If you are designing a site that will have 208V single phase power to feed the UPS, you should be aware of the nature of 208V power and how it works with different UPS models. This is one of the subjects that raise the most questions because it is different for the different UPS types, it is something you may not encounter on every project, and it has the potential to be a costly error if ignored.

As I said, this situation is not an issue in every case. SEPS has configured the standard part numbers to avoid potential issues, but some questions often remain.

The Nature of the Issue

Simply put, on some combinations of UPS model, bypass switch type, and input voltage, there exists the potential for an incompatibility. This all stems from the nature of 208V power and how it is affected by transformers.

120/208V Single Phase Power

120/208V single phase is derived from 208V three phase power. If viewed on an oscilloscope, the three voltage waveforms on the three “hot” conductors all peak at different times: each one is 120 degrees behind the next. When using 120/208V single phase, you will use any two of those same three hot conductors, with the two waveforms peaking 120 degrees apart from each other.

Since the two waveforms peak at different times, the combined power you get from them is less than the numerical sum of the two. When one waveform is at its peak, measuring 120V, the other is only part of way through its cycle, measuring only 88V. The sum of the two is 208V. As you would expect, if the two waveforms were coincidental (peaked at the same time), you would add both 120V peaks together to get 240V.

Make-Before-Break Maintenance Bypass Switches

The purpose of this device is to allow a path for the UPS protected equipment to be fed by utility power in the event of UPS service or replacement. It completely removes the UPS from the power path, enabling the service technician to work in the UPS without the worry of getting shocked. It also allows the UPS cabinet to be completely removed and replaced without shutting down the protected equipment, which is the reason Motorola requires this switch with every hardwired UPS.

When the switch handle is operated, the Make-Before-Break (MBB) switch overlaps the power on the “UPS” line with the utility power on the “Bypass” line. This momentary overlap does not cause a problem because it is so short, but it is necessary because it enables the switch to transfer sources with no interruption to the protected equipment.

Isolation Transformer

This device, whether internal to the UPS or a separate unit, recreates the utility power into clean, conditioned power. In cases where the utility power is “dirty” or “’harsh”, you should use a UPS that includes isolation to provide the maximum protection to your equipment. UPS systems that include isolation are considered separately derived power sources per Motorola’s R56 Standards. (For a more detailed explanation of isolation transformers, please see the article “Isolation Explained” in this section.)

Kirchoff’s Law states that the power into a circuit must equal the power out of a circuit. This includes transformers, where power on the primary (input) side must equal the secondary (output) side. Transformers cannot “create” power; they can only influence what’s already there. Therefore, if you put 120V in, you will get 120V out, 208V in gets you 208V out, 240V in gets you 240V out, etc.

Putting It All Together

When you run 120/208V single phase power into an isolation transformer, the two individual hot conductors of 120V are combined into one 208V waveform. Since most Motorola equipment requires 120V, we need to split the 208V back into two 120V conductors – but it can’t be done.

As stated earlier, the power that goes in must match that power that comes out without creating anything new. When the one 208V waveform coming out of the transformer is split into two conductors, only 208V of total voltage is available, with both conductors peaking at the same time. When one 120V conductor is created, the remaining voltage is only 88V, which cannot be used by any equipment.

The logical solution is to set the output of the UPS for 240V, because with 240V can be split evenly into two 120V conductors. This would work, if it weren’t for the Make- Before-Break maintenance bypass switch. When that switch is operated, it overlaps the power from utility (120/208V) with the power from the output of the UPS (120/240V).

Since the phase angles of the two voltages are different, a magnetic “crash” will result and the circuit breaker feeding the UPS will be tripped.

Solutions

In UPS systems with isolation, since the UPS output should be 120/240V to avoid the “88V leg” and the input cannot be 120/208V, then the input must be 120/240V. If this is already available at the site, then there really is no issue here. However, if the site voltage feeding the UPS is 120/208V, you will have to change it to 120/240V. This is accomplished in different ways with the FERRUPS and the 9170+ UPS systems.

FERRUPS

The FERRUPS has a ferroresonant transformer at the heart of its design, so all models automatically include internal isolation. This means that all FERRUPS models that would be fed with a 120/208V circuit will be affected by this. When you are looking for a FERRUPS model number, note that models 3,000W and larger are configured as both 208V source systems and 240V source systems, and you need to choose the appropriate one. The only difference between the two is that the 208V source systems include an external transformer to step the input voltage up to 120/240V.

As an alternative to the step-up transformer, you can opt to use the Break-Before-Make (BBM) maintenance bypass switch. The BBM switch does not overlap the utility and UPS output feeds when the switch is operated, so there is a break in power. Any equipment still operating during that break will lose power and crash, so all protected equipment must be shut down first. Motorola believes that the protected equipment should never be shut down, even for UPS maintenance, so the BBM is not offered in any standard model configurations. SEPS will quote you a special configuration if the external transformer is not a workable solution.

 9170+

The 9170+ includes an external isolation transformer for all models that end in “ISO”. This external transformer has multiple input taps to allow you to supply it with 120/208V or 120/240V power, with the output set at 120/240V. This transformer provides the isolation and provides 120/240V out to the bypass switch, so neither of the two issues described above are relevant.

If the 9170+ is not equipped with an isolation transformer, it can be supplied with either 120/208V or 120/240V power. The absence of the transformer eliminates all of the issues described above.

As an alternative to the external isolation transformer, if the site has 1220/240V power to supply the UPS, then you can opt for an internal isolation transformer (see the Powerware 9170+ part number lists for details). Again, this option is only available with a 240V feed because its use makes the 9170+ very similar to the FERRUPS – the issues with the 88V leg and the phase angle mismatch appear. If you are unsure of site voltage, SEPS recommends not opting for the internal transformer to err on the side of caution.

Download PDF: 208V-MBB-Xfmr