2
\$\begingroup\$

I'm designing a product that will be fed directly from 220V mains. The 220V goes into a PCB that has a high voltage section and a AC/DC converter module to generate low voltage to the microelectronics.

When using 110V I always use a input fuse for overcurrent protection in one of the mains wire (Live or Neutral - whatever...).

For 220V, since I have two 110V phases, should I use one fuse or two fuses (one in each phase)?

I think that using a single fuse should be suffice. However if one of the fuses blows and the other stays intact the board still have a hot main wire attached. Even if no current flows because the other phase is out, there's still possibility of electrical shock or worse... am I thinking right?

\$\endgroup\$
5
  • \$\begingroup\$ Why do you think you have two 110V phases? 220V is just a single phase. Anyway, you are thinking along the right lines, and one fuse only should be used. As you correctly identified, 2 fuses could be dangerous. \$\endgroup\$ – Steve G Jan 19 '16 at 19:30
  • 1
    \$\begingroup\$ Is this for use in Brazil? Is your 220V wiring like US wiring with neutral grounded and two 110V phases? Or is it like European? \$\endgroup\$ – Spehro Pefhany Jan 19 '16 at 19:33
  • \$\begingroup\$ Yep, for use in Brazil. Two phases 110V each (hot and hot). No neutral. Sorry not specifing before, didn't know that european and U.S. wiring were different. \$\endgroup\$ – RHaguiuda Jan 19 '16 at 19:38
  • \$\begingroup\$ 2 fuses won't help in the situation you describe. If a fault occurs which causes enough current to 'blow' a fuse, you cannot rely on both fuses blowing. It is entirely possible that only one will blow (the 'weaker' of the 2, even if they appear identical). \$\endgroup\$ – brhans Jan 19 '16 at 19:43
  • \$\begingroup\$ Yes, you are thinking right. But I have never seen the electronic parts having two fuses, even if they work from 85 to 275VAC, no matter what country. \$\endgroup\$ – Marko Buršič Jan 19 '16 at 19:44
6
\$\begingroup\$

If you really have a two-phase supply you need two fuses to protect in the event of a phase-chassis internal fault.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1 and 2.

In Figure 1 you need F1 to blow. In Figure 2 you need F2 to blow.

As per the comments, if you don't have two fuses then you need the RCD / GFID to isolate the power. If that doesn't work or isn't there you are depending on the upstream circuit-breaker / fuse which may have a relatively large rating and allow high fault current to flow for some time before tripping out.

\$\endgroup\$
8
  • \$\begingroup\$ This makes sense, seems the only right design, but have you seen it in some appliance before? I never did. \$\endgroup\$ – Marko Buršič Jan 19 '16 at 19:46
  • \$\begingroup\$ @MarkoBuršič similar to how a 3-phase device would be fused. You wouldn't skip the third fuse in that case, would you? In above design a single fuse migt suffice if the casing is made of plastic and no earth lead connected to the board. \$\endgroup\$ – jippie Jan 19 '16 at 19:57
  • \$\begingroup\$ For example, most apliances have one fuse only, in EU plug is symetrical you can turn 180 degrees and plug in two different ways. Once the live will be connected to the fuse, while in other situation the neutral is connected to the fuse and live is not fused. \$\endgroup\$ – Marko Buršič Jan 19 '16 at 19:59
  • 1
    \$\begingroup\$ I have seen double-fused appliances. I think they were medical devices. See C14 Male Straight Snap-In IEC Connector, Rated At 10A, 250 V ac. \$\endgroup\$ – Transistor Jan 19 '16 at 20:22
  • 1
    \$\begingroup\$ This is not really why one uses fuses. Fuses are designed to prevent fire in case the load fails short(ish), not protect the power supply from shorts to frame. If there is a short-to-frame, either the house breaker or ground fault interrupter should trip. Note that F1 and F2 cannot protect against the most-likely short to frame, which is abraded cable where the power supply passes through the frame! \$\endgroup\$ – William Brodie-Tyrrell Jan 19 '16 at 23:42
3
\$\begingroup\$

I think the OP is talking about a device that is pluged into a socket, therefore it is fused with circuit braker or by fuse (double, each phase one)already. If the case is metallic it has to be connected to the earth wire. If there is a dielectric break to the case then the circuit braker will diconnect the socket. The internal fuse of the device is just preventing a malfunctioning rise of current that would put on fire the device. So the conclusion is - you don't need two fuses.

\$\endgroup\$
0
\$\begingroup\$

I think you're confused bud. Most power generation systems are 3-phase. Wall sockets use 3 pins because 1 is ground, 1 is neutral, and 1 is power (HOT). You want your fuse to cut off the HOT line. Your circuit breaker is configured to provide either 120VAC or 240VAC to the specific plug but it still provides it to the HOT and NEUTRAL lines, not an additional pin.

There is a good summary in a DIY stack exchange if you want to learn more about 3-phase and 120VAC vs. 240VAC. https://diy.stackexchange.com/questions/42043/whats-the-difference-between-three-phase-240-v-and-standard-household-240-v

\$\endgroup\$
4
  • \$\begingroup\$ With respect to @transistor, you might have power going 2 phase (phase to phase), but more realistically, you are probably using a different transformer configuration in your circuit breaker to provide single phase 220VAC instead of dual phase 220VAC. \$\endgroup\$ – SpaceCowboyMDK Jan 19 '16 at 19:48
  • \$\begingroup\$ engineeringradio.us/blog/2012/02/… \$\endgroup\$ – jippie Jan 19 '16 at 20:02
  • \$\begingroup\$ @jippie Thanks, are you agreeing or disagreeing? The article is related to delta vs wye configurations and doesn't appear to add meat to the discussion relating to 3-phase power, wall sockets, fuses, or 220V wiring... \$\endgroup\$ – SpaceCowboyMDK Jan 19 '16 at 21:24
  • \$\begingroup\$ @SpaceCowboyMDK in Brazil 220V mains are 2 phases 127V, not a single phase of 220V like in Europe. So a 220V config here is 2 phases 127V + 1 ground pin. \$\endgroup\$ – RHaguiuda Oct 27 '19 at 16:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.