I was working in a Commercial building yesterday, hooking up some new mains.
In the bottom of the switchboard I saw this enclosure marked "Telecommunications Earth- Do Not Disconnect!".
And there was a 6mm2 wire leaving the box and running (I presume) out to an electrode outside.
I've never seen one of these boxes before, at least if I have, I've never taken any notice.
Why does the telcom's system need to be earthed?.
I thought it was a DC system, that was isolated from earth.
Could Paul or someone else please enlighten me?.
It just struck me as strange, it being the first time I've seen one!.

What's in the box? Probably a primary protector like we require here in the US. Protects the premises TELCO wiring, devices and building from damage caused by lightning and power crosses entering on the TELCO lines.

-Hal

The DC supplies used to power normal telephone equipment aren't isolated from earth. The norm is for the positive pole of the battery bank to be earthed.

The protection that Hal mentioned is one possibility, although I don't know whether this is commonplace at the subscriber's installation in New Zealand. It's not found in the U.K., but then we have a much lower incidence of lightning storms compared to most parts of North America.

Was there a PBX in this building, with its own power supply? If so, then this could be the main earth for the PBX. As well as the desire to securely ground the supplies to stabilize voltages to earth, there's also the point that some trunks to a PBX (either a regular PBX or a Centrex-type system) are ground-start lines. That means that to seize the trunk you momentarily apply a ground to one side of it instead of just completing the loop tip-to-ring as on a conventional line.

If this is a sizeable commercial place with its own PBX, then that telecoms earth is probably providing all three of these functions combined.

Yes Paul,
There was in fact a PBX system installed in this 3- storey Office block building.
Thanks for your help.

In many places, grounding of telecomm gear is almost taken for granted, but at times given great care in design, installation and maintenance. Serious problems can surface if grounding for AC switchgear, telecomm and lightning protection are not cross-connected. The “lower half” of Ben Franklin’s 1752 invention is usually alive and well in one form or another.

Although the North American continent has a wide range of lightning incidence, an “entrance protector” is a default item in all buildings, whether served from overhead or underground from telco central office. Routinely, surge protection above residential duty is furnished in 25-pair groups, and the protector modules are sacrificial, so usually made easy to replace; e.g., “5-pin” modules.

There are large varieties of protector modules, and entrance terminals that house the protectors. www.comm-omni.com/circa/ct3b1fs.htm slang: “3B/4B” module www.comm-omni.com/circa/1890.htm slang: “189” entrance block.

“Deluxe” ground electrodes can get fancy, like chemical rods planted in a conductive slurry. www.lyncole.com/rod.asp www.lyncole.com/p.lynconite.asp