The phone company delivers service via a pair of wires, which terminate at their Network Interface. Extension wiring is run from this as either a star on multiple pairs of wires, or as a chain on one pair of wires. I can see a grounding wire going into the telco side of the NID ... what is it connected to in the NID?
The short answer is that it connects the the NID grounding buss. The grounding buss is the bar were the screw post are mounted which are used to connect the discharge terminals of the line gas tube protector modules. The gas tubes themselves conduct current to their discharge terminals when there break over voltage is exceeded.
Earth ground is surge protection. A protector is effective when it connects short - typically less than 10 feet - to a central earth ground - the protection. No earthing means no effective protection. IOW phone lines are protected when connected directly to earth ground during a transient. Therein lies the purpose of that earthing wire.
Inside the NID is a telco provided 'whole house' protector - a low capacitance device that earths phone wires only during transients. But that protector, just like plug-in (UPS or power strip) protectors, will do nothing effective without short connection to earthing. Not just any earth ground. Must be single point earthing as demonstrated by these figures:
NEC calls for a less than 20 foot connection for human safety. For transistor safety, a shorter connection and quality of that earthing device determines NID protector effectiveness. Same is true of a 'whole house' protector for THE most common source of destructive transients - AC electric. Each incoming AC electric line (just like NID protector) must have a short connection to same single point ground - either by direct hardwire (neutral wire) or via the 'whole house' protector (both hot wires). IOW all utilities should enter at the same service entrance.
Thanks for this. I hope you don't mind if I ask a two-part supplementary question!
1) The ground wire from the NID (on the telco side) is a new wire, that runs to a junction box where it is joined to what I guess is the original ground. Is this safe, or to code?
2) The ground wire is not cross-bonded to the electricity ground. Is this contrary to code?
ehsjr posted this description of grounding in another thread. It also applies to 800.40(B)1 which defines grounding for Communication Circuits (except for h):
Should the phone line short to AC electric (a fault), then a path (short circuit) exists from AC electric hot to AC electric neutral via faulted phone line that will either trip out a circuit breaker or burn open the wire. Either way, excessive voltage - a human safety threat - would be removed by properly earthing phone wire. If that earthing wire was not connected to same ground as AC electric, then the 'short circuit for human safety' might not exist.
Code also says that if no grounding means exists, then a dedicated ground rod can be employed. But your building has grounding - AC electric ground. As the above example demonstrates, phone line ground must connect to AC electric ground so that a fault is eliminated by the short circuit created by safety ground. Next paragraph agrees.
Section 800.40(B)2(D) provides another way for both grounds to be common:
That addresses the human safety issue. Phone line ground must be cross bonded to AC electric ground.
I am not sure what is meant by 'junction box'. For this discussion, box must be defined in terms of length and wire gauge to earth ground. That connection being unique both in terms of human safety (NEC requirements) and of transistor safety (requirements that must exceed NEC to earth transistor destructive transients). Furthermore code states connection length from NID to earth ground. Section 800.40(A)4:
That length to meet human safety. To provide effective transistor protection, that 14 AWG or larger wire should be even shorter (less than 10 feet) with no sharp bends, no splices, and not bundled with other non-grounding wires. We want all earth ground conductors to meet at the single point ground - not even share a common conductor to that ground. Again, transient protection requirements for phone service that exceeds NEC requirements.