line interactive

the double conversion UPS converts the AC to DC to AC so you can then have a new neutral not connected to the existing neutral.

an isolation transformer ahead of your current UPS will also help to filter the line.

What's voltage tolerance? downstream circuit of UPS need no surge protection, upstream only need. Limit voltage for surge protection is 1.2kV or 1.5kV, must be complied to those electronic card specification.

Another alternative may be a ferro-resonant constant-voltage isolation transformer. I believe that this design uses core saturation to, maintain a near constant output voltage with an input variation of up to 20%. Look at the Ferrups brand for information.

JMM

line-interactive [UPS] is actually On-Line-UPS.
In this Schema Critical-Load runs on Mains-Supply & the UPS remains in Stan-by or On-Demand power. The main problem in this are:

1.
No regulation & No-Filtering of spikes & noise unless you have connected an AVR & a Power-Conditioner connected in Power-In or Power-Out side. This is a serious-threat for SENSITIVE-Electronics.

2.
As the UPS takes over after the Brown-out or Black-out, it may have following problems for the system:

a.
Interruption of power to Moving parts like Motors. As such-UPSs are not synchronised to the Mains-power the moving parts may have an electrical-jerk.

b.
As for as Electronics is concerned it may or may not be effected if the DC Supplies have large back-up capacitors, .. but

.. if any circuit depends on a signal-pulse from AC supply it may go out of Rhythm & damage may occur.


3.
In double conversion UPS the load is always on UPS' which ensures a clean & Regulated Power.
More-over remains Synchronised to Mains-Supply & shifts to Mains in case of any fault in UPS, so a smooth Change-over to Mains is ensured.

4.
The point of Guest in post #1 is not valid for

"a new neutral not connected to the existing neutral."

as all the neutral-Points are inter-connected.

combine, a UPS is a great solution for backup power but should never be mistaken for a protection or power quality technology. surge protection in devices like this is rudimentary MOV stacks typically which are nothing more than shunt devices. the problem with this solution, albeit mature, is that in the digital age of process automation, there is an avalanche of high frequency noise and this gives rise to Skin Effect and thus heat on conductors. i do represent a manufacturing R&D company so i am not going to push a product here, but if you are looking for a technology that absorbs surge and the subsequent ring wave that does the most damage at the power supply, programmable memory, and component level - such a solution does exist.

Good Luck!