PA Protect include:
1 Anticlick 5sec delay
2 DC cut-out 
3 Thermal cut-out on 90deg
4 Fan high speed on 60deg
5 Over current protect (power-on reset)

Conectors:
1 2x18V 500mA AC
2 +/-15V DC stab for XLR circuit
3 PRO overcurrent detect
4 Protect LED
5 Thermal sensor BD241C as diode on heatsink
6 24V DC Fan cooler (12V for low speed)
7 Speaker IN/OUT

http://www.diyaudio.com/forums/solid-state/164208-500w-pa-amplifier-limiter-12.html