I had a similar problem, the transistor for switching on the bed over heats and causes a short, so the green connecter melts.
I cool the replacement ramps board with two small fans blowing on the stepper drivers and switching transistors, and now the heat bed transistor only has to switch current
to the external relay I now use so it is not under any strain.
I use a 40a automotive relay , so more than enough for the job.