atcomsystems.ca/forum Forums General General Telephone & Systems Information NID Surge Protector/Circuit Protector??

I'm just a hobbyist, have always loved taking apart phones and learning about the telco system, from way back when I was a kid and #5 cross-bar was state of the art!

Anyway, I have a question. What surge protection/circuit protection is built into a standard telco NID? What would be the symptoms after an overload current? How can you tell if the NID took a "hit?" In such a situation is the NID repairable and, if so, how?

Thanks!

Johnny: The NID is provided by the customers telephone line provider, so if a problem exist with the line it is a point to test to see where might the fault be. Line provider or customer.
The Line provider should maintain the workings of the unit.
Test your line at this unit by opening up the circuit, and see
whom is responsible for the line fault.

The modern protectors are usually gas tube protectors. The older are carbon. They provide a simple shunt to a lightning induced surge to ground. They are kind of "brute force" and will only handle a "gross" surge. Your NID is designed to allow you to open the circuit and test inward, towards your telephone equipment. Some NID's also have testing ports to test for dial tone towards the Central Office. Gas tube protectors require special testing equipment. Carbon is a little easier as the effects of a hit are static, "frying or sizzling" sounds and disconnects or ground hums. In any event, if you suspect the NID has taken a surge and you want the protectors checked, you need to call your local telephone company and ask them for a service call to test your protectors.

Rcaman

What the LEC in my area primarily uses - TII 3700 series NID (they also use the 6 position and 12 position versions too)

And the protectors they use - TII AD-01W-FS (actually use the AD-01-FS but I can't find a link for that on TII's website)

I am not exactly sure how it works other than I think it shunts overvoltages to the ground lug (assuming a ground wire is connected to the NID which in my travels is like 75% of the time). I am not sure how to tell it has taken a hit or has failed. I assume if it has taken a hit and failed, it won't pass anything beyond it and remains open.

Since all of these items are field replaceable (the NID, the protector, and the bridge modules), the field tech from the LEC would simply replace the failed component(s) to restore service. A 216 tool is all you need now. No stripping wires back or screwing anything to terminals.

Johnny,

Since a T1 nid is line powered most will have a power led that glows green or a L (loop) led that glows green or in the case of the old two loop T1s L1 and L2 if all is okay. Usually if the facility or protectors take a hit the leds will start glowing a dull red. If the nid takes a hit you might see the dull red color on the nid or the nid might have no lights on at all.

The nid is kinda sorta a protective device in that telcos would rather have the nid take the hit and blow before cpe does.

Protection is one of the most poorly understood aspects of wireline communications, probably because it is not as sexy as iPhones, Fudgebook, routers in plastic housings styled to look like Bozo The Clown, etc. etc.

There are basically two things that a protector on a copper pair is protecting against: Lightning induced surges, and high-voltage power line "crosses," which can and do occur quite easily when aerial cable is carried on joint poles [poles that support both power and communications lines].

How they work is actually a lot more complicated and involved than could ever be explained in a brief thread on a forum, but here is an overview.

A standard protector on a residential POTS pair will consist of two main components. The first is an over-voltage protective element of some type [carbon block spark gap, gas tube, solid state device, or some combination of these] which is placed between the copper conductor and ground. There will be one such element for each conductor, so a typical single-pair protector block will contain two elements. As described earlier in this thread, these elements will conduct when the voltage gets high enough, and hopefully divert most of the potentially harmful current to ground. How many "hits" the protector can take before it is damaged or destroyed depends on the type of elements and the severity of the surges. Carbon block gap elements will fill up with particles that look like welding splatter, and show up as a partial [or hard] short to ground. Modern gas tubes and solid state elements are designed to fail as a short. Older gas tubes could fail open, and this was bad medicine, and a subject for a separate thread.

The second component in a protector consists of a fusible metal "plug" within the protector body. This is not a fuse like in your fusebox, although some older protectors did contain fuses [a whole separate discussion]. The fusible plug in a modern protector is designed to function in the case of a SUSTAINED fault, i.e., a high-voltage power line cross. Here is a typical scenario: Say a 7,500 volt primary feeder falls, and manages to miss the MGN bonding wire positioned below it, so the power company's protection never opens and it stays hot. Now say it happens to fall on a telephone drop wire going to a residence. The insulation on the phone drop does nothing to keep out this kind of voltage, so even if it doesn't make a dead short to the phone wires, there will still be thousands of volts present on the phone pair. Assuming the protector is properly installed and grounded, the over voltage protection elements will conduct, diverting most of the current to ground. At this point it is very likely the phone drop wire will burn up, which will break the circuit and keep the high voltage [and fire] out of your house. However, if it doesn't, the sustained current flow through the protector will cause it to heat up, eventually melting the fusible plugs. When the plugs melt, the house side of the protector becomes permanently open, and the line side of the protector becomes permanently grounded.

Now, just to make this thread a little longer...

The subject of antique fused protectors is a whole separate issue, and we can leave it alone for now, but when it was mentioned earlier that older protectors contained fuses, it implies that modern ones don't, and this isn't technically correct. Most modern residential NIDs do contain fuses in addition to the above mentioned components. The "fuses" actually consist of a short piece of VERY fine gauge wire [as light as 36 AWG], placed between the protector and the connection point for the house side. In the event that all else fails, and sustained high voltage makes it into the house, these fine gauge wires, often referred to as "fusible links," will melt and isolate the house side from the fault.

Yes, this is a long-winded post, but it barely even scratches the surface of this subject. If anyone wants to learn more [a LOT more], here are some sources:

1]Look online for some of the many BSPs on this subject.

2]Look online for some of the many REA/RUS publications on this subject.

3]Look online for "white papers" published by Lucent, Tii, Corning/Siecor, Bourns, etc. etc.

4]Search the archives of the Singing Wires group, and look for articles/posts on this topic written by Roger Conklin. Mr. Conklin worked in the industry for over half a century, and is considered by many to be the world's foremost authority on this subject.

I could provide links to some of this stuff, but I don't have the energy, and besides that would deprive you of the fun of doing your own research [it really can be fun, you know...]

Jim Bennett
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Please hit "Enter" twice, it's a long way to Utah.

CORRECTION...


This is incorrect, I didn't catch it on my proofread, and it was too late edit my earlier post. When the plugs melt, the protector will become permanently grounded, but this does not automatically open the house side. For that to happen, the fusible links [mentioned later] have to function.

Sorry, I am getting old, slow, and error-prone...

Jim
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Speaking from a secure undisclosed location.

" Sorry, I am getting old, slow, and error-prone..."
Amen brother.