SB General Info:10
09.01.2007 PK
Don’t break neutral in UPS installation
UPS neutral reference in TN-S systems
In a five wire TN-S distribution system [1,2], the neutral reference relies on the bonding of the neutral
conductor (N) to the protective earth conductor (PE) near the power source, which is usually a
transformer.
When a UPS is installed, it is important to remember that the UPS itself is also a power source, which
requires the bonding connection to be persistent in all operating modes to preserve the ground reference
of the downstream TN-S distribution system.
There are two main approaches to ensure this. In standard installations, the UPS output neutral reference
relies on the neutral reference of the input supply/supplies, in more elaborate installations the UPS
output is bonded to earth independently.
The input referenced approach is usually the most economical, but in these kind of installations care
should be taken that this neutral supply reference is not disrupted at any time whilst the UPS is in
service [3, 4] .
In other words, in all states and positions of the upstream breakers, switches, contactors etc., an
uninterruptible path from the N to PE bonding point to the neutral input terminal of the UPS should be
maintained, unless the device interrupting the neutral has an auxiliary contact operating the remote off
function (EPO) of the UPS.
Why is this important
The protection against earth faults in a TN-S system relies upon the bonding of the neutral to the
protective earth in order for protective devices such as fuses and breakers to operate as intended in the
case of an earth fault.
Also the safety and undisturbed operation of all connected equipment connected to a TN-S system is
based on the assumption that the normal potential of the neutral is near ground or limited. If the neutral
reference is disconnected while the system is under current, there is no control of the transient voltages
that can occur. This is different from a IT distribution system (isolated from earth), where all equipment
including both load and protective devices are intended for such an environment, and the floating
neutral voltage is defined by some constant impedance and/or voltage limiting circuit.
Transient or single fault conditions may exceed the functional range of EMC/RFI suppression circuits in
the load equipment, which are typically tested only according to the intended distribution system, and
unpredictable behavior can occur when the reference is lost, especially under dynamic circumstances.
Lastly, some equipment, including some UPS systems, include supervisory/protective circuits designed
to detect and even prevent operation, if abnormal conditions are detected (“Site Wiring Fault” alarm).
Clearly it is not good to introduce any of the abovementioned uncertainties intentionally, if it can be
avoided.
EATON POWER QUALITY OY, Powerware Division
Koskelontie 13, P.O.Box 54, FIN-02921 Espoo, Finland
Tel. +358 9 452 661 - Fax +358 9 452 66 396 - E-mail emea@powerware.com – www.powerware.com
VAT No. FI08436562
What to look out for
The most common reason for loss of neutral reference in UPS applications is the use of four pole
devices upstream of a TN-S UPS system.
When a four pole device interrupts the neutral reference of a UPS with the load under current,
uncontrolled transients will occur. High frequency oscillations appear, especially as the current of the
different poles will break at slightly different times, and the interrupted currents interact with the sudden
unsymmetrical high impedances to earth.
When the interruption is due to a source transfer, typically a transfer to or from generator, there is even
a stronger transient as the poles of the alternative source start to close before the first transient has
stabilized.
Securing neutral reference at the UPS input
The easiest way to eliminate the above problems is to use only three pole switching elements in the
upstream circuits (or bypass the forth pole). The neutral should be permanently connected all the way
from the UPS input terminals to the bonding point
When the upstream system consists of more than one power source, it may be required to have the
neutrals of two sources separated to avoid parallel earth paths. In this case a make before break function
of the neutral should be used during transfers. Doing this eliminates the risk of the abovementioned
source transfer transients.
Another approach with multiple sources is that the switchover is upstream of the PE-N bonding point.
In some installations it may be possible to regard the system upstream of the UPS as TN-C, making the
upstream PE and N conductors a PEN conductor (connected to UPS N and PE terminals). In a TN-C
system the PEN conductor must never be interrupted. The separation of N and PE for the downstream
TN-S system is then done at the UPS output or downstream distribution.
Bonding at the UPS output
The second approach for securing the neutral reference is to establish a permanent bonding of the N and
the PE at the output side of the UPS.
As the UPS output neutral is normally galvanically connected to the input neutral to facilitate internal
bypass operation, this normally requires at least one additional isolation transformer.
On the other hand this approach has the advantage that it provides a clean independent ground reference
for the UPS distribution system. The galvanic isolation prevents any ground currents from the input and
the load side to interact and thus prevents propagation of voltage disturbances.
In some locations and circumstances, a separate bonding from the UPS output directly to the power
source earth can or should be used, even with no galvanic isolation, but this depends on local code and
approval, since this will cause some current sharing of the earth paths.
Even if galvanic isolation is used and the UPS output is separately referenced to earth, the potentials of
the input and other power conversion parts are defined by the input neutral. Therefore it still makes
sense to provide a consistent neutral reference to the input of the UPS in the way described above.
References:
[1]
[2]
[3]
[4]
IEC 60364-3 Electrical installations of buildings - Part 3: Assessment of general characteristics
IEC/EN 60950-1 Safety of information technology equipment, Annex V
CEMEP European UPS Guide 12/98, 8-6
IEC/EN 62040-1-2 Uninterruptible Power Systems (UPS) – Part 1-2: General and safety
requirements for UPS used in restricted access locations, 4.9.22
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Don’t break neutral in UPS installation