Energy suppliers are using oil for the cooling of aggregates in transformer units. Normally pans which have to collect the oil are installed beneath the aggregates. The pan should avoid that in case of an accident oil could reach the ground and could pollute the groundwater. There is always a danger that a fire could arise due to the relatively low flashpoint of the oil and the high operating temperature. Bearing this background on mind it is more and more common to make use of fire retarding coverings to limit the chance of a fire in the oil collecting pan. In the past a layer of broken stones of about 20 cm thickness was used as a covering of the transformer.