Cause of NAS Battery Fire Incident, Safety Enhancement Measures and Resumption of Operations

June 07, 2012

NGK INSULATORS, LTD. has announced the cause of the fire that occurred on September 21, 2011 involving the NAS (sodium-sulfur) batteries for electricity storage that it has manufactured. NGK also announced safety enhancement measures and the resumption of factory operations.

NGK has worked to find the cause of the incident and implement countermeasures since it happened. Recently, a third-party investigation committee, with Japan's Hazardous Materials Safety Techniques Association serving as secretariat, reviewed the cause of the fire and containment measures, and judged that the details regarding both were reasonable and appropriate.

Based on the results of this investigation, NGK decided to voluntarily implement safety enhancement measures, including a monitoring system, under the guidance of the Fire and Disaster Management Agency.

By implementing these safety enhancement measures, customers can be assured of their using NAS batteries with even greater peace of mind because the batteries now have more safety features than previous ones.

Up until now, NGK has asked almost all customers to suspend using NAS batteries or to restrict their usage. NGK is working so that these batteries can be re-used after quickly implementing safety enhancement measures according to installation conditions, while following the guidance of the fire authorities having jurisdiction in the installation locations.

The main details of the cause of the fire and safety enhancement measures are as follows:

1. Cause of Fire (Refer to Diagram 1)
(1) The NAS battery system is made up of 40 battery modules. In one of these modular batteries, which are made up of 384 battery cells, 1 battery cell was faulty. That battery cell had a breach and leaked hot molten material.

(2) This molten material flowed over the sand filler portion between blocks inside the battery module, causing a short between battery cells in an adjoining block.

(3) Because there was no fuse installed between the battery cells that shorted, the short circuit current flowed continuously and emitted heat, which destroyed a number of other battery cells, which in turn caught on fire. This fire spread to the whole battery module in question.

(4)The combustion of the particular battery module released flames and hot molten material that melted battery cell casings inside battery modules installed above and below, causing the fire to spread further.

NAS Battery System (Pattern Diagram)
Diagram 1  Cause of Fire

2. Safety Enhancement Measures
1) Measures to prevent the spread of fire in modular batteries (Refer to Diagram 2.)

(1) Fuses will be added between battery cells in modular batteries to prevent a short circuit current from causing a fire.

(2) Insulation boards will be placed between blocks in battery modules to prevent leaking molten materials from causing a short circuit.

(3) Anti-fire boards will be placed between battery modules above and below to prevent fire from spreading to other battery modules.

Diagram 2  Fire Containment

2) Other safety enhancement measures

(1) The monitoring system will be strengthened to enable quick fire detection.

(2) Fire extinguishers and fire-prevention equipment will be installed and a fire-fighting structure put in place in preparedness for a fire.

(3) A fire evacuation route will be developed and a guidance system put in place in preparedness for a fire.

Since the fire incident, NGK has suspended operations at its NAS battery factory in Komaki City, Aichi Prefecture, Japan. However, operations will resume at this facility from June 2012 now that the cause of the fire has been investigated and safety enhancement measures devised.

For the time being, the plant will put priority on reforming NAS batteries that continue to be used by customers. However, NGK plans to commence production of new batteries in the second half of the current fiscal year, which starts on October 1, 2012.

NGK deeply regrets any worry or inconvenience that may have caused to customers and other concerned parties by the fire incident.

Global demand for large-capacity electricity storage batteries has continued to increase, driven mainly by the utilization of renewable energy, smart grids, and government energy policy reviews.

Looking ahead, NGK will work to implement safety enhancement measures and raise quality further as it works on customer service, while expanding business in NAS batteries, which have garnered high marks for their large capacity and highly efficient electrical storage capacity.

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