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Roger65
Contributor
8,635 Views
Message 1 of 17

AC line balance

Can AC line balance affect stability

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16 REPLIES 16
Geoff93
Recognised Expert
8,620 Views
Message 2 of 17

Re: AC line balance

Could you please elaborate a little more please?
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Roger65
Contributor
8,613 Views
Message 3 of 17

Re: AC line balance

freqent disconnection and slowing download speeds. I am led to believe that the ac line balance should be around 60 and mine is around 30.

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Somerled
Aspiring Expert
8,601 Views
Message 4 of 17

Re: AC line balance

 


@Roger65 wrote:

freqent disconnection and slowing download speeds. I am led to believe that the ac line balance should be around 60 and mine is around 30.


 

"AC line balance" could mean various things.  Could you explain what you are referring to ?

 

What units are you quoting ?  60 or 30 what?

 

Guru
8,599 Views
Message 5 of 17

Re: AC line balance

I think it must affect line stability as I believe there has to be some symmetry on the loop for it to function efficiently.

I recall an Openreach engineer commenting on one of my lines, and saying 60 is around the desired level, and closer to 70 gives an excellent connection. It's what I got after a pairs change ...

 

I can't really comment on the fact that yours is 30, other than to say it may be something to do with dslam card and exchange contention or imbalance due to resistance.

 

Perhaps any ex BTo people here can add more ....  

 

edit ... found this ....

 

"that is why telephone wires are twisted pairs, because by
twisting them at short enough intervals it guarantees that whatever
electrical exposure one wire has, so does the other.  They can
be run relatively close to power lines, for example, and pick up
considerable 60 Hz AC from them... but we don't hear it on our
phone line unless it becomes unbalanced for some reason.

That also explains why any mention of 60 Hz hum or buzz on a
telephone line (all of which run somewhere near some power line)
and the immediate response is to say that the line has something
causing it to be unbalanced.  Common problems are poor or wet
connections at junction boxes, or staples cutting through house
wiring.  Damaged insulation in a damp environment is another
major cause of unbalance.  Whatever the cause, the result is
usually hearing a 60 Hz hum.  But another thing it can cause is
hearing signals from other telephone lines, which are also
induced into the all of the cable pairs in a cable, but are
normally canceled out by common mode rejection."

 

 

I don;t think the 60hz reference is the 60 we're talking about ... but also I;m not sure whether it's voltage or restance it refers to ...

 

Do know it's measured between the MDF and NTE.

 

anybody?

 

 

 

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Somerled
Aspiring Expert
8,568 Views
Message 6 of 17

Re: AC line balance

 


 

Perhaps any ex BTo people here can add more ....  

 

anybody?

 


 

I am ex BT.....

 

I believe you're referring to "Longitudinal line balance", which is more commonly known as Common Mode Rejection Ratio (CMRR). The units are decibels (dB). It would take a rather long post to explain CMRR properly, so a visit to http://en.wikipedia.org/wiki/Common-mode_rejection_ratio will be a good start.

 

30dB is indeed a rather poor CMRR (how did you measure this ?). Your line will most likely be somewhat degraded at that figure. No point reporting it as a fault, though, as the Help Desk won't have a clue what you're speaking about.  If you have a poor CMRR, you will almost certainly hear background noises on telephone calls.

 

You may be able to improve things for yourself by installing an "I-Plate", which amongst other things, is designed to improve CMRR. See : http://www.productsandservices.bt.com/consumerProducts/displayTopic.do?topicId=25075&s_cid=btb_FURL_...  (The product description refers to problems in your house wiring, but its influence extends into the external network as well). 

 

The references you quote from are a discussion between some people in the USA. Hence the reference to 60Hz mains. The frequency of the UK electricity supply network is 50Hz.

 

Guru
8,556 Views
Message 7 of 17

Re: AC line balance

mystery solved ....  😉

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PheeragHfre
Recognised Expert
8,544 Views
Message 8 of 17

Re: AC line balance

 

You mean like this, a decent story anyway. Smiley Indifferent

 

Broadband Woes

 

 

"I have this awful feeling someone is watching every move I make (one of my pet hates is router location tagging)." Marvin (A paranoid Android)
Guru
8,535 Views
Message 9 of 17

Re: AC line balance

That's a good little link P  ....

 

Tells the story.  😉

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7,433 Views
Message 10 of 17

Re: AC line balance

Im a broadband engineer and i know only too well how AC balance can affect Broadband. characteristics are intermittent sync which occours randomly. AC balance is measured in dB and it is the ability that the copper pair has to resist an Induced signal and its ability to hold a steady DSL connection. if a line has had a line lest and there are no contitions on the line i.e No battery or earth contact then the AC balance should be the deciding factor (excluding line length) a poor AC is <50dB a good AC is >60dB.

hope this helps!