Asterisk fails when using APC power protectors

Asterisk is behaving very strangely when using APC PNET1GB protectors on the phone line.When I call from my mobile to the telco line I can hear it ring, but the phone does not ring. When I hang up my mobile THEN the phone starts ringing! Without the power protector everything is working fine. The power protectors are grounded, but I tried without grounding them and same thing. I’m using * 1.4.22 with a TDM400 card. How can I debug this? Where to start?

The page shows that the PNET1GB is for ethernet DATA ports, not phone lines. … ku=pnet1gb

Yes, but in the product description it says: “
Protection of data lines (Ethernet, Coaxial and Phone lines) ensures complete protection of your equipment from surges.”

In the short specification, it says:

The voltage on a PSTN line exeeds this when ringing; there is a standing 48 volts and up to about 60 volts of ringing on top of that.


Just to pedantic :smile: its -48v with ref to ground and the ringing is between 60 and 85v AC superimposed over it. So yes these will fail in use.

I have had many problems with inline data surge protectors full stop.


Ian, thanks for clarifying that. Do you know if the APC PROTECTNET DIGITAL TELEPHONE products are good for protection? If not, what other products would be best for protecting multiple lines?

Hi Are your lines fed overhead by a run of poles or are they fed underground ?


Hi, I don’t know what fed overhead and fed underground mean. How can I find out?

I believe Ian is asking if the lines coming into the location are buried in the ground or are they up on the electric poles. I’m interested in where Ian is going with this myself, maybe I’ll learn something new. :smile:

They are up on the electric poles. I know they say lighting does not strike in the same place twice, but I don’t wanna take a chance :smiley:


Ok this is based on over 25 years of telecoms and all of of that in rural England.

Firstly to find out how you are fed, Well look out of the window, do you see a wire leading to your house ? If you live in a City odds on you may well be fed underground.

Now to why it matters. Basicly the lightening surge/strike is looking for an easy path to ground for the induced voltage/current. So if you are fed by poles then the telecoms wiring will be that route as the cables are exposed . At the CO they will have surge suppression in place.

         A       B
         |        |
         |        -
         |       MOV
         |        -
         -        |  
       MOV        |-------Building Earth Point
         -        -
         |       MOV
         |        -
         |        |

So because they have suppressing in place and the strike is likely to be nearer you than them the surge will try and disipate to earth via your equipment.

Now if you are fed UG (underground) the likely hood of a surge is greatly reduced as there is much much more grounded metal work in the ground than hanging from poles, So the path to earth has many more routes.

What also has to be remembered that there are 2 types of surge Diferential and Common. they are pretty much what they say they are Common is a surge on both legs differential is a difference between legs.
In the above diagram A protects against Diff and B common surges.

With asterisk the most likely problem will be common surges, now these in a modem and I guess a TDM card will be protected against in part by the Line transformer, But in a rural area they wont stand a chance as the induced voltage is more likely to be in excess of the 1-2Kv that they are desiged to cope with. So some type of suppression is needed for example … 0Sheet.pdf

In a City Unless you have knowledge of people in your Buiding having surge issues I realy wouldnt bother as the induced surge will often be below the design spec of the card Surges have to be taken into accound at the design stage, You are more likely to have a mains surge and then you “should” get compensation from the electrical company for that.


The company that I am working for is established in a rural area and the phone lines are above ground. Do you know if the APC PROTECTNET DIGITAL TELEPHONE products are good for protection? I am asking because these are more common products and they are easyer to find in computer stores.

No Thats for E1/T1 if you want to get APC then go for … umentation

personaly I would get them from someone whos main business is surge suppresion.


Thanks for the effort Ian.
I appreciate it.


Ian, Thanks for the great write up!

A couple of points:

Surge protectors are designed to protect agains high power, medium energy pulses. Feeding ringing voltage onto them might put too much energy in and overheat them, so I wouldn’t re-use the surge protectors even for ethernet, without consulting the manufacturer.

On BT master sockets, the lightning surge protection is done with spark gaps, rather than semiconductors. Where you have a semiconductor, it tends to be there for fast protection, but with the spark gap taking most of the energy.