Report calls for arc flash review on distribution secondary equipment

In the 2012 National Electrical Safety Code, the approach for arc flash below 1000 volts V has changed. Now, utilities will have to review arc flash on distribution secondary equipment. The two main changes are at 480-V for metering and spot networks.

A recent report published by the Electric Power Research Institute EPRI titled, “480-V Distribution Arc Flash Updates” addressed these issues. The research conducted in 2011 concentrated on 480-V arc flash. For 480-V spot networks, research concentrated on information exchange and practices to manage arc flash in spot networks.

The main findings of this research were:

* IEEE 1584 is the predominate calculation method for spot networks with utilities assuming either an 18 or 24-inch 45.7 or 61.0-cm working distance.

* Many utilities are de-energizing the feeder for spot network work. This reduces fault current and energized buswork in protectors. Note that they are not operating a primary-side, oil switch.

* Work is manageable in many spot networks with heavy arc suits. 100 cal/cm2 suits are common.

* Utilities normally assume either a self-extraction time or assume that internal network protector fuses operate. Both assumptions allow work in many spot networks with available arc flash suits. Both assumptions also have disadvantages.

* External fuses or disconnects are a promising option to reduce incident energies and completely de-energize a network protector. These scenarios can be treated as open-air applications if the only exposure is line to ground.

For 480-V metering, several exploratory tests were performed on different meter styles to see if there were any units where the 20 cal/cm2 threshold would not apply. Tests showed that meters with significant internal busbar can have long durations and incident energies much higher than 20 cal/cm2.

Based on these new results, utilities should not work on 480-V meters with significant busbar that are energized without an analysis.