Issue:

• UPS output N-E voltage can be detected while UPS is running on battery.
• UPS output L-E voltage is higher when running on battery than when running online
• My load isn’t functioning correctly when the UPS is on battery.

Product Line:

Smart-UPS, Back-UPS

Environment:

All models, All Serial Numbers

Cause:

On November 17, 2000 UL's standard changed to have backfeed protection for stationary UPS's, with an output that is not a separately derived system must de-energize all supply conductors i.e. break both line and neutral. The backfeed protection requirement is listed below from UL 1778 Section 29. All paragraph references are to a section in UL 1778.

A UPS shall be provided with backfeed to prevent a potential involving a risk of electric shock(see Electric Shock Section 8) from being present on its input terminals during interruption of the input ac power.

With reference to the requirement in 29.1, performance of the protection is to be judged by conducting the Backfeed Protection Test, Section 48.

The protection mentioned above shall employ an automatic switching device having air-gap contacts such as an electromechanical relay for preventing a potential involving a risk of electric shock from appearing on the input terminals. A unit provided with a remote shunt-trip circuit breaker, which will open the input ac circuit when the input ac power is interrupted, complies with this requirement. If the remote shunt-trip circuit breaker is not provided with the unit, then a marking as described in 72.1.33 shall be provided.

Exception: An electronic control employing a solid-state power switching component and subjected to the tests described in 29.6 may be provided in lieu of an air-gap contact device.

The backfeed protection for a stationary UPS, with an output that is not a separately derived system, shall:
a) Open or de-energize all supply conductors; or
b) Make the blades of the supply cord inaccessible when the supply cord is removed from the receptacle,
Exception: This requirement does not apply to a stationary UPS provided with a single output receptacle.

One result of breaking the input Neutral and Hot connections when operating on battery is that you may be able to measure voltage between Neutral and Ground at the output receptacles when the UPS is on battery. This is phantom voltage which cannot damage equipment or give the smallest of electric shocks.

As a result the 230 / 120 Vac output of the UPS will float above earth.

The UPS output when on line would act like a standard TN network, but when on battery would act much like an IT earthing system where as no part of the secondary side of the transformer is connected to Earth so that there is a floating neutral in such systems. As there is no return path this system is very safe if installed properly. It is used in hospitals, mines and other special areas. It should have Earth Leakage Monitors installed as protection, an Earth Leakage monitor is not necessary for a UPS though.

IT Earthing

UPS On Battery

When the UPS is on battery with the neutral broken, Earth is no longer bonded to the neutral at the output, and the voltage measurements that we take when the UPS is online between L-E will no longer be the same as L – N when the UPS is on battery, and the N-E measurement will no longer be 0v but will be approx half of the L-N voltage.

Output voltages with UPS online

Output voltages with UPS on battery

There is equipment that may not be compatible or may exhibit issues with a floating neutral, where earth is used as a reference in its circuitry, such as audio equipment where noise would be introduced.

Resolution:

If any of these are an issue for the installation, we recommend adding an isolation transformer to the output of the UPS where a permanent neutral to earth bond can be had regardless whether the UPS is online or on battery without creating a ground loop.

Released for: Schneider Electric UK