1734-EPAC replacement

[Zoyalan]

Hello, I am kinda new to automation, I had an issue with 1743-EPAC which supply's 13 IO , when we inspected it there was a short circuit inside it, the components we're almost burned.
We had 4A CB protection on it , but it didn't work.
I have 2 questions :
1_ what could have caused the EPAC to short circuit only and the rest of components are intact
2_ we replaced the EPAC with EP24 and PFD
I read that both EPAC( convert voltage from 110 to 5V ) and EP24 ( convert voltage from 24 to 5V ) convert the voltage to backplane volt which is 5V if so why we needed the PFD ? Why couldn't we replaced the EPAC directly with EP24 since both of them convert the voltage to 5V .
Thank you .

 

[jholm90]

Point Io has field power and module/bus power on separate rails. The leftmost module/buscoupler provides 24vdc field power and 5v bus power. After a number of modules (varies based on module power consumption) the 5v bus runs out of power and additional power injection is required (1734-EP24DC/1734-EPAC). The DC24/AC also provides a new supply for field power and can be changed between DC/AC as needed. If there is no need for additional module/bus power there can be a 1734-FPD installed the provide a new source for field power. We might have 2 or 3 FPD installed on a rack with different power supplies for different safety zones on output cards.

 

[Zoyalan]

Point Io has field power and module/bus power on separate rails. The leftmost module/buscoupler provides 24vdc field power and 5v bus power. After a number of modules (varies based on module power consumption) the 5v bus runs out of power and additional power injection is required (1734-EP24DC/1734-EPAC). The DC24/AC also provides a new supply for field power and can be changed between DC/AC as needed. If there is no need for additional module/bus power there can be a 1734-FPD installed the provide a new source for field power. We might have 2 or 3 FPD installed on a rack with different power supplies for different safety zones on output cards.

What is the difference between field power and bus power, and what are the sources of theses powers