A factual comparison of the key technical differences between the Powervar Security II UPM and the Power Inspired TX Series UPS System.
| Feature | Security II UPM | TX Series | Comment |
| N-E Bonded | Yes | No | ➀ To Bond or Not to Bond |
| Isolation Transformer | On output | On input | ➁ Transformer on input or output? |
| Technology | Line Interactive (VI) | Online Double Conversion (VFI) | ➂ Quick explanation |
| Hot Swap Batteries | Yes | No | ➃ Hot Swap Batteries |
| Extended Run | No | Yes | ➄ Extended Run |
Isolation transformers will negate the effectiveness of Residual Current Devices (RCDs). Where an RCD has been specified either for additional safety or due to poor earthing then this point needs to be considered.
The advantage of making a Neutral to Earth bond is that it removes any voltage between the Neutral and Earth conductors. This is what the Security II UPM does. However, as a result of this the output live conductor is now referenced to earth directly. This means that in a fault condition it is possible to receive an electric shock between the output phase conductor and earth. Any protective RCD put in place to protect against this will not trip.
Where no bond is made, the output voltage is now floating with respect to earth. This means that any phase to earth fault would not cause an electric shock as the fault would simply reference the phase to earth. Again, no RCD would trip but this is irrelevant as no hazard exists.
See this article on isolation transformers for further info.
An issue with transformers is called “Regulation”. What this means is that the output voltage with no load will be higher than when the transformer is under full load. In line interactive topology such as the Security II UPM, the transformer on the input or the output does not make for much difference except when the unit is running from inverter. The problem is that the output voltage can be quite high under no-load conditions to being quite low under full load conditions. If the load is fluctuating this can cause the output voltage to be varying quite substantially.
This is also problematic where certain loads may have high start up currents, such as those with motors or pumps. The initial current surge causes the output voltage to fall resulting in under-voltage protections operating on the load.
In online units, such as the TX series, the load is powered from the inverter and hence is extremely stable. A transformer on the output would negate this benefit. The transformer on the input will still be subject to regulation issues however the UPS inverter will correct for any voltage variations. This means the output with online technology is fixed and unwavering – even with high start up current loads.
A line interactive UPS allows the input power to pass through to the output. If the voltage rises too much that the output voltage would become out of tolerance, then the UPS enters a “buck” mode to reduce the voltage by a fixed percentage.
The reverse is also true. If the input voltage is to become too low to remain in tolerance, then the UPS enters a “boost” mode to raise the voltage by a fixed percentage.
Such technology is referred to as “Voltage Independent” or VI. However in normal operation the output voltage is in fact passed straight through so the voltage regulation is somewhat minimal. Any distortions on the input power will also be passed through to the load.
The Powervar Security II UPM is a line interactive UPS.
Online double conversion units supply power to the load from the inverter at all times. This means the output voltage is fixed and of high purity. In fact, the TX series, being an online unit is capable of having an adjustable output voltage, or even frequency.
This feature of online UPS means this technology is referred to as “Voltage Frequency Independent” or VFI. In that the output voltage and frequency are in fact, independent of the input. However, note that it is common practice for online units to be in synch with the input frequency in case a revert to bypass is necessary. So technically, the frequency isn’t always independent.
Hot swappable is a term coined to describe that the batteries in the UPS can be replaced with the unit still powered up and connected. The important point is that this should be safe and be able to be performed without the use of a tool. It is advantageous in that it allows the replacement of batteries by unskilled persons and without powering down the load. If the UPS requires a service however, then this benefit is largely removed.
Extended run units allow for the connection of additional battery packs in order to extend runtime if required. Such units should have a higher capacity battery charger in order to recharge this larger battery bank. The TX series is an extended run unit with an adjustable battery charge current of up to 4Amps.