NOJA Power News

Issue 9

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Live Line Work - Is Your Switchgear Arc Fault Contained?

Using live-line techniques to install medium voltage reclosers is increasingly popular amongst electrical utilities and is done to minimize disruption to customers. Although common practice, what are the safety issues, and how can switchgear manufacturers design products which increase safety for live-line workers?

NOJA Power's Managing Director, Mr Neil O'Sullivan, says the answer is to use arc fault contained and vented reclosers that comply with the requirements of IEC60298 Annex AA.

“Not all reclosers on the market today are arc-fault contained and this should concern those responsible for the safety of live-line workers”, he said.

“NOJA Power has the only arc-vented, solid dielectric, SF6 free, product on the market today. In situations where SF6 gas is not desirable, and installation is to be live-line, our reclosers are the only safe choice”, he said.

Pole mounted switchgear have evolved dramatically over the years. Early switchgear used oil, while later designs used oil or SF6 gas for insulation combined with vacuum interruption. It has since been discovered that SF6 is a greenhouse gas and the by-products created when using SF6 gas for interrupting arcs are carcinogenic.

The challenge for pole mounted switchgear manufacturers has been to find safe, viable, long life alternatives to using oil and SF6 gas in their products.

This has seen the introduction of vacuum interrupted solid dielectric products into the market. The majority of these use vacuum interrupters embedded inside cyclo-aliphatic epoxy resin poles. Magnetic actuator mechanisms are used on each pole to operate the device. The actuators are normally encased in a mild steel or stainless steel housing which provides the base plate for mounting the poles.

This type of switchgear design is available from several manufacturers. Although it achieves the goal of eliminating harmful insulants such as SF6 and oil, is this really a safer, longer life product?

These products do not provide arc fault containment and venting as required by IEC60298 Annex AA.

It can be argued that it is not possible to achieve a phase to phase arc fault with three separate poles. However, it is still possible to achieve a phase to ground fault and this can propagate.

If this occurs, arc fault containment is essential for the safety of anyone in the immediate vicinity. Unfortunately, the design described above lacks this essential feature.

Electrical utilities must consider the risks of installing switchgear live-line which are not arc-fault contained and vented. A combined insulation system with arc fault containment is the safe answer to this issue.

The dielectric strength of a combined insulation system is higher than the insulating material itself. This effect is generated by the barrier hindering the chain reaction of molecule ionisation required for electrical breakdown across air.

The combined insulation system must provide the mechanical strength to withstand the forces associated with arc fault containment and fault level interruption. It must also be IP66 water proof and shaped to ensure electrical fields are minimised to withstand the required BIL/flashover levels.

Various insulation materials with different properties are used within NOJA Power's design. Silicone for joins and flexible areas, polymers for rigid structures that provide strength. High pressure processes are used for each material to eliminate voids that can cause partial discharge problems.

Reclosers will continue to play a critical role in the future of overhead line protection. Designs which offer a longer life and solid dielectric insulation systems with arc fault containment and venting should be specified by all electricity utilities when considering future recloser purchase specifications.

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