Fuses issue

Hi there,

My Demon battery/alternator harness has been modified to secure it.
Here is it :
Schéma3.jpg
But today, when I turned the key, nothing happened : motionless needles, no oil/brake red light, no starter.

The two 20A fuses which protect the system blew together.
I replaced the fuses and everything return to normal... the new fuses didn't blown and I was able to go back home.

My battery is recent, and the alternator delivers between 13 and 14 volts under normal driving.

What could have been the problem with the fuses?

:thankyou:
François