Capacitor Bank Failures Leading to Switchboard Fires

Forensic Services Newsletter

Capacitors can cause fires. This is true for small capacitors on printed circuit boards, and on large capacitor banks installed in industry and commerce. This newsletter addresses the latter.

Large capacitors are to be found in UPS* and power distribution switchboards in industry where there are large inductive loads, and in commercial installations with fluorescent lighting and air conditioning. Examples of the latter include shopping centres, hotels, colleges and office blocks. All of these will invariably have their switchboard connected to a power factor correction capacitor bank.

The hot gas layer generated when this capacitor bank burnt, spread through the attached switchboard, disabling a luxury spa resort at an international tourist 'hot spot'

In the case of large installations, unless they are generating their own power supply, this provision of power factor correction is not an option but imposed on the consumer by the power distribution authority. It does improve the power factor of the consumer’s installation immensely and this is reflected by the lowering of the power bills for the energy drawn by the consumer.

As an unfortunate expediency, in order to keep the costs down, the switchboard manufacturers invariably include the power factor correction facility in a cubicle attached to the array of switching cubicles comprising the switchboard suite.

Modern capacitor banks today are universally made up from modules built up by individual 'dry type' capacitors as they are relatively smaller, lighter and cheaper than their steel-canned, oil-filled predecessors. The dry type capacitor comprises of a bobbin of tightly wound, plastic film which has been metalised on one side and this is sealed within an extruded plastic cartridge enclosure. These dry type capacitors are able to ‘self-heal’ themselves a number of times when they are subjected to electrical trauma.

According to the old adage, “you don’t get something for nothing” and this holds true for dry type capacitors – they may be smaller, lighter, cheaper but they are far less tolerant of operation under elevated temperatures and as the temperatures rise their long term ability to self heal themselves rapidly diminishes.

When dry type capacitors are no longer able to 'self heal' themselves they fail, often in a catastrophic manner. These failures of dry type capacitors are more often than not accompanied by fire as their capacitive element and enclosure are both made from flammable materials.

The inclusion of the capacitor bank in the switchboard ensures that the switchboard is involved in the ensuing fire when capacitors fail.

The heat from this failed capacitor bank completely destroyed the attached main incomer 400volt switchboard for a very large shopping mall. The radiant heat from this fire also destroyed the control panel and switchboard for the emergency power generator that was located directly opposite.

The simple act of separating this cubicle from the switchboard in such a way that it is still close enough alongside for maximum benefit from the power factor correction unit to be applied directly to the main bus bars of the switchboard, but separate enough to prevent heat transfer from the cubicle, will ensure that the switchboard will be able to keep running, should a capacitor fire occur. If the power factor correction cubicle is lost the power bills will increase noticeably in size but at least the facility will be able to keep running until a replacement unit can be installed.

Correct design of power factor control capacitor banks, their cabinets and their environment (thus ensuring operation under optimum conditions) and pro-active maintenance techniques, can go a long way to preventing such losses from switchboard fires happening.

We are well-experienced in investigating capacitor bank failures and have attended switchboard fires throughout the S.E. Asian region. We can also carry out thermographic surveys of working facilities. Thermographic examination of power and control wiring and the capacitors themselves can indicate temperature differences originating from errant capacitor performance.

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UPS* Uninterruptible power supply