Linear heat detection cable alarm

Actuellement I have problem with thermosenstive cable installed around the bottom & top of floating tank.
the loop of LHD is connected with JB where there is security zener barrier, and from BJ we are connected to the PLC.
The main problem, is that I’m getting some short alarm "01mn to 5mn’’ periodacally, that’s mean we receive alarm in the control room, but after a moment it desappears, when I check on the field and I test the loop with multimiter i don’t find any short circuit.
How can we explain this problem?
thanks in advance **