When is dual fusing in power supplies really dangerous?

Many AC-DC electronic power supplies are now certified to meet the ITE (Information Technology Equipment) IEC 60950-1 standard and also the medical IEC 60601-1 standard. These supplies often have two internal fuses, one in the Line and one in the Neutral connection. This is to guarantee protection in a medical application where the polarity of the Line and Neutral could be interchanged due to an outlet socket or line cord wiring error.

If an electrical or electronic device consumes enough power to exceed the AC socket rating, it would need to be directly connected to the building wiring, or be permanently connected. In this case, polarity reversal is not possible. The NEC, CEC, IEE Wiring Regulations and IEC 364 though, specifically prohibit fusing the neutral in this type of equipment.

If a dual fused power supply was to be installed in permanently installed equipment, there is a 50% probability that either fuse F1 or F2 could open due to an overcurrent condition or fuse ageing. If F2 were to open, there is a risk that service technicians may believe there is no Line connection present in the equipment. If inadvertent contact was made to the Line, while touching the earthed equipment chassis, because the neutral is connected to ground, they would receive an electric shock.

When considering a power supply for this type of application, choosing a dual fuse power supply may cause an issue. One option is to select a single fuse product, or contact the manufacturer to have the Neutral fuse bypassed as a modified standard – although in this case the safety certifications may need updating.

It’s worth noting that some products like TDK-Lambda’s QM series of modular power supplies are safety certified with the option of single or dual fuses. This can be selected during the part number configuration.