http://aircraft-support.********.co....on-system.html
The are two types of system on the 727.
a) The standard 20-4 Joule system that has a single exciter box.
It's supplied by 115V AC and 28 V DC. The 20-4 joule exciter can operate in the intermittent or continuous mode. In the intermittent mode, it fires both igniters. In the continuous mode it only fires the igniter in combustion chamber 7.
b) The Dual 20 joule system has two exciter boxes bolted together (Sometimes called a Twin Pack).
Both are supplied by 115V AC. One fires the igniter in chamber 4 the other chamber 7. Two exciters can be operated individually or at the same time. Earlier models were for intermittent use only. Later models are continuous. Remember 10min on LH the 10min RH for the earlier systems.