Temporary Earthing Newsletter - Issue 4

March 2007

Portable Protective Earthing Workshops

Workshops were held on 21 March in Christchurch and 23 March in Auckland with over 100 attendees. The purpose of the workshop was to disseminate the EEA recommendations for portable protective earthing practices, and included was a description of the research and testing carried out, and an overview of the changes to the Technical Guide.

For copies of the workshop presentations, please contact the EEA Office.

Synopsis of Test Results

The field testing carried out indicated that:

1. In situ wooden and concrete poles are neither good insulators nor good conductors.
2. The surface resistance of the poles varies significantly depending upon a number of factors, including moisture.
3. Should single phase livening of a pole work site occur then hazardous voltages may appear between the conductors being worked on and the surface of the pole. To avoid this hazard, poles need to be effectively bonded to the conductors where practicable (equipotential zone) or steps taken to avoid simultaneous "bare hand" contact between the conductor and the pole by use of cover up gear, use of insulating gloves, etc.

Guide to Work on De-Energised Distribution Overhead Lines

A draft of the revised Guide to Work on De-Energised Distribution Overhead Lines was provided to workshop attendees. The Guide contains all guidance being provided by the EEA for work on poles, and will replace the current version of the Guide (previously titled 'Guide to Temporary Earthing of Distribution Overhead Lines') (Jan 2005) and the Notice to Industry (September 2005).

The draft was available for consultation, with comment due at the EEA by 27 April 2007. The EEA intends to publish the Guide by the end of May.

Amendments have resulted from research and testing carried out on wooden and concrete poles to determine the most practicable means of providing temporary earthing and the creation of equipotential zones. As a result of the work, practicable means are provided for creating acceptable earthed equipotential zones at work sites for wooden poles and concrete poles with integral earthing.

For existing concrete poles without integral earthing, a practicable method for bonding to the concrete in these cases was not found and hence different approaches need to be taken to manage the potential hazardous voltages which may appear during inadvertent single phase livening.

In addition, the structure of the Guide has been re-organized. It now commences with planning and hazard management, and requires the establishment of equipotential zones in conjunction with temporary earthing. The consultation draft of the Guide contains few diagrams, but it is intended that the issued version will contain more.

Those providing comment on the draft were also asked to provide:

• Examples of pole and conductor configurations against which the Guide can be checked, and which may also provide examples for diagrams.
• Suggested solutions, practices, work methods, or procedures for working on existing concrete poles without integral earthing.

New Concrete Poles

The testing carried out on concrete poles clearly demonstrated the effectiveness of bonding the pole reinforcing into the equipotential zone, and without the ability to bond the reinforcing the pole must be considered partially conductive and managed as described above. As a consequence, the EEA is recommending that all new concrete poles are manufactured with integral earthing, or an equipment earthing system. The requirement applies to pre-stressed poles as well as bulk reinforced poles. Details of the design method are to be arranged between the purchaser and the pole manufacturer.

Factors to be considered in the manufacture of new concrete poles include:

• Integral earths need to be able to carry a significant proportion of the fault current as the pole to earth resistance may be less than the driven spike to earth resistance.
• Preference should be given to using non-stressed conductors in pre-stressed poles.
• The effect of fault current passing through the reinforcing.
• Whether to fit only integral earthing or an equipment earthing system.
• Number of connections, eg for ease of connection and for continuity testing.
• Height of connections from the ground (to prevent contact by the public).

Consensus at the Auckland workshop was that the integral earthing connection point would be termed a 'bonding point'.

Existing Concrete Poles

Due to the risks associated with accessing reinforcing in poles, particularly for pre-stressed poles, expert advice has been requested on this practice. To date the indications are that a solution will not be feasible and therefore this solution will not be deemed practicable for field use. Any updates on this issue will be disseminated when they are available.

Asset Owner & Employer Actions

The actions which asset owners and employers need to take are:

• Provide feedback on the consultation draft of the Guide.
• Develop or revise procedures for field staff to use.
• Obtain appropriate additional equipment for temporary earthing and equipotential zone creation.
• Arrange for new concrete poles to have integral earthing or an equipment earthing system.

Programme for Outstanding Work

Work to be completed includes:

• Finalisation of Earthing Guide by end of May.
• Review section 6 of SM-EI Part 3.
• Complete consultation regarding possible means of accessing reinforcing on existing concrete poles.

When section 6 of SM-EI Part 3 is reviewed it is likely to be arranged to reflect the emphasis of the Earthing Guide.

Field Practices

The SSPG reinforces the priority which must be given to hazard identification, both at the work planning stage and when at the worksite. Temporary earthing and equipotential bonding is a necessary hazard control, but the focus must be on means to prevent inadvertent livening from occurring from any source including over or under passing live conductors and adjacent live conductors on the same structures.