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[sevenlayermuddle]

The initial "modern day" (i.e. post 1980) plug & socket system, before the genesis of the NTE5, was with the various line-jacks and those constructed as a "master" socket were fitted with a metal oxide varistor (MOV) for over-voltage protection. With age, they had a tendency to shunt the pair with non-linear frequency dependent effects.

I was thinking of GDTs, not MOVs.

MOVs  are sacrificial by nature and do degenerate with every surge that they arrest.   But these devices are more commonly found in mains extension leads and the likes, where some have an (unreliable) indicator to indicate the MOV has had its day.   I was not aware MOV had been used in master sockets, but I’d agree they would have been a very bad choice of device, as they would degrade with time and wouldn’t offer any real protection against lightning.


Re historical situation, stress again the link I posted (and Kitz posted too, in the earlier thread)?

https://www.britishtelephones.com/lightng.htm (my colour highlights)

Early this (20th) century the protection fitted to telephones was discontinued and the protection was fitted externally to the telephone.  At both ends of the line fuses and mica protectors were fitted.  This involved an earthed protector unit with fuses on the customer premises and in the exchange, fuses, protectors and heat coils.  The heat coils were to guard against low voltage contact (250 volts).

Ps: above link does seem to assert that original NTE5A ‘protection’ was GDT, not MOV.   That would seem more sensible, though still pointless with regards to lightning.

Edit: added ‘ps’.

[burakkucat]

I agree that the original NTE5 was fitted with a two pole GDT. Perform a forum search and you will find more than one post where I mentioned activating a GDT (with 500V DC from an Ohmmeter 18C) and described the discharge as a delicate violet glow.^*

It was before the NTE5, when the LJUs were current devices, that MOVs were used.

I can well remember the early 20th century protection devices at the subscriber's premises. Two fuses, one per wire of the pair, two shellac coated carbon blocks between each wire of the pair and the terminal connected to earth.


^* Here's four "mentions" --
https://forum.kitz.co.uk/index.php/topic,12515.msg237129.html#msg237129
https://forum.kitz.co.uk/index.php/topic,12660.msg242251.html#msg242251
https://forum.kitz.co.uk/index.php/topic,13644.msg257311.html#msg257311
https://forum.kitz.co.uk/index.php/topic,22990.msg390348.html#msg390348

[Chrysalis]

Weaver are you saying your equipment took 2 lighting strike's in the space of a week? that is extraordinary.

[sevenlayermuddle]

I agree that the original NTE5 was fitted with a two pole GDT. Perform a forum search and you will find more than one post where I mentioned activating a GDT (with 500V DC from an Ohmmeter 18C) and described the discharge as a delicate violet glow.^*

That make sense as I’d expect a MOV to be 100% incompatible with DSL owing to a pretty massive capacitance, even before any degradation. 

There’s nothing obvious to me about a GDT that would impact DSL, though I may be missing something.   Even if it’s a phenomenon related to ageing  I’d still be interested to understand the process,  I didn’t think they degraded like MOVs.   I will consult Mr Google and report back if I find anything of interest.

There are other technologies too of course.   I’m thinking about deploying TVS diodes in a small hobby project in which I’m currently indulging... a gadget containing static-sensitive electronics, that will hang on my garage wall with exposed contacts.   Not that I’d expect my home brewed protection to necessarily save anything, it’s a specialised area, but nothing lost by trying.

Weaver are you saying your equipment took 2 lighting strike's in the space of a week? that is extraordinary.

Worth pointing out that these would not be direct strikes, as a direct strike would probably have done much vastly damage.  And in the absence of full building lightning protection, possibly set the house alight.   More limited damage, like Weaver’s, can occur some distance from the actual strike and I believe can be managed more easily.

[Alex Atkin UK]

I thought GDTs were supposed to be MORE sacrificial than MOVs?  Maybe its just a certain type but I've seen "one shot" mentioned a lot, basically they die after the first surge whereas MOVs degrade gradually (not great in mains use as they can fail dead short).

[sevenlayermuddle]

I thought GDTs were supposed to be MORE sacrificial than MOVs?

That’s not my understanding, I thought GDT  returned to normal after the arc has subsided, provided the duration of the arc is short.

I think there is a challenge with using GDT on AC power lines, whereby an arc is started by a surge and then rather than subsiding, the arc can be sustained by the normal AC voltage, quickly causing burn out and failure. That wouldn’t apply to phone lines though, would it?

But whilst I believe myself to have some understanding, I’m trying to avoid claiming to be an expert in all of this, or asserting myself as a source of fact.   My posts are usually peppered with qualifications like “my understanding is”, “might stand corrected”, “I don’t think”, etc... 

[Weaver]

@Chrys - don’t understand. Recap: There was one lightning strike very near me in Heasta; the other three next day were a few miles north of me.

[Alex Atkin UK]

That’s not my understanding, I thought GDT  returned to normal after the arc has subsided, provided the duration of the arc is short.

I think there is a challenge with using GDT on AC power lines, whereby an arc is started by a surge and then rather than subsiding, the arc can be sustained by the normal AC voltage, quickly causing burn out and failure. That wouldn’t apply to phone lines though, would it?

But whilst I believe myself to have some understanding, I’m trying to avoid claiming to be an expert in all of this, or asserting myself as a source of fact.   My posts are usually peppered with qualifications like “my understanding is”, “might stand corrected”, “I don’t think”, etc... 

Looked at a data sheet and its kinda as expected, depends on the surge current. eg https://www.littelfuse.com/~/media/electronics/datasheets/gas_discharge_tubes/littelfuse_gdt_cg_cg2_datasheet.pdf.pdf

[sevenlayermuddle]

Interesting reading from ITU-T, who ought to know a thing or two about phone lines, telling us how lightning protection should be done...

”Principles of protection against overvoltages and overcurrents”
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-K.11-200901-I!!PDF-E&type=items

And a scarier document with a broader scope, “Protection of Customer Premises...”
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-K.66-201911-I!!PDF-E&type=items

Other K series rec’s also of interest, check out the references section in above.  Such as K.12, which deals specifically with GDTs, browse itu.int to find.

But whilst I’ve no intention to read these publications cover to cover it’s clear to me that this is a non trivial topic.   Interesting to discuss but not something we should be thinking of actually implementing, with DIY hardware designed by forum contributors on the back of an envelope.

[Chrysalis]

Weaver what I mean most people would perhaps have equipment affected by a strike once in their entire lifetime, so for you to get two in a week is really unlucky.

[Alex Atkin UK]

Weaver what I mean most people would perhaps have equipment affected by a strike once in their entire lifetime, so for you to get two in a week is really unlucky.

I hope so, I've already had mine.

[Weaver]

Yesterday and today line 4 very sickly, down for 5 hours, today ‘dripping blood’ (ie AA CQM red colouring indicating PPP LCP ping echo loss). AA has booked BTOR engineer to come out tomorrow afternoon. I have also asked AA to look at the other lines, especially line 3 whose modem was also damaged.

[sevenlayermuddle]

You could ask the engineer to consider the ITU-T document(s) I posted earlier.  A recurring theme is that protection should be a balance between cost of protection, consequences of damage and cost of repairs, based on factors including probability and past experience of lightning damage.   Your recent and past experience might be a fairly compelling qualifying factor, imho.

Of course, tHe ITU-T documents are only recommendations.   It is up to regional operators, such as BT, to decide whether and how to incorporate these recommendations into their own standards.  They can ignore them if they like, on aspects like this.

[Weaver]

@7LM thanks for those docs, I somehow missed that post earlier- was very out of it much of today, exhaustion for no good reason

[Weaver]

I need to point out that this kind of lightning damage is just the general experience out here. My neighbours had an electric shock inside the house and one or two houses (memory fades) near here have caught fire. Each year we narrowly avoid or fail to avoid equipment damage.