POINT I/O FPD and EP24DC placed next to eachother

We have a few control panels with POINT I/O that were designed by an outside engineering firm. As we are adding new equipment centers, I try my best to adhere to standards and design philosophy applied to the original control system.

Can someone help me understand why many of the POINT I/O racks are designed with an EP24DC and an FPD right next to eachother? My understanding is that both devices split the power distribution for field devices. The EP24DC provides blackplane continuity so faults to the right of the EP24DC will fault the entire rack. The FPD does not.

By placing an FPD after the EP24DC, doesn't this defeat the purpose of the EP24DC? Is this set up redundant, or am I missing something?

I've attached a screenshot of one of the schematics.

Ill take a shot at it
It looks to me like the 1734 EP24DC is used to add additional power to the modules on it's left. with 10 modules they are about at the limit for the power supply they just wanted to make sure there was enough power available to power the modules

The 1734-FPD would be used to isolate the modules on the right to a separate power supply
I have used 3 1734-FPD's in one rack to isolate the supplies in that case I had 3 separate control boxes I needed to control but the needed to be isolated from each other

The 1734-EP24DC both provides additional power to the POINTBus and interrupts the Field Power Bus.

https://literature.rockwellautomation.com/idc/groups/literature/documents/in/1734-in058_-en-p.pdf

But it does so from a single connection to its terminal block; the same power that you connect to the EP24DC goes to the "brains" of the modules to the right via POINTBus and to the "brawn" of those modules via the Field Power bus.

Adding an additional 1734-FPD allows the designer to provide an isolated (maybe fused ?) power supply to the Output circuits that is separate from the logic power that runs the POINT modules.

*If field power must be interrupted or cycled, the 1734-FPD must be used. The 1734-FPD only separate the field power and does NOT affect the Communication Backplane Bus.

1734-EP24DC or 1734-EPAC: When to Use the Expansion Power Unit

https://rockwellautomation.custhelp.com/app/answers/detail/a_id/57904

Check also this thread
http://www.plctalk.net/qanda/showthread.php?t=108019

Conceptually that makes sense. Here is how the two are fused:



All outputs here are for a pneumatic valve bank. So the idea is that if the MCR is opened that power to the FPD and thus the bank is cut. Since the backplane power is sustained by the EP24DC, the modules would remain powered and wouldn't know the difference.

The EP24DC exists simply to power more modules, while the FPD allows the field power to be removed without the entire rack being shut off or faulting. Does this seem plausible?

Thanks for the link. That pretty much answered my question before I finished my last post.

Yup; an individually fused and MCR-switched Output power source is exactly why you would use a 1734-FPD in this application.

Separating the Output power that serves inductive devices like valve coils also gives you the opportunity to keep their back-EMF spikes from affecting Input or data bus circuits.

I would suggest you use Rockwell's Integrated Architecture Builder (IAB) to check anything you might build. Blindly slapping in PointIO will eventually cause you problems. You need to understand what you need PointIO to do for you, and what it can do for you so you design it right. IO modules vary quite a bit in power requirements.

IAB will automatically add in power modules based on module power consumption of the configuration, and FPD modules best it can. But it isn't perfect. Such as the case with 24VDC control for everything, in this case, the FPD isn't automatically included in the IAB because it's all 24VDC and isn't aware of MCR or fusing considerations.

A more obvious example using IAB, mix in 120VAC modules and you'll immediately see the FPD get dropped in.

There is also the designer's discretion. While PointIO has modules that support 8 IO points, there is something to be said for lower count modules which can accommodate source/common/ground on the same terminal block, can make it a bit easier to install in the file especially if you are bringing in multi-conductor cables.