Last month I wrote about why using a
dual input fused medical/industrial power supply might cause an issue in some
industrial applications. Shortly after
publication a customer requested an application note showing the fitting an
external fuse in the Neutral connection of an industrial DIN rail power supply
application. DIN rail power supplies are
rarely certified to the medical safety standards and usually only have a single
input fuse.
Upon investigation, the application was
in this case operating a power supply phase to phase from a 3-phase WYE
configuration in North America, see Figure 1.
Figure 1: Phase to phase connection (208Vac)
Connecting phase to phase enables the equipment
to be supplied with 208Vac which draws less input current than using 115Vac. This allows the use of smaller wire gauges
and connectors which saves money and are easier to install. The power supply is connected as shown in
Figure 2.
Figure 2: Power supply connected phase to phase
In the UL safety report Conditions of
Acceptability, or Technical Considerations section, reference is made to all
testing being performed on a protected branch circuit rated for 20A; this section also lists the items that
a user should consider before applying power to the unit.
If a short or overload inside the power supply occurs across the Line and Neutral or Line and Ground, the internal fuse F1 will open. If the short occurs from Neutral to Ground, the fuse or breaker at the distribution panel will open.
If, however, the equipment connection to the building installation wiring is made via a non-industrial plug and socket, then an external fuse or breaker (F2) has to be added in the Neutral line as shown in Figure 3. This applies to input voltages of less than 240Vac +10%.
Figure 3: The installation of an external fuse
or breaker in the Neutral
Always have your equipment installed by
a qualified electrician and checked by a safety engineer for compliance to the
relevant building and electrical codes!
Power Guy
