Capacity. The TX91 starts at 3.8 kVA, so below that there is no single-box isolated online UPS in the Xtreme Power range. A pair fills that gap. There is a second reason too: with a pair you can size runtime and isolation independently, replace either half without replacing the other, and add isolation to a P91 that is already installed. A TX91 cannot be unbundled.

After. The transformer belongs downstream, between the UPS output and the load. Putting it upstream cleans the power going into the UPS, which is not the problem you are solving, and leaves the load exposed to whatever the UPS output and the building ground are doing. Downstream is what breaks the common-mode path to the protected equipment.

No, and the difference is the whole point. An autotransformer uses a single shared winding, so it will change voltage but the input and output stay electrically connected. It provides no galvanic separation and does nothing for common mode noise. An isolation transformer has separate primary and secondary windings with no electrical path between them, which is what breaks the common-mode path and establishes a clean ground reference. Autotransformers are lighter and cheaper for the same kVA, so if a quote looks unusually good on either count, confirm which one you are being offered.

To the load in watts, not to the UPS nameplate. If the instrument draws 900 W, an XPD-IT15LV at 1.5 kW is correct whether the UPS is a P91-1500 or a P91-2000. Sizing off the UPS rating is how you end up with a transformer running near 100% continuously. VA ratings are not much help here; most modern loads are close to unity power factor, so watts is the number that matters.

It defeats most of the point. Isolation is established at the transformer output and nowhere else. A downstream strip or PDU with its own ground path reintroduces exactly what the transformer was installed to break. Connect the sensitive equipment directly to the transformer receptacles. This is the single most common reason an isolated installation fails to deliver what was paid for.

Yes, on the number that matters. The J60-350 is 350 VA / 200 W and the JX-200 is 200 VA / 200 W. Both are 200 W devices. The VA figures differ because VA and watts diverge on low power factor loads, which is largely a legacy of older equipment. Size the protected load at 200 W and the pair is matched. The JX-200 is also built to the J60-350 footprint, so they stack or mount together.

As close as you can practically get it. Everything downstream of the transformer secondary is unprotected cable: it picks up interference from whatever it is bundled with, and it adds voltage drop. Isolating at a panel and then running a long branch to the equipment gives back a good part of what the transformer bought you. Same rack, same cabinet, or same counter as the load is the target.

Often, yes, and this is the largest single application for the 1.5 kVA and larger models. Hum in audio and banding in video usually trace to a ground loop between components fed from different circuits, which is common mode noise described in different words. A galvanic isolation transformer breaks the loop at the source. It is a proper fix rather than lifting a ground pin, which defeats a safety ground and does not address the underlying condition.

Common mode noise, measured between ground and either neutral or line. It is the tenth of the ten common power problems and the only one that online double-conversion alone does not address. Galvanic separation breaks the common-mode path between source and load, limits harmonic coupling from equipment sharing the supply, and gives the load a stable ground reference. If your site has shared neutrals, ground loops, or instrumentation with a low noise floor, that is the condition to look for.

Yes, on the load side. Leakage current is what escapes to ground through insulation and filter capacitors rather than following the intended path, and separating the primary and secondary windings reduces how much of it reaches connected equipment. That is a large part of why isolation is specified around diagnostic imaging and laboratory instrumentation. One caution: this is not the same as a medical isolated power system. Patient-care areas covered by NFPA 99 use isolated power panels with a line isolation monitor, and an isolation transformer on a UPS does not replace one where code requires it.

Yes. A transformer couples through alternating magnetic flux, so direct current does not cross from primary to secondary. DC offset on a supply, which can come from certain half-wave loads or from failing equipment upstream, will saturate transformers downstream of it and produce audible hum and extra heating in anything that has one. The isolation transformer stops it at the primary.

An online double-conversion UPS on its own addresses nine of the ten common power problems, which is enough for most equipment. Add isolation when the load is sensitive enough to see ground-referenced interference in its output rather than in its power: analytical instruments, mass spectrometers, electron microscopes, medical imaging. For general IT and network equipment it is usually not needed, and specifying it anyway buys weight, depth, and cost for no measurable benefit.

No. Each bundle is two line items, which is deliberate: it lets you match the transformer to the actual load rather than to a fixed pairing, and lets either half be replaced or upgraded on its own schedule. Quote both together and we will confirm the sizing before you order.