We have a similar request from a client presently, however it's not in the US.
There have already been several posts indicating that it violates NFPA 72 but NFPA 72 does have a loop hole called "...acceptable to the Authority having jurisdiction..." (AHJ). When someone attempts to navigate through this loop hole, it's usually followed by some question as to who the AHJ is, and the answer depends...If it's in the US, OSHA has the final say and what they say trumps the local Fire Marshal, as I witnessed last year.
Assuming that a legitimate AHJ has granted permission to use the UPS, you need to know...
- Is the UPS power going through an Inverter and to the panel as 120 VAC, or is it DC?
- Is the UPS Grounded or Floating?
- Will the UPS shutdown below a given voltage and if so, what is that voltage?
Many UPS System have an inverter so the power to you're system would be AC. This doesn't solve the issue of not connecting batteries to keep the panel out of fault, but a resistor across the battery circuit may help. NFPA 72 also has requirements that state the system has to be designed at "...85%..." of the name plate rated voltage. This raises many questions about the UPS, that are out of your control. Who's to say that someone doesn't add loads to the UPS, which might compromise the standby duration?
One issue you'll likely have is that if it is DC, most UPS systems use a "Grounded" power supply where as a fire panel needs a "Floating" power supply. Fire panel power supplies are "Floating" so they're isolated from ground. With a grounded UPS, you may have a ground fault on the fire panel.
UL requires fire panel power supplies to cut off after a given voltage so as not to "...perturb..." the processor (didn't make that up, it's happens to one manufacture and the result was leathal).
You may find it difficult to provide external 24 VDC to a conventional fire panel as the termination point in the panel is usually the battery charger.
..anyway I just want to tell that this is possible..