RCD kept tripping due to underfloor heating; I have had resistance tests, earth leakage tests, rcd tests all to no avail. Currently running two mats that produce 15.6 amps and had no trouble for 18 months. Each mat on their own appears to cause no problem and has baffled the sparks who have come to test everything, There is no pattern to the tripping it can run for a couple of days with no issues, it can trip when mats are warm or cold. I have since put a separate 16 amp RCBO in and the following picture if it works has happened after 24 hrs, Cannot keep paying for sparks to come out and scratch their heads; any help greatly appreciated.
You say two 16A mats so assume there are two thermostats? If not that's clearly a problem. Lose connection most likely causing the problem with melted wires. Much depends on type of mat the Raychem type give a varying load and don't need a sensor to same degree as simple resistance wire type. But with the resistance wire type you need to ensure it does not overheat by having a sender in a pocket between the heating elements so two mats means two senders and two pockets they can't be both controlled by same sensor.