ECN Forum Forums General Topics of the Trade Non-US Electrical Systems & Trades New Houses with the bare minimum Mains

That sounds as though those cables are way undersized.

Most immersion heaters here are 3kW and fed with 2.5mm cable (12.5A load). What loading is the electric range and what size is the OCPD? 2.5 would generally only be suitable for the "Baby Belling" type portable units here. Most ranges are fed with a 30A fuse or C/B and 6mm cable.