An electrical ark occurs when the electricity flowing through a heating wire finds a ‘short cut’ wither to another section of the heating system, an earthing wire within the system or some other earthed object.
Arks can be very dangerous and can cause electric shocks if you come in contact with one and the RCD or fuse does not switch/blow. Additionally they can burn or melt the area around them. All Heat Mat’s underfloor heating systems have an integrated earth so they can not ark without tripping the RCD.
Unearthed underfloor heating systems, such as carbon films or heating ribbons, can ark if they are damaged during installation and if they are mains voltage this is presents a safety issue. Heat Mat’s advice is to ensure that all underfloor heating systems are fully earth protected, ideally with an integrated earthing shield but at the very least with an earthing grid.