is much more likely that you will need to have an isolation transformer in the line.
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Miguel Dahl

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #2 on: August 21, 2018, 03:11:28 pm »

Depends on the input.  if it is transformer isolated no problem.  It is much more likely that you will need to have an isolation transformer in the line.

Thanks, but I feel it didn't answer my question regarding what "answer I seek", the difference between a 3p iso and just a pin one iso.
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Pete Erskine

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #3 on: August 21, 2018, 03:42:21 pm »

Thanks, but I feel it didn't answer my question regarding what "answer I seek", the difference between a 3p iso and just a pin one iso.

A transformer is a 2 pin iso.  Lifting pin 1 just disconnects the ground but if your equipment is NOT transformer isolated, pin 1 does not really isolate it, particularly if it is and electrally balanced I/O.

If you have transformer and pin 1 lifted it is a 3 pin iso.
« Last Edit: August 21, 2018, 03:44:23 pm by Pete Erskine »
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John Roberts {JR}

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #4 on: August 21, 2018, 04:25:25 pm »

This might be a stupid question, but I'm curious about the difference.
perhaps...  do you have a problem?
Quote
If running a signal line to several active speakers, where one of them is powered from a different outlet with a different ground potential. Is there a difference in using a galvanic isolation between all three XLR-pins and just "lifting" pin 1? on the signal feeding the speaker on a different ground potential relative to the ground potential to where the signal originates from?
If the active speakers have a well designed input there will be no difference, if they don't the answer is not to float the ground (pin 1), but maybe see if the speakers float in the canal out back.

JR 
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Jonathan Johnson

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #5 on: August 22, 2018, 12:37:07 pm »

perhaps...  do you have a problem? If the active speakers have a well designed input there will be no difference, if they don't the answer is not to float the ground (pin 1), but maybe see if the speakers float in the canal out back.

JR

OK, let's assume that we've got balanced in/out, and they are well-isolated, so there's no impact on the audio. If there is voltage potential between the two grounds -- "pin 1" -- could you conceivably get a current flowing on the shield? What is the likely current? Could it be high enough to lead to undesirable heating of the shield?
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John Roberts {JR}

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #6 on: August 22, 2018, 02:06:46 pm »

OK, let's assume that we've got balanced in/out, and they are well-isolated, so there's no impact on the audio. If there is voltage potential between the two grounds -- "pin 1" -- could you conceivably get a current flowing on the shield? What is the likely current? Could it be high enough to lead to undesirable heating of the shield?
sure... mike probably has pictures of melted stuff over in the power forum from rogue ground currents.

JR
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Jonathan Johnson

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #7 on: August 22, 2018, 03:49:56 pm »

OK, let's assume that we've got balanced in/out, and they are well-isolated, so there's no impact on the audio. If there is voltage potential between the two grounds -- "pin 1" -- could you conceivably get a current flowing on the shield? What is the likely current? Could it be high enough to lead to undesirable heating of the shield?

My point being that there may be reasons other than sound quality to isolate or lift pin 1.
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John Roberts {JR}

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #8 on: August 22, 2018, 04:01:01 pm »

My point being that there may be reasons other than sound quality to isolate or lift pin 1.
If the safety ground is drawing excessive current you have a larger problem that needs correction.

JR
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Jordan Wolf

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #9 on: August 22, 2018, 10:11:32 pm »

sure... mike probably has pictures of melted stuff over in the power forum from rogue ground currents.

JR
Indeed, this Sticky is all about that, due to a RPBG and the snake wiring was the fuse.
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Mike Sokol

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #10 on: August 28, 2018, 12:05:38 am »

sure... mike probably has pictures of melted stuff over in the power forum from rogue ground currents.

JR
Like this one?
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John Roberts {JR}

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #11 on: August 28, 2018, 09:29:00 am »

Like this one?
exactly...

I'm probably repeating myself but UL performs a ground bonding test on any external product connection labelled as ground, to carry something like 50A fault current with single digit voltage rise. That wire melting is obviously a failure, but that audio jack may not have been labelled "ground". The intent is to protect humans irrespective of where the fault occured, and trip the breaker.

JR   
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Pete Erskine

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #12 on: August 28, 2018, 09:46:08 am »

exactly...

I'm probably repeating myself but UL performs a ground bonding test on any external product connection labelled as ground, to carry something like 50A fault current with single digit voltage rise. That wire melting is obviously a failure, but that audio jack may not have been labelled "ground". The intent is to protect humans irrespective of where the fault occured, and trip the breaker.


50 mA
« Last Edit: August 28, 2018, 10:37:51 am by Pete Erskine »
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John Roberts {JR}

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #13 on: August 28, 2018, 10:04:39 am »

50 mA

??

Not sure what you are saying...

I had to redesign a PCB inside a small install mixer/amp product when the UL ground bond test completely evaporated a PCB trace off the circuit board. I had the option of relabelling the audio termination 0V instead of "ground", but did the right thing and passed the test with a new fatter ground trace on the PCB.

JR
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Pete Erskine

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #14 on: August 28, 2018, 10:39:14 am »

??

Not sure what you are saying...

I had to redesign a PCB inside a small install mixer/amp product when the UL ground bond test completely evaporated a PCB trace off the circuit board. I had the option of relabelling the audio termination 0V instead of "ground", but did the right thing and passed the test with a new fatter ground trace on the PCB.

JR

the UL standard is 50 mA not 50A for a dangerous current
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Pete Erskine
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John Roberts {JR}

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #15 on: August 28, 2018, 11:22:01 am »

the UL standard is 50 mA not 50A for a dangerous current
50mA will not trip any fuse or circuit breaker I ever saw.

Ground bond test has to pass several tens of amps with low single digit voltage drop.

The first commercial ground bond tester I found on google provides 10A to 30A test current.

Quote from: ESTGUIDE.P65
Ground Bond
A ground bond test verifies integrity of the
ground path by applying a high current, low
voltage source to the ground path circuit,
typically a 25 or 30 A current.
This test is similar to the ground continuity
test with the additional benefit of verifying
how a product will perform under actual fault
conditions.
When a ground fault occurs, current starts
to flow through the ground circuit. If the
current-carrying capacity is high enough
and the circuit resistance low enough, the
system operates properly and the user is
protected from shock.
If, however, the ground circuit cannot carry
enough current or has too high an electrical
resistance, the circuit breaker may not trip
or the fuse may not blow. If this occurs,
voltage can build up to a point where current
will flow through the userís body instead
of the ground circuit.

I try not to be too pedantic but ground safety is important.

JR
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Mike Sokol

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Re: Galvanic isolation of all three pins in XLR vs ground lift
« Reply #16 on: August 28, 2018, 09:01:37 pm »

50mA will not trip any fuse or circuit breaker I ever saw.

Ground bond test has to pass several tens of amps with low single digit voltage drop.

The first commercial ground bond tester I found on google provides 10A to 30A test current.

I try not to be too pedantic but ground safety is important.

JR

I'm with JR on this one. Remember, this isn't continuous current, but it needs to be long enough in duration to trip a circuit breaker. That's the primary job of the EGC (Equipment Grounding Conductor). The NEC could care less about reducing hum in your sound system. They just don't want anything to have a elevated voltage due to a line-to-chassis short.
« Last Edit: August 28, 2018, 09:39:46 pm by Mike Sokol »
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