TDK-Lambda Americas Blog

When are fuses needed in Line and Neutral for industrial applications?

Fusing Safety Certification

August 30, 2019

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!