TELECOM ACCESS STANDARDS NEWSLETTER NO. 153

December 2004

CONTENTS
1. PREMISES WIRING SAFETY CONCERNS
2. ADSL MODEMS – LIKELY CHANGES TO PTC 270
3. PRIVATELY-IMPORTED CDMA DEVICES
4. HOME CABLING NETWORKS
5. SEASON’S GREETINGS
RETURN TO MAIN INDEX



1. PREMISES WIRING SAFETY CONCERNS

We have had a recent report of both telephone cabling and 230 V cabling being installed within the same flush box and with a telephone jackpoint and a 230 V outlets being fitted on the same faceplate. The only mechanical separation between the two operating voltages being via a rubber boot over the telephone socket module.

This practice hardly complies with the “spirit” of Electricity Regulation 59, which requires that electrical installations shall be installed such that there is “no significant risk of injury or death to any person”. To achieve this objective there must be effective “segregation” between different classes of operating voltage.  

The segregation requirement is followed through in “Wiring Rules”, the common name for AS/NZS 3000: 2000, more correctly titled “Electrical Installations”. Clauses 1.10.4 (e), 3.9.9.3 and 7.7.4 of this Standard deal with the segregation issue. As a means of ensuring safety, PTC 103, our Code of Practice for Residential-type Premises Wiring, requires either that the two classes of wiring are installed in separate compartments; or that there is a “horizontal separation of at least 200 mm” between 230 V hardware and telecommunications jackpoints if the two classes of wiring share the same wall cavity.  This rule was adopted as a simple means of ensuring that there is minimal risk of unsheathed TNV and Low Voltage conductors coming into contact.

A significant safety issue for Telecom is that any 230 V contact with the premises telephone wiring can extend hazardous voltage into our cable network, with potential risk for staff and contractors at some distance from the source of that contact.

The “200mm rule” obviously means that telephone and data socket “modules” may not be mounted in the same faceplate as any 230 V wiring components. 

An added complication is that Telecom’s contractors do not generally hold either full or limited electrical registration and cannot legally access or replace a faulty jackpoint if it is mounted along with Low Voltage (230V) components. As a result, the terms and conditions of our Residential Premises Wiring Maintenance Service specifically exclude cover for any jackpoints and wiring “mixed” with 230 V wiring and components.  PTC 103 is currently focused around the use of BT sockets, but increasingly, RJ 45 socket modules are being used for both telephony and data applications in the home. For data use, these sockets are not subject to our Telepermit requirements or to PTC 103 and there is a risk that they could also be mixed with 230 V fittings on the same faceplates. However, AS/NZS 3000 and the electricity regulations still apply.

Data cablers should be complying with AS/NZS 3080, Appendix ZA, of which clause 3.1 calls up AS/NZS 3000. Clause 3.2 of this Appendix requires increased segregation between 230 V and data cabling beyond the mandatory safety clearances in order to minimise noise coupling.


2. ADSL MODEMS – LIKELY CHANGES TO PTC 270

The success of Telecom’s current broadband promotion is likely to advance the need for improved power control of both downstream and upstream signal levels. Telecom’s PTC 270 specification has, from the outset, defined maximum permissible signal levels as a means of limiting crosstalk (in the form of interference between the different signals on cable pairs within the same sheath or cable unit - typically 25 or 50 pairs). However, “ADSL 1” technology leaves a modem operating at full power independent of whether it is carrying any data. As more pairs in the cable carry ADSL signals, the “noise floor” rises and all ADSL users on that cable may experience poorer service. This reduction usually has little effect on those customers who are subscribing to a 256 kbit/s service, but it could mean that they are unable to up-grade to a faster service in the future.

Advances in ADSL technology have included various means of controlling the power output of both DSLAM’s (the network end of the link) and the ADSL modem. The basic concept is to reduce power on short lines with low attenuation, or to reduce power during “no data” periods, dropping back to “sleep mode” when such periods are prolonged. These steps help reduce the overall crosstalk levels in the cable such that more cable pairs in the unit can carry ADSL with much reduced mutual interference.

PTC 270 will need to be revised, or an equivalent replacement specification will need to be published to cover these developments in due course. Meantime, modem suppliers may be interested to look out for ADSL 2 or ADSL 2+ chipsets that have passed inter-operability testing, operate with our current ADSL-based services today and be able to support the power reduction features when they come into force.

We will set up a new PTC-series for modems meeting the revised specification so that customers and our service staff can easily determine the capabilities of a “new generation” ADSL modem.


3. PRIVATELY-IMPORTED CDMA DEVICES

This matter was raised in Newsletter No. 137, two years ago, but we still get enquiries about “converting” overseas-purchased CDMA handsets and data products for use on our “027” network.

Clause 2.4 of PTC 256 provides the following warning:-

“CDMA devices purchased from other countries are generally incompatible with Telecom’s network and are not able to be connected to our network unless “Roaming” from a partner Carrier."

Even where an imported CDMA device is identical in hardware aspects to that already Telepermitted Telecom is unable to provide connection of that device. This is due to the software of the device being incompatible with that approved by Telecom. Telecom is unable to assist in connecting such devices as these devices are “locked” by the original carrier to prevent them being exported.

As the PTC process is expensive and time consuming Telecom will not accept PTC applications for one off devices or small quantity of devices”.

Testing alone costs a lot more than the price of a typical new handset.  Even if an overseas device is not “software locked”, suitable Telecom network compliant software may not be available or local agents may be unwilling to re-programme it.   Add the risk that an overseas handset can’t be modified for use here for some reason and the logical conclusion is that purchasing a locally available “Telecom approved” handset is the only way to be sure that it will work correctly on the Telecom “027” network.

Further advantages are that a new handset purchased in New Zealand comes with a full warranty and often a heavily subsidised price.


4. HOME CABLING NETWORKS

Further to my frequent recommendations that new home builders install plenty of star-wired Cat 5 cable in their walls before they are lined, I note that I had omitted a recommendation to terminate the cable with one of the rectangular series of Telepermitted 2-wire jackpoints where 4-pair cable has been installed. These have the big advantages that they can be replaced later by RJ 45 jackpoints, which are commonly available as modules to fit in the same size of faceplate and that they match the other 230 V switch plates and sockets.

One point to watch for long-term reliability is the type of flush box used. Many are simply folded brackets providing no protection, while others are virtually “a box full of holes”. Telecom recommends that a substantially enclosed flush box is used as the clearances between the contacts on RJ 45 sockets are much tighter than those of the BT jackpoint and generally represent a higher risk of corrosion. Our climate and building practices can lead to a lot of drafts in wall cavities such that dirt and dust can be deposited on the contacts. In combination with damp conditions and salt-laden air, this could prove to be a recipe for disaster unless there some protection is afforded by the flush box or the socket is specifically designed for such operating environments.


5. SEASON’S GREETINGS

Richard Brent, Alan Reedy, Marilyn Humphrey and I all wish our readers a Happy Christmas and a prosperous New Year. This has been another year that seems, on looking back, to have taken about 6 months to pass.

For readers in the Northern hemisphere, please note that Access Standards will be closed for business from 24 December until 10 January, so there may be some delay in getting answers to emails and other correspondence.

Meantime, we and no doubt everyone else in New Zealand are looking forward to some good weather for our holidays. Like many parts of the world, things have been a bit variable recently and we are certainly not supposed to get mid-winter weather at Christmas time.



Doug Burrus
Manager
Access Standards