Can a short circuit between live and ground destroy a telephone line ?

[tigrou]

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?

[radiolistener]

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.

No, it can't damage telephone line.

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?

No, it can't damage telephone line.

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?

if 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.

[tigrou]

if 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.

The 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).

[IanB]

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.

[radiolistener]

The 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).

It'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.

[floobydust]

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.

[tigrou]

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.

The router is connected to a small +5V power supply having only two connectors, it's not grounded

It'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.

I was thinking you were talking about no grounding at all or wall sockets with no proper grounding.

[madires]

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.

A 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.

[radiolistener]

I was thinking you were talking about no grounding at all or wall sockets with no proper grounding.

No, 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.

[SteveThackery]

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:

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.

A 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.

[tigrou]

No, 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.

Can ground wires be fused ? I'm not an expert, but that does not make sense to me.

[IanB]

No, 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.

Can ground wires be fused ? I'm not an expert, but that does not make sense to me.

I 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.

[IanB]

I admit to being puzzled by your story and cannot explain it.

It 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.

[ArdWar]

You can fuse GND, but you can't fuse PE.

Wait, what are we even talking about here?

[radiolistener]

Can ground wires be fused ? I'm not an expert, but that does not make sense to me.

The 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.

[radiolistener]

You can fuse GND, but you can't fuse PE.

On 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.

[IanB]

As you can see, it is possible to have a fuse or/and circuit breaker on the Neutral wire.

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.

People are talking about protective earth, and you keep saying neutral. They are not the same!

[radiolistener]

People are talking about protective earth, and you keep saying neutral. They are not the same!

sorry for mixing the terms. The protective Earth often is just missing, for example see the photo of mains socket above.

[themadhippy]

Neutral also can be connected through circuit breaker, though this is not allowed

protective devices on the neutral is fine, as long as the device also breaks all  live conductors on that circuit

[JustMeHere]

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.

[SteveThackery]

... it can cause a problem because mains current may flow into the grounding system through a different path, such as through a telephone line.

No, 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.

[tigrou]

The 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.

I 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.

[SmallCog]

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/

[SteveThackery]

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?

I'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 is extremely unlikely indeed.

[SmallCog]

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?

I've already dealt with this higher up the thread. It's better to read all the replies before adding your own.

I read your reply, I considered it, and then decided to add my own anyway.

Is there something wrong with doing so?