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Author Topic: Telephone drop wires resistance to earth and lightning strikes  (Read 5817 times)

les-70

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Telephone drop wires resistance to earth and lightning strikes
« on: September 17, 2016, 09:47:29 AM »

  I am puzzling over whether telephone drops wires and overhead wiring are liable to attract lightning strikes.  At one stage I got some SNRM improvement by earthing the spare unused pair on my wire drop wire, it happens to be very easy to do in my property.  However I got worried over making the drop wire into a lightning conductor and disconnected it. Lightning did not seem something I wanted to mess with!  I am now wondering whether my fears were misplaced either because the AB wires are already referenced to earth or that telephone cables are not especially c.f. TV aerials or chimneys, prone to attracting strikes to houses. I do however recall that being on the phone in the middle of a storm is supposed to be risky.  I can't seem to find or remember the resistance to earth status of a live telephone circuit with its 0v and 50v A and B connectors. 

  I hope someone can offer offer advice or thoughts. 
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d2d4j

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #1 on: September 17, 2016, 10:11:05 AM »

Hi

Mythbusters, an American program did a section on this, and in normal wiring conditions, were not able to create the myth

You may want to watch it, which may be shown by looking up google

Many thanks

John
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Dray

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #2 on: September 17, 2016, 10:19:49 AM »

Quote
The MythBusters zapped a makeshift house with 300,000 volts of electricity to chart lightning's potential path from the ground wiring to the phone held by ballistics gel dummy Buster. The gel inside Buster shares the same electrical resistance as human flesh, and a heart monitor hooked to him measured how big of a zap his body received from the hot phone.

When they released the electricity, it quickly hopped from the phone wiring to Buster's heart, blowing out the monitor device. Since the machine maxed out at 40 milliamps, the amount of electricity pulsing through the conduit must've far exceeded the lethal 6-milliamp mark.

Unfortunately, this myth also has been confirmed the hard way: About two people die from lightning strikes through phone lines each year
http://www.discovery.com/tv-shows/mythbusters/mythbusters-database/phones-and-thunderstorms/
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les-70

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #3 on: September 17, 2016, 11:17:41 AM »

  That link confirms that lines do get lightning hits and suggests using wired phones lead to 2 dead a year in the US.  I guess the issue for me whether the line already has an effective earth and whether or not earthing an unused pair is an extra risk.  If it does not increase the risk of a strike it might have a positive effect by giving a safer path to local earth should one occur.
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Dray

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #4 on: September 17, 2016, 12:05:31 PM »

Earth won't protect you from a lightning direct hit
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les-70

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #5 on: September 17, 2016, 02:31:12 PM »

  I am not trying to protect from lightning.  i am just trying to make a good judgement of whether Earthing an unused drop wire pair would increase the risk of a strike.  My guess is that so long as the connected A and B wires are fixed with respect to Earth at the exchange it won't make much or any difference to the risk.
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Weaver

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #6 on: September 30, 2016, 11:08:48 PM »

> I can't seem to find or remember the resistance to earth status of a live telephone circuit with its 0v and 50v A and B connectors. 

SIN 349 says 100 kΩ
    http://www.btplc.com/sinet/SINs/pdf/349v2p5.pdf

Presumably in the case of ground potential rise, where you have a long line and the potential of the local earth at your house is different from that at the far end (i.e. at the PCP or at the exchange or whatever, don't know which is earthed), then you're going to heat up the conductors with current flowing trying to level the difference, aren't you? I'm not sure it's going to make much difference concerning lighting strikes, somehow I would think not. I wouldn't do it myself, but not because of lightning.

It's very interesting that you got an SNR improvement, makes sense. I should have said that if the far end isn't even earthed at all, then that's a different matter of course.

I have three ‘lines’, delivered as three pairs in two drop cables, and the one with the fastest downstream sync rate is the one that's alone in its drop cable. The difference is slight, but always consistently there.
« Last Edit: July 04, 2018, 11:19:04 PM by burakkucat »
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burakkucat

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #7 on: September 30, 2016, 11:56:12 PM »

SIN 349 says 100 kΩ
    http://www.btplc.com/sinet/SINs/pdf/349v2p5.pdf

It actually says that the insulation resistance, for an isolated MPF (i.e. one that is disconnected at both the EU's NTP and at the exchange MDF), will be greater than 100 kΩ for --
  • A-wire to B-wire
  • A-wire to earth
  • B-wire to earth
  • A-wire to battery
  • B-wire to battery
SIN 349 only defines the parameters of the MPF and not a service (i.e. PSTN and/or broadband) carrying circuit.
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Weaver

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #8 on: October 01, 2016, 12:09:59 AM »

@burakkucat - good point, you're talking about how it might be in use, which is more relevant surely. That's perhaps what les-70 needs.
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burakkucat

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #9 on: October 01, 2016, 12:26:43 AM »

Yes, indeed.

Try as I might, I am unable to find any relevant data.  :no:

I have a memory that each and every circuit which enters an exchange building is fitted with some form of rapid-action over-voltage protection device. If the maximum potential is exceeded on either or both legs of the pair, both legs are simultaneously clamped to earth.
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benji09

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #10 on: October 02, 2016, 09:55:03 PM »

   Surely if you are referring to telephone circuits, in the old days, the line would have been fed via a relay to the -50V supply and earth. I can't remember the relay coil resistance now, but assuming the maximum current of 50mA, ohms law will provide relay coil resistance. I think now the relay may have been replaced with a constant current source instead in modern electronic exchanges?  I am not sure now if lightning arresters are still fitted to lines these days where the lines are likely to be in buried cable to the customer?  As far as earthing the unused pair in the drop wire, I can't see it would act as a lightning conductor. But if you line actually got struck by lightning you would have no cable left to worry about anyway.......... I think most storm damage is due to the build up of static, or a pulse of energy from a near miss. It would not be unusual for people to damage equipment by plugging an aerial back in AFTER a storm.       
« Last Edit: October 03, 2016, 09:20:45 PM by benji09 »
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burakkucat

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #11 on: October 02, 2016, 10:27:14 PM »

To the best of my knowledge, all System X, System Y and remote concentrators have constant current sources driving the pair of each MPF.
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Black Sheep

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #12 on: October 03, 2016, 07:39:52 AM »

Yes, indeed.

Try as I might, I am unable to find any relevant data.  :no:

I have a memory that each and every circuit which enters an exchange building is fitted with some form of rapid-action over-voltage protection device. If the maximum potential is exceeded on either or both legs of the pair, both legs are simultaneously clamped to earth.

Over voltage protection modules.  :)
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burakkucat

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #13 on: October 03, 2016, 03:59:01 PM »

Thank you.
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les-70

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Re: Telephone drop wires resistance to earth and lightning strikes
« Reply #14 on: October 03, 2016, 04:30:48 PM »

  If I understand correctly it sounds as though the used lines have high impedance to earth with surge protection at the exchange.  I therefore think I will be wise to continue not to earth the unused pair that is currently disconnected at both the pole and in my house.  As I said before lightning is not something to mess with.
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