I've been working for a company that sells a line-switching device for use by the utility industry. We routinely get boxes returned to us that have been fried by voltage spikes, and it occurs to me that a central office must take a lot of surges, spikes, etc. That's a paradox, because I never heard of a CO being damaged by anything but a direct lightning hit the whole time I was a tech.
We had, of course, "carbons" and "heat coils" at the frame that were intended to take care of spikes coming in on the cable, but to gauge by the way today's electronic boxes get burned up so easily, I'm really surprised we didn't have a lot more trouble with lightning. Ergo, I'm wondering what other equipment was used to keep the CO's from suffering at Zeus' hands.
The old electomechanical switches (including the analog ESS) were pretty much immume from anything that got through the carbon blocks. The heat coils were for overcurrent protection due to a short in the cable plant.
Modern digital line cards have "secondary protection" circuits which are specifically design to deal with anything that can get past the gas tubes that replaced the carbon blocks. Don't forget the gas tubes short the pair to ground when activated (as did the carbon blocks). Gas tube protectors are more closely controlled as to breakover voltage than the old carbon blocks. That and maintenance issues are the reason they replaced the carbon blocks. This secondary protection often uses high voltage tolerant technology and can be designed to be very effective. The heat coils are still in place to protect from shorts in the cable plant, but modern digital line cards also current limit the loop current to something in the 20-40 mA range.
CO bonding and grounding is also very good compared to most subscriber locations.
Of course the cable plant is shielded (except, perhaps, the drop) and twisted to minimize capacitive and inductive coupled surges. Where poles are shared, then the power company conductors are above the telco wiring, acting as one of old Ben Franklin's lightning rods... Underground cable sees large surges only in the area of a strike being grounded, so they see less of the effect of lightning.
In short, if things are properly designed for the environment, then things short of a direct strike are usually pretty harmless.
BTW, subscriber telephones seem to do pretty well behind the telco protection blocks (at least the good ones)....
Perhaps the boxes you see are being fried by power line surges (which can be large due to either switching actions at substations or power line faults). COs run on batteries and have some decent isolation from power line surges.
On Tue, 07 Dec 2010 20:25:28 -0500, Bill Horne wrote: ..........
I know for a fact that all incoming connections in Australian COs are protected by heavy duty surge protection devices (and have been since the exchange equipment went digital decades ago). These devices clipped directly onto the Krone termination blocks along with a good ground connection.
These will protect the electronic exchange ports from anything bar a direct hit, but at the other end of the cables things like ADSL modems will just get progressively degraded by induced spikes if they do not have some protection.
One just has to frequent various forums and see the posts of people complaining about their ADSL services immediately after storm activity in their local area, but it takes a while for them to stop blaming their ISP and realise that their equipment "...but it's been working perfectly for years..." is the problem.
There are only about two things one can do to 'protect' equipment from spikes/surges/etc.: 1) give the 'excess' voltage/current a 'more attractive' path to 'ground'. 2) deny it a path 'in'.
(1) typically involves a 'gap' between the cabling and ground. sufficient to be an effective insulator at 'normal' voltage, but, at moderate over-voltage, it will 'arc over', and provide a near 'zero-ohm' path to ground. In the simplest form it is just a calibrated "air gap", fancier forms use gas discharge conductivity similar to a NE-2 indicator bulb, or something else that 'avalanches' into conduction when a threshold is crossed. (2) is the equivalent of a fuse, albeit in a variety of forms. Upon detection of the overload condition, it -open- the circuit, preventing the spike from going further.
Then there are combinations of the above. 'crowbar' circuits are a 'selective' short to ground, installed 'downstream' of a fuse type device. The crowbar detects a 'slight' over-voltage, and provides a massive short to ground, causing an immediate reaction from the circuit interruptor. The radical increase in the overload brought about by the crowbar causes the interruptor to react much faster than it would have to just the 'natural' overload itself.
Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.