TELECOM ACCESS STANDARDS NEWSLETTER NO. 156

September 2005

CONTENTS
1. POTENTIAL WIRING HAZARD
2. RJ45 JACKPOINTS IN HOME SITUATIONS
3. 2.4 GHz WIRELESS INTERFERENCE ISSUES
4. PTC 220 INSTALLATION COMPLIANCE
5. NEW FAX NUMBER
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1. POTENTIAL WIRING HAZARD

Telecom has received reports from its service contractors that there have been increasing cases of telephone or data jackpoints being mounted on a common faceplate with 230 V outlets, switches, lamp modules, etc. This practice is in conflict with clause 3.2 of the joint Australia/New Zealand Standard ASA/NZS 3112, “Approval and test specification - plugs and socket outlets”.

While this is physically possible with the now widely-used modular electrical components, mixing TNV and LV on the same faceplate brings the risk of 230 V contact with the telecommunications wiring. Telecom is obviously concerned at any risk of 230 V passing into the Telecom cable network, potentially creating a shock hazard for staff far from the actual point of contact.

Telecom’s Code of Practice for residential premises cabling, PTC 103, requires a 200 mm horizontal separation between 230 V fittings and jackpoints connected to the Telecom network unless there is a physical barrier, such as a wall stud, between the two classes of hardware.

From purely practical viewpoints, any jackpoints mounted on the same faceplate as a 230 V fitting and failing in service cannot be replaced by Telecom contractors, most of whom do not hold electrical registration. Even mixing jackpoint modules with other ELV modules, such as co-axial connectors, aerial sockets, etc, complicates servicing, as Telecom’s contractors do not usually hold stocks of jackpoint modules and can only replace a failed module with the closest Telecom equivalent jackpoint.

TNV/ELV mixing will arise with RJ45 modules, which we expect to often be fitted in a common faceplate with a coaxial “F” connector, but this does not introduce any potential hazards. Also, as Telecom’s wiring maintenance service currently applies only to the standard 6-pin BT-style sockets, it is expected that most RJ45-based home installations, like commercial installations, will be maintained by an independent contractor.

Electricians probably do most telecommunications wiring in new homes these days, as it is most convenient to do this work as part of the electrical sub-contract. While their 230 V wiring is required to comply with AS/NZS 3000, this standard is not particularly clear on segregation requirements. Hopefully, this point will be addressed at the next revision of the Standard.  


2. RJ45 JACKPOINTS IN HOME SITUATIONS

The 8-way RJ45 socket is now the “international standard” for generic or “structured” cabling installations and has been used in commercial buildings for many years. The RJ45 has gradually been introduced into home cabling, mainly for home LAN’s and data applications, but we are now seeing a lot more “generic” home cabling. Modern generic cabling systems in homes are designed to support a wide range of services from POTS, through to data and video, and operating at speeds of 10/100 Mbit/s or even 1 Gbit/s. Use of these RJ45 sockets for POTS applications means that the centre pins can be connected to TNV3 (Telecom line voltage plus ringing voltage and possible voltage surges from lightning or mains contacts).

The usual safety requirement for TNV3 is that any exposed conductive parts at that voltage shall not be accessible with a standardised Test Finger. However, such a test finger can contact the centre pins of an RJ45 socket. The safety standard for Information Technology equipment, AS/NZS 60950, relaxes this requirement such that sockets on the actual equipment are subject to testing with a Test Probe, which does not have a tapered tip and cannot contact the centre pins of an RJ45.

However, AS/NZS 60950 does not apply to premises cabling, so RJ45 sockets fitted to premises wiring connected to a Telecom line should comply with the Wiring Rules (AS/NZS 3000). Clause 1.4.50 of this Standard defines an “Exposed conductive part” as one which can be touched with the standard test finger specified in AS/NZS 3100. The AS/NZS 60950 test finger is virtually identical.

Strictly speaking, this means that any use of RJ45 sockets connected to a Telecom line operating at TNV3 is in breach of the Wiring Rules, despite the fact that RJ45’s have been widely used in this way for some years. Commercial premises are usually occupied by adults, whose fingers are somewhat larger than a test finger, and most adults would not poke a finger into a socket in any case. However, the home situation is different, as small children are often present.

To ensure that the installation of RJ45 sockets and generic cabling in new homes is not going to cause any safety concerns, this matter was raised with the Energy Safety Service. However, where there is any concern over the safety of children, the simplest answers are to either use shuttered sockets or to insert a “dummy” plug in any unshuttered sockets likely to be “explored” by small children.  


3. 2.4 GHz WIRELESS INTERFERENCE ISSUES

The 2.4 GHz band is one of those covered by a General User Radio Licence (GURL) in that anyone can use this band for a range of services, subject to their not exceeding the defined power output limits and not transmitting spurious radiation from harmonics. Specific site or user licences are not required, but it does mean that interference is a distinct possibility with some combinations of products using the same radio band.

With the prices of equipment dropping steadily, this band is already used for a wide variety of services. Not only WiFi, but 2.4 GHz and wideband DECT cordless phones, baby monitors, video senders, security cameras and wireless alarm systems, Bluetooth devices, wireless door bells, microwave ovens, etc, may all be using this same band. The following explains how such interference may impact on Telecom services. Bluetooth and many cordless phones in use today may severely interfere with a WLAN in the same vicinity. It has been claimed that some 802.11b WLANs have been shut down by simply answering a nearby 2.4GHz cordless phone.

Bluetooth and 2.4 GHz cordless phones typically use frequency hopping spread spectrum (FHSS) technology that operates by hopping from frequency-to-frequency across the entire 2.4GHz spectrum. 802.11b WLANs, on the other hand, use direct sequence spread spectrum (DSSS), which provides for three 22 MHz wide bands within the 2.4 GHz spectrum and transmits over just one of those bands. As FHSS jumps across the entire spectrum while DSSS stays in only one part, a FHSS 2.4 GHz device can interfere with an 802.11b network, causing lower bit rates or even failure in extreme cases.

The easiest and cheapest way of minimising mutual interference between a cordless phone and a WLAN – if it works – is to change the location of the WLAN access point and/or the cordless phone base station to maximize the distance between them.  If this does not work, it may be necessary to replace the cordless phone with one operating on a different frequency band.

Bluetooth operates at much lower power levels and will generally only have any impact on devices located close by. Where the WLAN device is operating at maximum distance from the access point, it will be most susceptible to interference.  If it is essential that the WLAN service is available without interference in such locations, it may be necessary to install a WLAN repeater to improve the signal level. On the other hand, the WLAN can also interfere with Bluetooth to some extent, slowing down its effective bit rate.

We would be interested in any feedback on other situations in which interference may be occurring.  


4. PTC 220 INSTALLATION COMPLIANCE

As explained in Newsletter No. 155, we are seeing a lot of VoIP system test reports that indicate compliance of a specific combination of terminal equipment without the actual transmission settings being defined. Further complications arise where it seems clear that different settings may have been used for different tests, such that there is no one lot of settings that satisfies all PTC 220 requirements.

Actual installations in New Zealand have resulted in a range of performance variations. These have come about because the installers were not given clear instructions. It is appreciated that different manufacturers use different terms for their system settings and these are not always very clearly related to the actual functions performed. These differences can also cause unnecessary confusion.

What we are aiming for with any VoIP Telepermit application is:-

a. a clear statement from the manufacturer or test laboratory of the “specific settings” that have been used during all of the testing (written in the same terms as covered in that particular system’s documentation) and giving the actual values for that setting; and

b. assurance that these same settings will be clearly communicated to the installers of that equipment when it is to be connected to the Telecom network; and

c. that the supplier has a reasonable system for ensuring that these settings either not adjustable in the field, or that they are not adjusted by any party without the express permission of the Telepermit holder.

The aim of “b” above is that instructions given to the installer or party commissioning the system would include the same terms and values as were determined to provide compliance in the test conditions in “a” above.   


5. NEW FAX NUMBER

We have recently been re-located into Unit 4 of the Tory Street Telecom Centre. As a result, we now have a new fax number (04) 801 7496, as shown on the front page of this newsletter.



Doug Burrus
Manager
Access Standards