Why fuse always placed on line not on neutral, 2 pin plug

[Vindhyachal.takniki]

1. I have seen some people recommended to always place to place fuse on line and not on neutral (220Vac,50Hz). I have application which has two pin plug which is plugged into main socket.

2. I mean what difference it make whether I place it on neutral or line. Since end user can plug 2 pin plug into socket in any direction. i.e socket line can be plugged into plug netural & vice versa.

3. Moreover why in ac line & neutral are different. Isn't we can plug our equipment in any direction & it works.

[Brumby]

When you have a non-polarised plug/socket arrangement, you are correct - it makes no reliable difference.

In Australia (and some other countries) the plug and socket are polarised, so you can reliably know which pin carries the active and which carries the neutral.  (There is the possibility that the socket could have been wired incorrectly, but that is not really the main point here.)

When the active line is fused, the safety aspect is clear.

[Brumby]

3. Moreover why in ac line & neutral are different. Isn't we can plug our equipment in any direction & it works.

The difference is that the neutral is tied to earth at some point.  This does not matter for the operation of the equipment - it could be totally floating for all it cares - but it does have the function of holding the voltage on the neutral side to zero volts with respect to earth and thus holding the active to 115v, 230v (or whatever) above earth potential.

Tying the neutral to earth also allows a chassis safety earth to function as well as offering the circuit path for RCD breakers to function.

[vk6zgo]

Yes,with a system having the Neutral side earthed either at the inlet to the building,or at some other specified point,it is common practice to connect the metal cabinet of a device to earth.

Assuming this kind of connection,with the fuse in the Active,if there was a breakdown of insulation between the Active  & the device cabinet,the fuse would  blow,

If the fuse is in the Neutral side,it will not  blow,& you would be relying upon the building fuse,which may be much slower than the device fuse,with resultant risk of fire.

Fuses sometimes fail from "old age".
If it is in the Neutral side,this may cause wiring which you would reasonably expect to be at a low voltage w.r.t.earth to be at,or near the full Active potential,making troubleshooting more hazardous.

For instance,a lightbulb with a blown fuse in the Neutral---you would get a shock by touching the Neutral side of the lamp connection.

There are other "gotchas"---240v AC in North America is supplied by means of a centre-tapped pole transformer.
Both pins  are 120v "hot" w.r.t earth.
Equipment for such use often has a fuse in each leg.

Such equipment is compatible with UK,Aust,NZ,EU use,except:-

(1) In Australia,it is against the regulations to fuse the Neutral (obviously one side of the 240v supply is Neutral in Oz)
That said,I've seen plenty of such devices around.

(2) EMI filtering on the NA device may contain capacitors from each leg to earth,which although adequately rated in the intended service,with 120v  across them,may not be satisfactory in Australian service with  approx zero volts across one set of caps & 240V across the other.

[Gyro]

The fuse should always be in the line side of the supply to break the current under fault conditions. A fuse in the neutral has no protective value, and as others have said, can actually increase the hazard.

Unfortunately in the modern world there is no excuse for having non-polarized connectors, other than national inertia and the drag of not bringing 'old' standards up to date.

[BobsURuncle]

To put it simply, an overload current will always flow through the hot line, but not necessarily through the neutral line.

[Vindhyachal.takniki]

I will place two fuses one in each line & neutral since plug can be reversed.

[jitter]

The fuse should always be in the line side of the supply to break the current under fault conditions.

In a polarised system, I would agree, but more from a standardisation point of view than from a safety point of view.
In theory, the only situation  that does profit from having the fuse in the L is that in which an unearthed (class I) device is used on a socket without earth and in a building without RCD/GFI. In reality most devices are class II (double insulation, electrification of the case is improbable) and/or buildings are equipped with earthed sockets and RCD/GFI (although earthed sockets are not necessarily equipped with a RCD in old buildings!).
Typically, our RCDs in residential buildings are 30 mA.

A fuse in the neutral has no protective value, and as others have said, can actually increase the hazard.

Not agreed. It protects against an overcurrent just the same, and that's by far its most important task.

Unfortunately in the modern world there is no excuse for having non-polarized connectors, other than national inertia and the drag of not bringing 'old' standards up to date.

The reality is more like that regulations are put in place to prevent common or easily made mishaps. If having only one fuse in a device that happened to be on the N had caused numerous problems, then yes, a regulation would probably have been put in place to either polarise sockets or use fuses on L and N, or both.

The British system uses a belt and braces approach that's theoretically the safest of all systems around, and yet I believe it has its own problems, like e.g. breaking one side of the ring main and then draw all the current through the other side that's now too thin for the task.

[Gyro]

@jitter:

Yes I agree that, for a class II (double insulated) appliance, the fuse is just as effective in protecting against overcurrent. My point is that in a class I (earthed) appliance, if the fuse is in the neutral then there is nothing to break line-case faults other than the main circuit fuse (especially if the plug isn't fused  ). In this situation the risk is that the line cord, case earth connection etc. might give way before the circuit fuse. The fuse in the UK plug is rated to protect the line cord - as you say, the appliance fuse is there to protect against internal overcurrent.

The introduction of RCDs has significantly improved safety in such situations but these are nowhere near universal in (existing) installations yet.

Yes, the possibility of a break in the ring main may seem like a flaw in an otherwise "belt and braces" system but in practice isn't quite the risk that it sounds. The 2.5mm' cable used on our 30A ring circuits is rated a 26A so in theory if the broken ring is loaded to maximum current at a single point this would be a problem. The maximum current that can actually be drawn at a single point is limited to 26A (a dual 13A socket), 3 or more way sockets are available, but they are required to include a 13A fuse in their faceplate. In practice the ring is loaded more evenly and to nowhere near its 30A rating. Very high current devices (electric showers and cookers) are fed from dedicated 4mm' 30A radial circuits rather than the ring mains.

[jitter]

@jitter:

Yes I agree that, for a class II (double insulated) appliance, the fuse is just as effective in protecting against overcurrent. My point is that in a class I (earthed) appliance, if the fuse is in the neutral then there is nothing to break line-case faults other than the main circuit fuse (especially if the plug isn't fused  ). In this situation the risk is that the line cord, case earth connection etc. might give way before the circuit fuse. The fuse in the UK plug is rated to protect the line cord - as you say, the appliance fuse is there to protect against internal overcurrent.

Apparently it is deemed safe by the regulatory authorities as that is the common situation in The Netherlands, Germany and probably lots of other continental European countries. We don't have polarised sockets, they are not switched and there are no fuses in plugs.
We do limit 2.5 mm² wiring to 16 A, though, and the 16 A circuit breakers intervene rather quickly.

I have a 150 A MIG-welder running off single phase 230 V/16 A that theoretically should work on such a circuit breaker, but won't in the higher settings. When starting the weld, the current peak is enough to trip a 16 A breaker. My old apartment has (16 A) fuses instead which lets me use the welder without problem.
But even with those slower fuses, I have yet to hear of melting power cords, including the thinnest ones.

The introduction of RCDs has significantly improved safety in such situations but these are nowhere near universal in (existing) installations yet.

Yes, indeed. I live in a building built in the late 1950s, and it doesn't have RCDs in installations that haven't been renovated yet. Curiously enough there were earth wires tucked away behind the unearthed outlets of the unrenovated part of my apartment. Curious, because in those days, having earth wires in living rooms or bedrooms was pretty uncommon. When I discovered that extra and unexpected lead, I quickly changed to earthed outlets throughout the apartment. And yes, those wires were already connected to earth on the other side.

Yes, the possibility of a break in the ring main may seem like a flaw in an otherwise "belt and braces" system but in practice isn't quite the risk that it sounds.

And I have the same views on having only one fuse in that in practice it hasn't shown to be a risk of any significance.

[retrolefty]

I will place two fuses one in each line & neutral since plug can be reversed.

 I don't think that adds to safety but rather worst. One can have little faith in which of the two fuses blows open first in a overload condition and you could very well end up with the nuetral fuse blown and the line fuse unblown thus will having high voltage potential still supplied to the load.

[jitter]

I will place two fuses one in each line & neutral since plug can be reversed.

 I don't think that adds to safety but rather worst. One can have little faith in which of the two fuses blows open first in a overload condition and you could very well end up with the nuetral fuse blown and the line fuse unblown thus will having high voltage potential still supplied to the load.

I don't think I agree. Most of the devices the company I work for manufactures have IEC mains entrances with fuses in both L and N, but they also use switches that switch both L and N, and not just L. Okay, they are earthed and have three pin plugs, but those also go both ways into an outlet.