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Chapter 12 Helpful information
Because of the expected high leak current it is not recommended to use an upstream RCD /
differential circuit breaker. If, however due to legal requirements an RCD must be used, then a
D curve 420 mA RCD is to be installed. Please note that an RCD has to be tested once a year
by pressing its test button. A better choice to RCDs are RCBOs, that is Residual current
operated Circuit-Breaker with over-current protection, which are also known as GFCI (ground
fault circuit interrupter). See also
information.
During normal operation these UPS units usually have a small leak current, quite often below
400 mA. The maximum defined leak current is the worst case scenario. Therefore the
maximum leak current must be considered before installing the UPS unit.
When an electrical device draws more current than the circuit breaker is rated for, then the
circuit breaker will trip. The same happens with an RCD. When the current difference is larger
than what the RCD allows for, then the RCD will trip.
Measuring the leak current
Attention! The measuring and certification of leakage current values is a task and involves a lot of set
up work. Installing the hardware and starting measuring leakage current will not yield the
desired results.
The leak current is influenced by a number of parameters such as:
Was the measurement taken when load was attached to the UPS unit? If yes, then this
•
load contributes to the leak current; that is the leak currents of all devices are summed
up.
What electrical grounding system is in use? For example, TT, TN-S, etc.
•
Leakage current can also come from the source side of the UPS.
•
Leakage current can come from anywhere in the utility power network.
•
If a leakage current has been measured without load attached to the UPS unit, then this is
indicative that leakage current from other devices is measured at this point in time. In that case,
an isolation transformer must be placed between the source utility power and the UPS unit.
If a leakage current has been measured without load attached to the UPS unit, then this is
indicative that leakage current from other devices is measured at this point in time. In that case,
an isolation transformer must be placed between the source utility power and the UPS unit.
226
Earth leakage considerations using an RCD
6U Rack or Tower UPS RT8kVA and RT11kVA IUOMG
for additional
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