Cabling between buildings

The new customer cabling rules came into effect in July 2013. With the new rules came a new clause, 17.3.1, that sets out requirements for cabling between separate buildings. To some cabling providers, this clause seemingly precludes the use of twisted pair cable for cabling between buildings. However, this is a misinterpretation of the clause. This articles clears up some prevalent misconceptions.

The abovementioned clause is poorly written, which is presumably why it is being misinterpreted. I am allowed to be critical about the composition of this clause because I am the one who drafted it. In paraphrase, what the clause really states, after factoring in the cross-references and the notes but omitting a few esoteric details that most cabling providers won’t understand, is this:

Where any equipment is to be interconnected between separate buildings, the connection may be made by one of the following methods:

(a) using cabling that does not contain electrically conductive elements (eg, optical fibre cable with non-metallic strengtheners);

(b) using wireless technology;

(c) using coaxial cabling, in which case: the coaxial cable should be connected to the indoor cabling via an isolator that provides a minimum isolation of 3 kV rms and 7 kV impulse; and the outer conductor of the coaxial cable should be earthed on the outdoor cabling side of the isolator in accordance with Clause 20.18.4;

(d) using twisted pair cabling, provided that:

      (i) the manufacturer or supplier of the equipment to which the cabling is intended to be connected has not indicated that: only ‘internal’ (indoor) cabling is to be connected to the equipment or an equipment port; or ‘external’ (outdoor) cabling is not to be connected to the equipment or an equipment port; or

      (ii) if subparagraph (d)(i) does not apply:

• the cabling provider assesses the need for surge suppression for the protection of the end user as required by Clause 10.1; and
• if the risk of injury to the end user is assessed as high under Clause 10.1, as required by Clause 10.2 the cabling provider installs surge suppression in accordance with Clause 10.2 and the applicable requirements of Clause 10.3 and Clause 10.4; and
• the cabling is connected to equipment or an equipment port that is classified as ‘external’; or
• if the equipment or an equipment port is classified as ‘internal’, the cabling should only be connected via a suitable line isolation unit (LIU).

In the case of subparagraph (d)(i), the cabling provider is precluded by Clause 5.2 (b) from using twisted pair cabling between the buildings to interconnect the equipment. Most ethernet equipment, ethernet ports and IP-based (eg, VoIP) service ports are classified as ‘internal’ and are not designed to be connected to outdoor cabling.

There are no mandatory requirements in Clause 17.3.1 itself, merely cross-references to other mandatory clauses that apply independently of Clause 17.3.1, namely Clauses 10.1 to 10.4 (surge suppression) and Clause 5.2 (manufacturer’s instructions). Only Clause 5.2 (b) may preclude the use of twisted pair cabling between buildings for the connection of certain equipment. Clause 5.2 states:

Manufacturer’s instructions

Cable and equipment installed for connection to a carrier’s telecommunications network shall be installed:

(a) in accordance with the manufacturer’s instructions including, in the case of cable, such things as cable bend radius, tension, cable tie pressure, colour code, etc; and

(b) in accordance with the instructions of the manufacturer or supplier of any equipment to which the cable or equipment is to be connected.

Under Clause 5.2 (b), if the installation instructions or operating manual for the equipment states that the equipment must only be connected to metallic cabling that does not leave the building in which the equipment is located, this becomes a mandatory requirement under AS/CA S009.

There are circumstances where it is perfectly safe to use twisted pair cables between buildings. There are also cases where the use of twisted pair cabling is essential, eg, to remotely power IP cameras or other low-power devices using Power over Ethernet (PoE). For these reasons, the use of twisted pair cabling between buildings is not prohibited by AS/CA S009. However, if the purpose of such cabling is to connect equipment whose manufacturer or supplier expressly forbids the connection of metallic outdoor cabling to the equipment (eg, some Telco FTTP NTDs), the cabling provider should use another medium (such as Wi-Fi or optical fibre) to make the connection.

FTTP NTDs are usually earthed, which makes them particularly susceptible to damage from lightning or power system surges induced into outdoor cabling - and they are expensive to repair or replace. Other equipment with ethernet or VoIP ports, such as modems, gateways, routers and switches, are not usually earthed yet they still may be damaged by the same surges - but these are cheaper to replace and are the customer’s responsibility.

The installation of twisted pair cabling between buildings can be risky for the end user. Any cabling installed between buildings will have some exposure to possible lightning activity and, if both buildings have mains power connected, power differentials. These can cause what are broadly described as surges or overvoltages. There are three things to consider in the case of lightning or power surges/overvoltages (in order of importance): the safety of the end user, the risk of fire and equipment damage.

AS/CA S009 directly addresses (a) and is indirectly concerned with (b) and (c) simply because fire is a risk to personal safety and property, while (c) can ignite a fire which invokes concern (b) and may also affect end-to-end performance (network integrity).

Let’s look at each of these in the context of AS/CA S009.

Safety of the end user

Clause 10.1 of AS/CA S009 requires the cabling provider to assess the need for surge suppression for the protection of the end user of a telecommunications service in accordance with Australian Standard AS 4262.1, Telecommunication overvoltages Part 1: Protection of persons, where: twisted pair customer cabling is provided to a building or structure; the network boundary is not located in or on that building or structure; and the cabling will be used to connect ‘terminal equipment’ in or on that building or structure.

‘Terminal equipment’ is defined as peripheral equipment operated by the end user to access a telecommunications service, such as a telephone instrument, headset, fax machine, modem or other equipment that may be handled by the end user. Equipment that is not usually handled by the end user, such as a fixed IP camera, is not terminal equipment.

Where the risk of injury to the end user is assessed as high, Clause 10.2 requires the cabling provider to install surge suppression in accordance with the relevant requirements of AS 4262.1. In assessing the safety of the end user per AS 4262.1, it is always assumed that the end user is in an earthed environment, ie, standing on a concrete floor or touching an earthed object like plumbing or an earthed electrical appliance (refrigerator, toaster, etc) and touching telecommunications equipment at the same time (eg, a telephone handset). There are three potential sources of overvoltage that may cause injury to the end user in such circumstances, explained below.

1. If there is a power surge or a lightning strike to the power lines (anywhere within the power grid, not just at the premises), the overvoltage will be shunted to earth via the mains supply neutral which is earthed via the neutral-earth (MEN) link at the electrical switchboard in the premises. This will momentarily raise the local earth potential many thousands of volts. This means any object in electrical contact with the local mass of earth (eg, concrete floor slab) or connected to the protective earth of the electrical switchboard (eg, refrigerator, toaster, etc) will be at the raised earth potential. If the end user also happens to be touching an object electrically connected to a telecommunications line (the most common example being a telephone handset) and that line is remotely connected to an earth (eg, at the distant telephone exchange or at another building), there will be a potential difference (voltage) between the locally earthed body of the end user and, via the telecommunications equipment, the distant earth on the telecommunications line. Most overvoltages originate from the power lines, not the telecommunications line. Experience has shown that significant overvoltages may occur regardless of whether the telecommunications lines or power lines are aerial or underground. Without protection measures, voltage V may be tens of thousands of volts.
2. If there is a lightning strike to the telecommunications line (anywhere within the telecommunications network, not just at the premises), the overvoltage will appear on the telecommunications line and, if the end user is in contact with the local earth, as in item 1, there will be a potential difference (voltage) between the locally earthed body of the end user and, via the telecommunications equipment, the overvoltage on the telecommunications line.
3. Finally, if there is a nearby lightning ground strike (including a strike to a tree or structure), the electrical potential of the local mass of earth - and, via the local electrical earth electrode, all earthed electrical appliances - will be momentarily raised many thousands of volts and the effect will be the same as described in item 1. The remedy to protect the end user from any of the abovementioned overvoltages is to install a surge suppressor on the telecommunications line that will operate (conduct) under overvoltage conditions to bring all of the abovementioned elements (including the end user) to about the same electrical potential.

In reality, the voltage difference will, at best, be limited to about 1500 V due to the threshold (‘trigger’) voltage of the gas arrestor and the inductance of the bonding conductor. In Australia, the handsets of telephones are required to have a minimum breakdown voltage of 7000 V. A handset is the most likely telecommunications object that the end user will be holding. Surge suppression is designed to limit the potential difference (voltage) between the local earth and the telecommunications line to less than 7000 V to minimise the risk of injury to an end user holding a handset to his/her head. If any equipment that the user may touch has a breakdown voltage of less than 7000 V (most other equipment), the user may receive an electric shock. If the end user is holding a cordless handset (which is not electrically connected to the telecommunications line), the risk of electric shock is eliminated - unless the user also happens to be touching the cordless base station.

The voltage depends on the length of the bonding conductor between the surge suppression device and the earthing bar of the electrical switchboard. In cases of lightning-generated overvoltages, the voltage drop along the bonding conductor (due to inductance) will be significantly greater than the surge suppression device trigger voltage. The total value of V is assumed to be about 1500 V peak but must not exceed 7000 V peak. The bonding conductor length must not exceed 10 m or voltage V will exceed 7000 V.

The same factors that may create a shock hazard for the end user in the main building are present in the separate building, namely: an earthed environment; a metallic telecommunications line entering the building; and usually, electrical power lines entering the building.

Therefore, assessment of risk at the separate building is required in accordance with AS 4262.1. A momentary voltage difference between the separate earthed environment in each building or between the metallic line and one or both earths may create a hazard.

Risk of fire

The surge suppression described above will not entirely prevent overvoltages occurring between any telecommunications conductors or between the conductors and earth. At best, the overvoltages may be reduced to around 1500 V (although a ‘spike’ exceeding this voltage may also get through the surge suppression due to the time it takes to operate). Therefore, if the equipment or equipment port is not designed to withstand an impulse voltage of at least 1500 V, damage to the equipment may occur. In some cases, this could result in arcing or sparking that could ignite a fire. However, such cases are rare for lightning-induced surges, which are of a very short duration.

Another overvoltage condition that is more likely to cause a fire is referred to as ‘HV contact’, ie, where an aerial high-voltage power line falls onto LV power lines or telecommunications lines carried on the same poles. These may produce a more prolonged, low-frequency overvoltage condition that is more likely to ignite a fire at the customer premises. This situation is unlikely to occur in customer cabling.

AS/CA S009 does not directly address the risk of fire in the above cases but there is a general mandatory requirement that would apply if there is a known or suspected risk of fire, property damage or injury to any person. This is Clause 5.1, which states:

5.1 Safe and sound practice

Customer cabling shall be installed in accordance with principles of safe and sound practice.

Equipment damage

Apart from the slight possibility of fire as described above, there is generally no significant safety risk associated with damage to the equipment (ie, usually it simply doesn’t work anymore).

The concerns are primarily operational and economic, ie, loss of service and the cost of repair or replacement. However, this does affect end-to-end performance (network integrity) and it is incumbent upon the cabling provider to minimise the risk of damage to cabling or equipment in accordance with Clause 5.4 of AS/CA S009, which states:

5.4 Protection against damage

All parts of an installation shall be adequately protected against damage which might reasonably be expected to result from mechanical injury, exposure to weather, water or excessive dampness, corrosive fumes, accumulation of dust, steam, oil, high temperature, or any other circumstance to which they will be exposed under the conditions of their use. It is the last provision in this clause that may be relevant to damage caused by overvoltages (ie, any other circumstance to which they will be exposed under the conditions of their use)