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Please refer to the following drowing:

enter image description here

Imagine from utility to the electrical outlet we receive line, neutral and earth as shown above in Figure 1. This is connected to a device such as a power supply with a load R inside a chassis. F depicts the fuse of the device.

If we swap the power plug to the outlet or socket, we will obtain Figure 2.

Does that mean the device fuse will not work in this case?

When making a power plug like the one below, does it matter which pin is connected to line and which pin is to neutral?

enter image description here

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  • \$\begingroup\$ Remember, current only flows through the Earth conductor if a fault occurs - the Earth is a safety feature. \$\endgroup\$
    – Attie
    Commented Sep 5 at 9:55
  • \$\begingroup\$ As far as I know, Live and Earth are also not typically bonded as you've shown - Earth is a "local steak in the ground" (i.e: at the property) while neural is a "distant streak in the ground" (i.e: at the transformer) \$\endgroup\$
    – Attie
    Commented Sep 5 at 9:55
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    \$\begingroup\$ @Attie May depend where you live. In case which you speak of, if your device had a fault between Live and Earth, it would require fault current to flow through soil, and the soil resistance must be low enough for enough current to pass to blow a fuse. Say more than the default of 10A or 16A we have. \$\endgroup\$
    – Justme
    Commented Sep 5 at 10:35
  • \$\begingroup\$ @Justme - Indeed, I should have clarified... it's referred to as "TT" and I believe this is (was) fairly common in the UK, especially for more rural properties. I've just done a bit of reading, and it sounds like it is indeed not the preferred / standard option (my mistake, it's what I'm most familiar with) \$\endgroup\$
    – Attie
    Commented Sep 5 at 10:57

3 Answers 3

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Does that mean the device fuse will not work in this case?

It depends what you mean by 'work'.

The fuse WILL work to protect the wiring from over-current.

The fuse WILL NOT work to disconnect the chassis from live in the event of some internal faults to chassis.

If the L and N are likely to be swapped, then the appliance should be a 'double insulated' type. If it is not double insulated, then L and N should be scrupulously routed to their correct pins on a polarised plug.

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  • \$\begingroup\$ That plug is unpolarized so wiring it to correct pins is impossible. \$\endgroup\$
    – Justme
    Commented Sep 5 at 10:26
  • \$\begingroup\$ @Justme so 'L and N are likely to be swapped' then, let's hope the appliance is double insulated. \$\endgroup\$
    – Neil_UK
    Commented Sep 5 at 10:29
  • \$\begingroup\$ That plug is also a grounded plug so the device is clearly not your average plastic hairdryer i.e. one with no touchable parts that can become live. For example your average desktop computer is a metal cube which has that plug, the metal case being earthed. Double insulated devices like hairdryers don't need earth so they have unploarized and non-earthed plug. \$\endgroup\$
    – Justme
    Commented Sep 5 at 10:42
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Yes the fuse will work no matter what (regarding the load between Live and Neutral), and no the plug is unpolarized so which way Live and Neutral go does not matter.

You have posted a picture of Type F (German Schuko) plug.

You can plug it in in either way, which means, your device must be built with the assumption that either wire can be Live or Neutral and it must be just as safe when plugged in both ways. Cheap devices may not do that and are likely against rules for electrical safety and must not be sold - which is why you should not buy cheap power strips from online stores and take the risk of importing it yourself.

Of course the fuse is useful. It prevents overcurrents between Live and Neutral themselves. It prevents the device electronics from taking excess current.

A fault can happen before the fuse, so in that event, the fuse will not blow, but a circuit breaker or fuse in your electrical panel will. The fuse inside the device is not for protecting the cable wiring from making contact with metal chassis.

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Yes, the fuse will work just as well on the neutral as on line as current flows in loops. In countries with unpolarized plugs, this is expected. In countries with polarized plugs, fuse on the live is required in the cases I've been involved with.

With fixed wired appliances, rules are usually a bit different and the fuse is always placed on live.

If you open up several appliances, you will find that many SMPSes marks one incoming wire as L and the other as N, despite unpolarized plug. From there, fuse on L.

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