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Author Topic: Is Blue/Brown in 4pair dropwire A/B or B/A?  (Read 525 times)

H4rry

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Is Blue/Brown in 4pair dropwire A/B or B/A?
« on: March 23, 2017, 02:12:38 PM »

I'm currently drawing a diagram of connections to figure out a separate issue and need some clarification please.

The house is served (by BT) to the Wall block terminal (BT66B) by a 4pair cable coming underground from the pavement DP.

I'm assuming this underground cable is a dropwire cable as it has the following pair scheme which is indicative or Dropwire 14 or 15 spec:
Pair 1: Orange/White, Pair 2: Green/Black, Pair 3: Red/Grey, Pair 4: Blue/Brown

The Blue/Brown pair is the one serving the house and I need to clarify which of the 2 colours should be going to the NTE5 A/B terminals?
« Last Edit: March 23, 2017, 02:14:51 PM by H4rry »
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j0hn

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #1 on: March 23, 2017, 02:50:54 PM »

The Blue/Brown pair is the one serving the house and I need to clarify which of the 2 colours should be going to the NTE5 A/B terminals?
Either. It makes absolutely no difference.
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licquorice

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #2 on: March 23, 2017, 03:13:24 PM »

Older dropwire didn't even have colour codes so wasn't even possible to determine A and B legs. The only time polarity mattered was in the good old days of shared service when ringing was A leg to earth for subscriber A and B leg to earth for subscriber B.
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H4rry

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #3 on: March 23, 2017, 03:19:45 PM »

I see...

Does that mean that BT are serving telephone lines with random DC polarity for each line (because it doesn't matter)?
Or is the polarity switched at the exchange by way of detecting the wiring scheme?
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licquorice

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #4 on: March 23, 2017, 03:20:46 PM »

Because it doesn't matter.
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tubaman

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #5 on: March 23, 2017, 03:21:36 PM »

The polarity as it enters the premises does not matter.
What can muck things up is if the polarity gets reversed between the master and any extension sockets (or between extension sockets).
 :)
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H4rry

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #6 on: March 23, 2017, 03:25:38 PM »

....So a -50V is as valid as a 50V line and it's pot luck what you get?....
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licquorice

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #7 on: March 23, 2017, 03:29:43 PM »

Not sure what you mean, there is a potential difference of 50V between the legs, one leg is at -50V the other at 0V, it doesn't matter which is which. Telephony equipment is not polarity conscious.
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tubaman

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #8 on: March 23, 2017, 03:36:37 PM »

....So a -50V is as valid as a 50V line and it's pot luck what you get?....

All lines are supplied as -50V as that is the exchange voltage.
It just doesn't matter which way around you connect it when it arrived in your house.
 :)
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H4rry

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #9 on: March 23, 2017, 03:38:15 PM »

I understand that telephony equipment is not polarity conscious. I'm just wondering if there's any standard that is (even loosely) adhered to when an engineer is wiring up a new socket.
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tubaman

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #10 on: March 23, 2017, 03:56:38 PM »

If you want to be strict about it then I believe the A leg would be the 0V side and the B leg -50V.
I think that would show as the "correct" polarity on the old test handsets (butt set) that the voice engineers had. (If they still have a polarity test feature I imagine its just to check that its the same throughout an individual installation?)
But it really does not matter nowadays.
 :)
« Last Edit: March 23, 2017, 04:01:47 PM by tubaman »
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H4rry

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #11 on: March 23, 2017, 04:02:07 PM »

OK, thanks for the clarification. :)
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licquorice

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #12 on: March 23, 2017, 04:30:30 PM »

If you want to be strict about it then I believe the A leg would be the 0V side and the B leg -50V.
I think that would show as the "correct" polarity on the old test handsets (butt set) that the voice engineers had. (If they still have a polarity test feature I imagine its just to check that its the same throughout an individual installation?)
But it really does not matter nowadays.
 :)

Indeed that is the case, but as to which colour is which..................
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burakkucat

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #13 on: March 23, 2017, 05:18:50 PM »

The house is served (by BT) to the Wall block terminal (BT66B) by a 4pair cable coming underground from the pavement DP.

A small point worth noting. A drop wire (or drop cable) is an aerial feed. An underground cable is best described as a service feed (cable).

Both CW1417 and CW1420 specification cable use --
  • Pair 1 Orange/White
  • Pair 2 Green/Black
  • Pair 3 Red/Grey
  • Pair 4 Blue/Brown
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Black Sheep

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Re: Is Blue/Brown in 4pair dropwire A/B or B/A?
« Reply #14 on: March 23, 2017, 05:48:41 PM »

A small point worth noting. A drop wire (or drop cable) is an aerial feed. An underground cable is best described as a service feed (cable).

Both CW1417 and CW1420 specification cable use --
  • Pair 1 Orange/White
  • Pair 2 Green/Black
  • Pair 3 Red/Grey
  • Pair 4 Blue/Brown

Slight amendment .............. Green/Black is the last pair when working on these cables. This kind of colour coding pair-count (although now seen in the newer cables), is actually a throwback to the old-style 'Concentric cables'.

It matters not what the size of the cable ...... Orange/White is always PR1, followed by Red/Slate, then Blue/Brown, then Red/Slate, Blue/Brown .......................................................... until you get to the last pair (marker), which is Green/Black.

It is a quality failure should an auditor spot an engineer has used the Green/Black as PR2 instead of Red/Slate.  :)

To the OP ............. as mooted above by many, the telephony systems of today are not polarity conscious, but there are still odds and sods of kit out there that require it to be so ........... old business systems, some pay-phones and also 'Caller Display Units' have all had us scratching our head at one time or another.  ;) :)
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