Cable Shield Bonding

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davet

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Saw the Video. Q: is there any automatic way ( device ) or method to prevent or isolate or interrupt etc. the increased voltage/amperage from frying the shield of the cable etc. and preventing a fire, when you lose the neutral ? or is this impossible to do ?
 
I don't know the video to which you refer, but it is customary in my industry to connect only 1 end of a shield.
 
Only one end connected if it's only a used as shield, both ends have to be connected when it's also a conductor (e.g. coax cable).

It would be possible to build that device- not that difficult actually, but if the shield is a conductor you have to account for how the signals will behave across the switching element; if nothing else, there will be a blip in the impedance.
 
zbang said:
Only one end connected if it's only a used as shield, both ends have to be connected when it's also a conductor (e.g. coax cable).

It would be possible to build that device- not that difficult actually, but if the shield is a conductor you have to account for how the signals will behave across the switching element; if nothing else, there will be a blip in the impedance.
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To pick a nit, the shield is not always a signal path in coaxial cable. For unbalanced connections is is but for balanced connections it isn't.
 
davet said:
Saw the Video. Q: is there any automatic way ( device ) or method to prevent or isolate or interrupt etc. the increased voltage/amperage from frying the shield of the cable etc. and preventing a fire, when you lose the neutral ? or is this impossible to do ?
Click to expand...

What "cable" are you referring to? If it's a CATV drop (cable from the street to the house) they make isolators that do exactly what you are talking about. We have discussed them here before. And IMO they should be included as part of the normal cable installation by the cable company. Problem is the cost a little more than a ground block.

-Hal
 
Look for a coaxial cable ground loop isolator.

I've no direct experience with these, so cannot tell you what to look for quality wise. But I know they exist.

-Jon
 
What we are discussing is generically called a "DC block". Some only block DC on the inner conductor, and some have blocking on the outer conductor. For the purposes that the OP mentioned, an isolator that's designed specifically for the intended application and frequency range should be used, as hbiss has mentioned. Ones for CATV are widely available and relatively low cost.
 
zbang said:
Accepted, OTOH how much coax is used as balanced? (Is even possible? I expect you can with triax but I've never thought about it.)
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Balanced microphone cables are coaxial with two inner conductors. Maybe that's what you mean by "triaxial" but I have never heard them called that.
 
Balanced mic cables wouldn't be coax(ial) at all since the conductors don't share a common axis. I suppose you could refer to low-frequency shielded twisted pair (e.g. mic cable) as "twinaxial" but I can't think of anyone who does.

IME "twinax" refers to a balanced twisted pair enclosed in a shield and designed for >1MHz signals; an early example is the cabling for IBM 5250-type terminals and IBM mid-range systems (System/3, /34, /36, etc). IIRC it's also used in 10G ethernet DAC cables and other fancy stuff.
 
Triaxial cables are a special type of coax, with a second outer shield with an insulation layer between it and the shield surrounding the core. Used for broadcast TV cameras. Starting to be replaced with hybrid cables using fiber for the digital video and copper pairs for power.

As to twinax, when the IBM stuff went away, I converted a lot of twinax into pull rope for CAT5 cable. :)
 
zbang said:
Balanced mic cables wouldn't be coax(ial) at all since the conductors don't share a common axis. I suppose you could refer to low-frequency shielded twisted pair (e.g. mic cable) as "twinaxial" but I can't think of anyone who does.

IME "twinax" refers to a balanced twisted pair enclosed in a shield and designed for >1MHz signals; an early example is the cabling for IBM 5250-type terminals and IBM mid-range systems (System/3, /34, /36, etc). IIRC it's also used in 10G ethernet DAC cables and other fancy stuff.
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Irrespective of what you call them, for unbalanced connections through shielded cable with a single inner conductor the shield is part of the signal path and must be connected at both ends, while for balanced connections through shielded cable with two inner conductors, it isn't and should only be connected at one end if the devices at both ends are connected to AC power through a three prong plug. This is true for audio, anyway.
 
ggunn said:
while for balanced connections through shielded cable with two inner conductors, it isn't and should only be connected at one end if the devices at both ends are connected to AC power through a three prong plug. This is true for audio, anyway.
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While some experts feel that in some situations, a XLR balanced interconnect cable may have the shield connected at the send end only.
While other experts feel that the shield should be connected at both ends.
Microphone cables often require the shield connected at both ends for phantom power.
If the shield is connected only at the send end, then the receive end often has a hybrid shield connection of a radio frequency capacitor.
 
In the broadcast world, line-level balanced lines typically have the shield connected at one end only. Mic cables need to have a shield connected at both ends, but typically only one end is bonded to ground...the other end is the microphone itself.

In a high-RF environment, you might have line-level shields connected at both ends, or one end capacitively coupled...whatever it takes to beat any RFI that's getting into the system.

As to the OP's question, the instructions for the Jensen CATV isolator show the incoming drop connected to a ground block first, then the isolator goes between the ground block and the customer's equipment. So, foreign neutral current could still flow through the drop and the customer's ground system (and N/G bond) in a code-compliant install even with the isolator.
 
grich said:
As to the OP's question, the instructions for the Jensen CATV isolator show the incoming drop connected to a ground block first, then the isolator goes between the ground block and the customer's equipment. So, foreign neutral current could still flow through the drop and the customer's ground system (and N/G bond) in a code-compliant install even with the isolator.
Click to expand...

That's perfectly useless. Bonding is sufficient for protecting the inside coax from neutral current. It just doesn't protect the drop.
 
Speedskater said:
While some experts feel that in some situations, a XLR balanced interconnect cable may have the shield connected at the send end only.
While other experts feel that the shield should be connected at both ends.
Microphone cables often require the shield connected at both ends for phantom power.
If the shield is connected only at the send end, then the receive end often has a hybrid shield connection of a radio frequency capacitor.
Click to expand...
But microphones are not connected to AC power, so yes the shield can be connected at both ends and usually is. It has to be for phantom power to work, but even with dynamic mics there is no reason for it not to be.

If the devices at both ends are connected to AC power with grounding the shield should only be connected at one end because if they are connected at both ends it likely will result in a ground loop with current flowing in the shield.
 
ggunn said:
If the devices at both ends are connected to AC power with grounding the shield should only be connected at one end because if they are connected at both ends it likely will result in a ground loop with current flowing in the shield.
Click to expand...
While this may be true in a large multi-buiilding studio (but experts have different views on this). Correctly wired components shouldn't have 'ground loop' problems with it. The 1995 'Pin 1 Problem' papers covered correct design.
 
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