The siemens sentrons i've installed are only good to 200K IR ......how does one combat infinity?
Not sure what you mean.
There would be few, if any, distribution transformers capable of delivering more than 200,000 amps fault current. The 200k AIC Sentrons would be OK almost anywhere.
My point was simply this - Current limiting fuses do reduce the available fault current that downstream equipment is exposed to.
And, (this is an observation, not a criticism) the NEC people are being very cautious in recognizing series rated combinations.
Ed
