Can a short circuit between live and ground destroy a telephone line ? - Page 1
Forum section: electronics / beginners

author: tigrou
author_rank: Regular Contributor
timestamp: September 03, 2024, 06:48:38 pm
content: I installed a new light fitting in the house. Due to an error, there was a short-circuit between the voltage and earth, and the fuses blew.The installation was repaired.Later that day, I realized that the internet was no longer working. Despite numerous attempts (restarting the router, checking the line connection as best I could), it didn't work. The router couldn't synchronize with the ADSL signal.We called a technician (who wasn't briefed on the whole story) and he discovered that one pair of the phone line was dead. The other pair was working, he made the switch (it didn't cost anything) and the problem was solved.I'm now trying to figure out what happened:1) A “mobile coverage extender” was added to the router earlier this afternoon. This device is connected to the router via an RJ45 cable. This device didn't work because it was too old / probably incompatible. I left it for a few hours because it kept flashing. I don't think it caused any damage.2) EMI and induced voltages: I don't think the phone line runs close to a grounding cable, but could it be possible that the current flow created strong electromagnetic fields around the electrical wiring and induced voltages?3) Common ground: is it possible for the house equipment and the telephone lines (installed elsewhere in the village) to share a common ground? Would there then be a direct path between the two?

author: radiolistener
author_rank: Super Contributor
timestamp: September 03, 2024, 07:01:08 pm
content: Quote from: tigrou on September 03, 2024, 06:48:38 pmNo, it can't damage telephone line.Quote from: tigrou on September 03, 2024, 06:48:38 pmNo, it can't damage telephone line.Quote from: tigrou on September 03, 2024, 06:48:38 pmif your ground is bad or disconnected (for example with blown fuse), there is possible ground path from remote side equipment through telephone line. In this case it can damage it, it happens when mains current flows through telephone line. But with proper connections it shouldn't happens. If it happens, this is an indication that something is wrong with your telephone line connection.

author: tigrou
author_rank: Regular Contributor
timestamp: September 03, 2024, 07:10:33 pm
content: Quote from: radiolistener on September 03, 2024, 07:01:08 pmThe ground is properly connected, it has been verified two times recently: one after solar panel installation (one year ago), second time by another electrician few months ago (electric installation was completely reverified).

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2024, 07:20:48 pm
content: If you have a short circuit between mains and protective earth/ground, then depending on the return resistance, the short circuit current can elevate the ground potential for a short period before the fuse blows or the breaker trips. If some other delicate electronic device is connected to the same ground, then the elevated voltage could damage it.There is maybe a possibility that this might happen to a telephone circuit, if perhaps it is plugged into a modem or router, and if that router shares the same ground that experienced the short circuit.

author: radiolistener
author_rank: Super Contributor
timestamp: September 03, 2024, 07:24:27 pm
content: Quote from: tigrou on September 03, 2024, 07:10:33 pmIt's hard to discuss abstract connections. If you want to verify all mains connections and possible ground loops through telephone line, it's better to draw complete schematic of all your connections. It should include all mains fuses.

author: floobydust
author_rank: Super Contributor
timestamp: September 03, 2024, 07:41:14 pm
content: As I fuzzily recall, telephone "tip" is earth-grounded at the local phone exchange, can be a few km away of 20AWG.So a ground fault in your home, if ground kicked up and also found a path to the telephone line could give high current in the phone wiring. It's over 880A to fuse 20AWG at 32msec.I would guess your home's PE ground is bad, and something dumped that ground rise over to the telephone line.Most phone surge protection is a GDT but it could have been a TVS that did it. Where the phone surge protection is, I have some in power strips and a UPS but don't connect it.Check the home's grounding and neutral tie as well.

author: tigrou
author_rank: Regular Contributor
timestamp: September 03, 2024, 07:47:54 pm
content: Quote from: IanB on September 03, 2024, 07:20:48 pmThe router is connected to a small +5V power supply having only two connectors, it's not groundedQuote from: radiolistener on September 03, 2024, 07:24:27 pmI was thinking you were talking about no grounding at all or wall sockets with no proper grounding.

author: madires
author_rank: Super Contributor
timestamp: September 03, 2024, 08:08:19 pm
content: Quote from: floobydust on September 03, 2024, 07:41:14 pmA telephone line surge protector could be the missing link. Usually telephone lines aren't grounded at the customer's side, and the router/modem connects to the telephone line via a transformer.

author: radiolistener
author_rank: Super Contributor
timestamp: September 03, 2024, 08:53:20 pm
content: Quote from: tigrou on September 03, 2024, 07:47:54 pmNo, I'm talking about bad grounding or the case when fuse on ground wire was burned first, so it leads to missing ground at all and the current start to flow through parasite ground paths, which can include telephone line. It may lead to burned wire.There are too many cases that can cause it, so with no schematic of your connections it's hard to say what may going wrong exactly.

author: SteveThackery
author_rank: Super Contributor
timestamp: September 03, 2024, 10:28:34 pm
content: Here in the UK the telephone line is not connected to ground at the customer end.It is extremely rare nowadays, but occasionally you might come across a lightning protector device at the customer's end of the line. However, these need a higher voltage than the mains to break down and conduct. The chances of finding such a device these days are very small.Incidentally, your initial description could be ambiguous:QuoteA phone line requires one pair. Modern dropwire has two or four pairs in it, but only one of those pairs is used for a single line. From your description it sounds like one of the pairs in the dropwire was faulty, so he switched to a spare pair.I've never known a phone line to be damaged by an electrical fault at the customer's premises. It's a very different story if there is a lightning strike nearby.To close: I want to emphasise that telephone lines are never* connected to earth at the customer's premises, even if they go into a modem/router. One good reason for this is that one leg of the phone line is earthed at the exchange, and the polarity of the line at the customer's premises is unspecified so it can be connected either way round. This precludes the possibility of an earth connection at the customer end.I admit to being puzzled by your story and cannot explain it.*Many decades ago there was a signalling system from the exchange to the customer in which a low frequency (sub-audio) common-mode voltage was applied to the line at the exchange, and a detector at the customer end was connected to both legs and a local earth. This system has been obsolete for four, probably five decades.

author: tigrou
author_rank: Regular Contributor
timestamp: September 03, 2024, 10:29:05 pm
content: Quote from: radiolistener on September 03, 2024, 08:53:20 pmCan ground wires be fused ? I'm not an expert, but that does not make sense to me.

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2024, 10:59:41 pm
content: Quote from: tigrou on September 03, 2024, 10:29:05 pmI don't think so either, but I declined to comment. Protective earth is a safety feature, and it should not have breakers or disconnects on it.

author: IanB
author_rank: Super Contributor
timestamp: September 03, 2024, 11:02:41 pm
content: Quote from: SteveThackery on September 03, 2024, 10:28:34 pmIt could be a coincidence. One time my phone line went dead, and when the phone company came to investigate, they found the wire was broken/detached at their cabinet out in the street.

author: ArdWar
author_rank: Frequent Contributor
timestamp: September 03, 2024, 11:32:41 pm
content: You can fuse GND, but you can't fuse PE.Wait, what are we even talking about here?

author: radiolistener
author_rank: Super Contributor
timestamp: September 04, 2024, 01:23:58 am
content: Quote from: tigrou on September 03, 2024, 10:29:05 pmThe mains socket has three wires: Line, Neutral, and Ground. Neutral and Ground can be separated or/and connected together at the mains electrical panel. There are different standards for their connections, as well as for wall sockets and equipment sockets. Therefore, it's possible to encounter a mix of different standards, making it difficult to predict which one is used in your building.Ground should be always connected, but some equipment or building wiring may not use this wire at all, so it may be just missing.Line should be connected through circuit breaker on mains panel.Neutral also can be connected through circuit breaker, though this is not allowed, but may be used.In addition, there may be a fuse in the equipment, and it might be randomly placed on the Neutral line because many mains sockets do not have strict orientation requirements for their connections, so you can't predict which wire will be connected to Line and which one to Neutral.As you can see, it is possible to have a fuse or/and circuit breaker on the Neutral wire. If it breaks the circuit before the circuit breaker/fuse on the mains Line wire, it can cause a problem because mains current may flow into the grounding system through a different path, such as through a telephone line.

author: radiolistener
author_rank: Super Contributor
timestamp: September 04, 2024, 01:44:50 am
content: Quote from: ArdWar on September 03, 2024, 11:32:41 pmOn EU mains sockets there is no way to predict which terminal will be connected to Line and which one will be connected to Neutral.And since equipment has a fuse it may randomly appears on any wire, include Neutral.Also there is possible incorrect wiring on mains panel, so Neutral may have circuit-breaker.As I said, there are possible many cases, so it's hard to discuss with no schematic.

author: IanB
author_rank: Super Contributor
timestamp: September 04, 2024, 02:11:51 am
content: Quote from: radiolistener on September 04, 2024, 01:23:58 amQuote from: radiolistener on September 04, 2024, 01:44:50 amPeople are talking about protective earth, and you keep saying neutral. They are not the same!

author: radiolistener
author_rank: Super Contributor
timestamp: September 04, 2024, 02:17:41 am
content: Quote from: IanB on September 04, 2024, 02:11:51 amsorry for mixing the terms. The protective Earth often is just missing, for example see the photo of mains socket above.

author: themadhippy
author_rank: Super Contributor
timestamp: September 04, 2024, 02:44:33 am
content: Quoteprotective devices on the neutral is fine, as long as the device also breaks all live conductors on that circuit

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 04, 2024, 05:33:20 am
content: Make sure you don't have a mains line running parallel with your low voltage stuff. An idiot did this with cable coax in my old house. It was blowing cable boxes, modems, the port on the utility pole, and even my neighbor's equipment.

author: SteveThackery
author_rank: Super Contributor
timestamp: September 04, 2024, 07:10:15 am
content: Quote from: radiolistener on September 04, 2024, 01:23:58 amNo, not through a telephone line. There is no electrical connection between a phone line and the local ground. The only time that could happen is if a voltage surge breaks right through all the insulation, and you're looking at a lightning strike for that.

author: tigrou
author_rank: Regular Contributor
timestamp: September 04, 2024, 01:12:07 pm
content: Quote from: radiolistener on September 04, 2024, 01:23:58 amI don't know if helps but during short circuit it's not the main circuit breaker which tripped but differential protection equipment (If I remember correctly there is two of them for the whole house).Those have been installed recently in the house (few months ago) as it's now a requirement to have them.The telephone line is as old as the house itself which is from the 70's. I am pretty sure short circuits like that occurred many times in the past and never damaged telephone line.

author: SmallCog
author_rank: Frequent Contributor
timestamp: September 05, 2024, 01:29:53 am
content: Firstly I'm not a data/voice cabler in my country and I have no idea what is typical in your country, however...IF there is a fault with the protective earth of the building, perhaps a high resistance joint, and IF there is surge suppression fitted to the phone line that is intended to discharge to earth, then would it not be possible for some of that short circuit current imparted into the protective earth cabling to be dissipated through the surge arrestor into the telephone wiring?A lot would have to go wrong for this to happen though...Example of the surge suppressor I'm thinking of:https://www.lightningman.com.au/product/surge-protection-hsp10k36-high-speed-line-protector-10-pair-suit-krone-lsa-copy/

author: SteveThackery
author_rank: Super Contributor
timestamp: September 05, 2024, 05:35:34 pm
content: Quote from: SmallCog on September 05, 2024, 01:29:53 amI've already dealt with this higher up the thread. It's better to read all the replies before adding your own.Decades ago a few (very few) telephone lines had lightning protection fitted (I'm talking about here in the UK). There were two types: a pair of gas discharge tubes, one from each leg of the line to a local earth. The other type was cheaper - I'm not sure of the technology, but once triggered they remained low resistance to earth, thus requiring a call-out to fit replacements.I also described how - even longer ago than that - a prehistoric signalling system used a low frequency common mode signal, which required a path to a local earth at the customer's premises.Both of these are exceptionally rare or quite possibly non-existent nowadays (both systems were officially withdrawn and all instances should have been removed). Both systems required MUCH more than mains voltages to break down. The telecommunications company was obsessed with keeping dangerous voltages off the telephone lines because the workforce uses no protection while working on it.Therefore I judge that the probability of the OP's local mains fault damaging the phone line is not literally impossible, but it isextremely unlikely indeed.

author: SmallCog
author_rank: Frequent Contributor
timestamp: September 06, 2024, 12:43:37 am
content: Quote from: SteveThackery on September 05, 2024, 05:35:34 pmI read your reply, I considered it, and then decided to add my own anyway.Is there something wrong with doing so?

author: radiolistener
author_rank: Super Contributor
timestamp: September 06, 2024, 04:39:47 am
content: Based on the description, the most likely cause of the telephone cable damage is that during the short circuit, a large current began to flow through the telephone pair, leading to the burning out of the wire.

author: SteveThackery
author_rank: Super Contributor
timestamp: September 06, 2024, 11:40:25 am
content: Quote from: SmallCog on September 06, 2024, 12:43:37 amThe idea of a conversation in a forum like this is for us all to share our ideas in such a way that collectively we home in on the correct solution. Putting aside misconceptions is how we make progress towards the truth.What you did was reintroduce a misconception that had been dealt with much earlier in the thread, and therefore you undid a significant amount of the progress we had made. This required someone (me, in this case) to waste their time explainingagainwhy your suggestion had already been considered and put aside as extremely unlikely.Your post added nothing useful (because it was simply repeating someone else's post) and actually detracted from the progress of the thread by throwing in a distraction that could mislead people.Obviously anyone can post anything anywhere (moderators permitting), and I wouldn't dream of trying to stop you. But you should expect to be challenged.

author: SteveThackery
author_rank: Super Contributor
timestamp: September 06, 2024, 11:44:03 am
content: Quote from: radiolistener on September 06, 2024, 04:39:47 amAt first glance, yes, obviously. But we have the challenge of working outhowthe mains fault could lead to a large current in the telephone wire. That's the bit that needs a credible explanation.

author: floobydust
author_rank: Super Contributor
timestamp: September 06, 2024, 02:01:42 pm
content: Part of the picture is still missing.Quote from: tigrou on September 03, 2024, 06:48:38 pmAt the house you have access to the other pair but how did it get switched at the other end? My phone exchange is 2km away.

author: tigrou
author_rank: Regular Contributor
timestamp: September 06, 2024, 06:40:17 pm
content: Quote from: floobydust on September 06, 2024, 02:01:42 pmThe phone exchange box is 500m away from the house. Switching pairs is probably as simple as unplugging 2 wires and plugging two others into some slots.Not sure if it anwser your question.

author: floobydust
author_rank: Super Contributor
timestamp: September 06, 2024, 07:07:30 pm
content: I wondered how far the tech had to walk/drive to switch to a new pair.If it failed due to corrosion or a loose terminal, it would have been acting up earlier possibly as slow (DSL) data, or you would hear crackling if a landline.Over-current melting the wire (fusing) would make smoke and burn conductors adjacent to it. So that seems unlikely it was destroyed that way.Second, so far we have no electrical path between your home's PE ground and the phone line.

author: tooki
author_rank: Super Contributor
timestamp: September 06, 2024, 07:28:28 pm
content: Quote from: SteveThackery on September 06, 2024, 11:40:25 amI feel it important to point out that you two didnotdiscuss the exact same thing. Steve, you spoke specifically of “lightning protectors”, while SmallCog spoke of “surge protectors”. And this matters because those aren’t the same thing!A kinder way of dealing with this might have been to say “oh, when I said ‘lightning protector’ above, I actually meant surge protectors, so we already discussed that a bit”.And unless I am overlooking something, I don’t see the topic discussed or “dealt with” above other than saying that it’s extremely unlikely that such a device is installed. To me that doesn’t constitute a misconception, nor a resolution to one.

author: SteveThackery
author_rank: Super Contributor
timestamp: September 06, 2024, 09:02:24 pm
content: Quote from: tooki on September 06, 2024, 07:28:28 pmThere are no surge protectors on telephone lines, and as of several decades ago no lightning protectors either. (Actually there may be the odd one that didn't get recovered, but they didn't trigger on mains voltages anyway.)

author: SteveThackery
author_rank: Super Contributor
timestamp: September 06, 2024, 09:06:16 pm
content: Quote from: tooki on September 06, 2024, 07:28:28 pmIt is resolved in as much as there is nowhere further to take it, because we cannot know for sure. What we do know for sure is that those protection devices weren't triggered by mains voltages anyway. As I say, resolved in the sense that it's a dead end.

author: Gregg
author_rank: Super Contributor
timestamp: September 07, 2024, 03:08:47 am
content: Here is a picture from a telco main office where a sparky grounded out a bus bar in a 48 volt nominal 53 volts battery float power distribution panel and blew a 600 amp fuse.