Cabling rules: how to assess the risk of injury?

In the last issue of ECD Solutions, we explained the requirements for customer cabling between separate buildings and why the use of metallic telecommunications cabling between buildings may be risky. This issue, we explain how a cabling provider can assess that risk.

Assessment of the risk of injury to the end user of a telecommunications service, as required by Clause 10.1 of AS/CA S009, is carried out in accordance with Australian Standard AS 4262.1, Telecommunications overvoltages Part 1: Protection of persons.

Factors affecting the probability of a lightning-generated overvoltage occurring are listed in AS 4262.1 as follows:

(a) known lightning damage or injury history within the vicinity of the premises (eg, if Telstra has routinely installed lightning protection on the lead-in cable or is known to routinely install it in the area);

(b) number of thunder days per year for the geographical area (a thunder day is a day on which thunder is heard at least once);

(c) building density - the number of buildings within 100 m of the location under consideration (this provides a measure of the number of connections to earth of the local electricity distribution system);

(d) soil resistivity - the higher the soil resistivity, the greater the area of influence of a lightning strike to ground and the higher the risk;

(e) exposed terrain - elevated locations over 1000 m above sea level or prominences in the terrain, such as cliff tops, ridges, bluffs and hills, are considered to be at greater risk;

(f) building construction - concrete slab and metal-frame construction create a greater earthed environment, increasing the risk that a person may be in direct or indirect contact with local earth during a thunderstorm;

(g) aerial telecommunication cable construction - the risk is considered to be higher if the premises is fed by more than 200 m of aerial cable within approximately 2 km of the premises;

(h) isolated telecommunication service - a service located in a building or structure that is not connected to any public electricity supply system (or where the public electricity supply earth is more than 100 m from the building or structure at which the service is used) is considered to be at higher risk.

How the above factors are applied to determine the level of risk for a particular location is conveyed in AS 4262.1 in the form of a flow chart.

In the case of doubt or uncertainty in assessing the risk, surge suppression should be fitted.

Assessment of the risk of injury due to lightning overvoltages

Examples

If there is no known history of lightning damage or injury in the vicinity and the number of thunder days in the region is less than 10 (eg, Hobart) then the risk is deemed to be low and there is no need for further assessment.

Conversely: if there is a known history of lightning damage or injury in the vicinity or the number of thunder days in the region exceeds 40 (eg, Darwin), the risk is deemed to be high and surge suppression needs to be installed.

Further:

• if there is no known history of lightning damage or injury in the vicinity; and
• the number of thunder days in the region is between 10 and 40 (eg, Sydney); and
• the number of buildings within 100 m of the location under consideration exceeds five, the location is deemed to be low risk and there is no need to consider the remaining factors.
• However, if you get to point 3 (building density) and the answer is ‘No’, the remaining factors must be assessed and, if the answer to any of the questions is ‘Yes’ or ‘unsure’, surge suppression needs to be installed.

Relevance of AS 4262.1 to cabling between buildings

The same principles apply to cabling between two separate buildings as with an incoming landline from a carrier’s telecommunications network. The only ingredients needed to create a hazard are an ‘earthed environment’ in each building, a separate metallic link (telecommunications line) between them and a momentary voltage difference between the earthed environment at each building or between the metallic link (line) and one or both earths. Earthed environment means that the end user may be in physical contact with an earthed object (eg, concrete floor slab, plumbing, etc) or may be touching an earthed electrical appliance (eg, refrigerator, toaster).

Where twisted-pair cabling is to be installed between buildings, assessment in accordance with AS 4262.1 must be applied for the separate building and all factors are relevant.

Notes:

1. Absence of power in the separate building does not eliminate the risk.
2. If there is a separate power MEN system (ie, electrical earth electrode) at the separate building, there is no bond between the electrical earths at the separate buildings and the risk of one earth being at a higher potential than the other is higher.
3. It is important that surge suppression is installed at the separate building as well as the main building.

Risk of damage to equipment

As already discussed, if any equipment connected to the twisted-pair cabling has earthed elements (eg, FTTP NTDs), such equipment is inherently more susceptible to damage from overvoltages than equipment that does not have an earth connection. However, this does not mean that unearthed equipment is immune to damage for the following reasons:

• Transverse (line-to-line) overvoltage induced into the cable may exceed the maximum operating voltage of the equipment and cause component failure.
• All powered equipment is connected to earth via the neutral conductor of the 230 V power (eg, via the MEN link at the electrical switchboard) and all electrical insulation between the neutral conductor and any other conductive element of the equipment has a finite breakdown voltage. Often this breakdown voltage is less than the surge suppression overvoltage limit for the protection of the end user (1500 - 7000 V). Therefore, there will be cases where equipment with a double-insulated power supply that is connected to twisted-pair cabling is damaged even if surge suppression is installed for the protection of the end user.

To reduce the risk of equipment damage, it may be necessary to install ‘secondary’ surge suppression (ie, for the protection of equipment) in accordance with Australian Standard AS 4262.2, Telecommunications overvoltages Part 2: Protection of equipment. However, that is another story.

This article was first published on TITAB Australia, a not-for-profit cabler registry, and is reprinted here with permission.

www.titab.com.au