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Fire testing of a profile plank for a transformer pit

Created on Tuesday, February 17, 2015 and posted in Industry News
Fire testing of a profile plank for a transformer pit

On behalf of Meiser Vogtland OHG, SP Fire Technology has tested the fire protection effect of a Profile Plank installed at the top of a transformer pit.

The traditional way of improving the fire safety of transformer stations is to fill the transformer pit with stones.  The purpose of these stones is that, in the event of an incident involving leaking and burning transformer oil, the oil should be cooled as it comes into contact with the stones and that the limited amount of oxygen would help to avoid a long lasting fire when the oil runs into the transformer pit.  An alternative to filling the pit with stones is to cover it with a Profile Plank.  This has the advantage of allowing the entire volume of the pit to be available for rainwater and for any oil, instead of having much of the volume filled with stone.  Another advantage is that it is easier to perform service work on the transformer station with a smooth Profile Plank as the base instead of stones.  It is also easier to clean and restore the transformer pit after an oil leak.

No technologically neutral description is available today of the requirements for extinguishing burning oil in a transformer pit.  Swedish standard SS 421 01 01[1] states:  “Preferably arrangements that contribute to extinguish the fire in the leaked liquid shall
be used, for example the use of a layer of stones (approximately 300 mm deep and with a
grain size of about 40/60 mm) that extinguishes the burning liquid that enters the layer.”

A search of the literature, both national and international, standards and guidelines, finds that several describe the problems of burning oil, but that none of them states specific requirements for fire extinguishing.  SP Fire Technology has therefore performed fire tests simulating a transformer failure by tipping burning transformer oil into a transformer pit covered by Profile Planks.

Figure 1    The transformer pit covered with the Profile Planks.  Instrumentation consists of thermocouples and gas sampling probes.  The tippable trailer on the left was used to contain the oil while being heated.

The test setup is shown in Figure 1.  The transformer pit was 4 m long, 3 m wide and 1 m deep.  Instrumentation consisted of thermocouples at various heights above and below the Profile Plank, and gas sampling probes mounted 5 cm below the plank.  The oil was stored in a tippable trailer, and heated by means of an LPG burner.  Three tests were carried out, as follows:

1.    With the oil heated to 90 °C.
2.    With the oil heated to 90 °C.  The transformer pit had been filled with water to a depth of 19 cm to represent rainwater.
3.    With the oil heated to 140 °C.

Figure 2    A series of pictures from Test 2.  The time, in minutes and seconds, can be seen on the display in the bottom right hand corner of the pictures.  It can be seen that the flames are extinguished within a few seconds of tipping the burning oil into the pit.

Figure 2 shows a series of pictures from Test 2, from which it can be seen that the flames were extinguished within a few seconds of tipping the burning oil into the pit.

Figure 3 shows how the temperature rises rapidly as the burning oil contacts the Profile Plank.  The flames disappear quickly, and the temperature above the plank falls back to a low level.  Beneath the planks, the temperatures remain at elevated values for a longer period of time.  For Tests 1 and 2, this is due primarily to the limited ventilation through the grating, but in Test 3 (in which the oil was heated to 140 °C) there was some heat release for about two minutes.

Figure 3    Temperatures in the centre of the pit, 50 cm above its bottom (i.e. beneath the planks), and 50 cm above the planks.

Figure 4    Oxygen concentration at the centre of the pit, 5 cm below the planks.

Figure 4 shows the oxygen concentration, which quickly falls to low levels beneath the planks and helping to extinguish the fire.

Summarising, a method has been developed to evaluate the fire protection performance of covered and/or filled transformer pits.  The result for the particular planks that was tested shows that the flames above the planks are extinguished within a few seconds. For more information and a complete description of the tests can be found in SP Arbetsrapport 2013:09, which can be downloaded from

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