Poletop Fires

Western Power was recently plagued with approximately 70 poletop fires that caused widespread outages and damage to the network. Around 47,000 homes and businesses were without electricity initially with some not getting power restored for days.
Western Power advised that each pole replacement could take up to 8 hours to repair and some customers were likely to be without power for an extended period due to the sheer number of poles affected.
Of course, we are talking about the tops of wooden poles catching fire. These can be dangerous to repair if the risks are not adequately managed. I remember in the 70’s that a colleague suffered back injuries when a crossarm broke as he was trying to put out a poletop fire – he never worked again.
Poletop fires mostly occur with extended dry periods as dust, salt spray and/or other pollutants collect on the surface of insulators. Fires occur with fog, mist and dew but mostly with the onset of drizzle or light rain. This allows leakage currents from the powerlines to pass over the surface of insulators. These leakage currents cause heating, charring and burning at high resistance locations and where the leakage current concentrates such as where:

• Insulators are held on wooden crossarms or poles;
• Arm braces connect to the crossarm or pole;
• The crossarm is held at the pole by the king bolt.

Slightly damp wood chars readily but wet wood doesn’t. Therefore, fires are likely to start where the wood is protected from rain, such as those areas listed in the bullet points above, and this is also where the leakage current concentrates as previously mentioned.

Wooden poles and crossarms generally shrink after installation and this shrinkage also helps to create high resistance joints at those areas listed in the bullet points above.
Poletop fires can also start where conductors fall off or are dislodged from the insulator (see the photo opposite).

The amount of damage at the poletop may range from charring of the crossarm and pole to burning through of the crossarm or complete destruction of the pole if the fire is left unchecked (see the photo opposite).
Western Power control measures include insulator replacement with silicon insulators which are less prone to dust and pollution buildup (long-term ‘fix’). They also use insulator washing and siliconing by helicopter and EWPs (short-term ‘fix’).
I remember sitting in on a conference in Brisbane where a powerline engineer from one of the Melbourne poles and wires company was giving a talk on improvements to network performance that they had made. In passing, he mentioned some poletop fires that they had experienced. Emeritus Professor, Matthew Darveniza, was the first to comment after the talk, where he said in no uncertain terms, that they solved poletop fires in the 70’s.

Some of these solutions developed in the 70’s by Professor Darveniza and others include:

• Strong galvanised spring washers on insulator hardware and on the crossarm and arm brace bolts;
• Galvanised nailed plates are used where insulators attached to the crossarm and at the junction of the crossarm and pole. This ensures a good connection between hardware and the crossarm and between the crossarm and the pole, helping to distribute the leakage current to prevent charring and burning and reduce the contact resistance at these points;
• Metal strips or braids can be attached under an additional nut on the insulator hardware and this strip is then nailed into the timber crossarm;
• A metallic strip can be wrapped completely around the timber crossarm and bonded to the hardware;
• Conducting paint can be applied at wood to metal connections on crossarms or the complete crossarm may be painted (this is not one of the better control measures and no longer used but some crossarms may still have it);
• Washing of insulators and the application of silicon grease as previously mentioned. This can be carried out with the electricity supply still on using ‘live line’ techniques;
• Phase bonding may also be used. This is where a metallic connection is made between the bottom of the pins or fixing bolts of all insulators of the same circuit.

Professor Darveniza’s book, ‘Electrical Properties of Wood and Line Design,’ details that pole fires can start with as little as 8 mA and are maintained at levels as small as 3 mA. It can be seen how poletop fires start with leakage currents based on these values.

Conclusions
Poleop fires mostly occur after extended periods of dry weather in light rain or drizzle due to leakage currents flowing from the powerline, over the surface of the insulator to ground through timber crossarms or poles.
Poletop fires are minimised by good line design, and construction techniques and through maintenance activities such as insulator washing and siliconing and hardware tightening.

References
Electricity Supply Association of Australia’s “Insulator Pollution Guide”.
Western Power Distribution Equipment Bulletin Issue 9.
Electrical Properties of Wood and Line Design – M. Darveniza.