To recieve my Master degree I have analyzed the working of hydrant network in the area of Trnovo Residence quarter. The analysis has been done with the assistance of QGIS programe, version 2.10.1. The programe is available on network. The first part of my work concerus with a theoretical description of the workings of drinking water supply system and the water supply in general. The geographical information systems and the basis of fire – security planning and all due precautions are given, too. The discription of legislation in Republic Slovenia follows. The continuation is dealing with the analysis of the hydrant networks in Trnovo Residence quarter, where the hydrants are defined rather suitable or unsuitable source of fire – water supply. The aim was to find out how the whole community is covered with the sources of water – the hydrants. According to the results which I have obtained, the estimation of fire – threatening of the whole residence community Trnovo – the population as well as buildings, was made. It was found out that the quarter is fire – endangered to the fourth degree – which means middle to higher fire risk. In this residence quarter there are public as well as higher residential buildings which are most endangered. Therefore the operator of the water system JP Vodovod Kanalizacija Ljubljana simulated 2 hour – deprivation of fire – water in case of 2 hours lasting fire at the top of the daily consumption. The drinking water supply system proved to be capable of such load.