Fuse

[Tom749]

Why are the reasons for using the fuses at my attached drawing? (Especially the quantity and the position) In my opinion those should be installed at the both of the two lines such as the number (1). (I can not understand the quantity and the position of the fuses from number (2) to number (5).) Is it right that all lines should have the fuses in the cases of AC and DC power source? Is there any rule when the fuse is used for the protection? Thanks for your help in advance. Edited July 6, 2011 by Tom749

[ianbuckley]

Note that the transformer is grounded at X2 - providing a single phase secondary. So the wires connected to X1 have potential in relation to ground, where the wires connected to X2 do not (they are directly connected to ground). If you were to fuse the "other" half of the circuit (AC neutral or DC common), you are interrupting the path to ground for that circuit, which leaves the possibility of electrical potential trapped in that circuit - a potentially shocking development. The only time you would normally switch (fuse or breaker) a neutral wire is if the neutral is not locally grounded. As for general fusing guidelines, check your local electrical laws and/or a fuse manufacturer catalog. The general idea is to interrupt the connection to the power source in a safe manner if excessive current is drawn by a downstream device, but there are other reasons as well (circuit branching, etc.).

[Tom749]

Thanks for your reply very much. Could you give me more help at this attached drawing? What is the reason for the three fuses are used in two cases? (Just for the branch circuit and the Motor starter...) Thanks for your more help in advance. Edited July 7, 2011 by Tom749

[BobLfoot]

Tom - my response is based on my reading of the USA - NEC 2008 and may not be relevant to you in kuala lampur but the basic logic still holds valid even if the local regulations are more or less stringent. Fuse 1 - Is sized based on Table 450.30(B) or at no more than 125% of the rated primary current. - Assuming the 3000 VA transformer and 240 volts we get 12.5 amps which when multiplied by 1.25 yields the 15.625 amps. The nearest standard size is therefore 15 amps and is used. Fuse 2 - IS to protect the Power Supply and its input wiring and sized most likely from the manufactureres recommendations. Fuse 3 - Is sized based on the rating of the power supply and to blow before the supply is subjected to crowbarring. Fuse 4 - Is sized based on the loads fed down that wire. NOTE: the XFMR can supply 24 amps at the reated 3000 VA. To protect Filed loads and wiring a fuse is used. Fuse 5 - Is sized to support the non-power hand tools that the designer wants plugged into the outlet. I suspect a laptop PC. And as others have said there are no fuses on the X2 connecting wires because X2 is a neutral with ground reference. And you almost never fuse a neutral. Hope this helps.

[BobLfoot]

Again for Americans we would say that these fuses are the required OverCurrent Protection Device which is required for almost each and every motor branch circuit {I'm sure there are exceptions, but few come to mind}. The design of which is specified in Artivcle 430 Section III of the National Electric Code. It sounds like you bought a machine built to to US Standards and are learning the standards "on the fly"

[Shiner]

In regards to the second drawing two things. The fist example shows a motor feed that is of lower current rating than the main feed. This is a simple protection of the motor from overcurrent (the line will supply way more current than the motor can handle). However, I see in the notes that the disconnect is rated for 30A yet the wire used is 14awg. The standard here (again I know you are in a different country and standards may be different) for 30A is #10AWG. putting 30 amps through a 14 guage wire will make some heat when it is a load such as a motor. The second example shows that the motor is fused at what the line wires are rated for (#14AWG is good for 15A). If something is fused already at one rating, why fuse it again you may ask. This is because the load (motor) is subject to different types of failure. If the motor short circuits, then the fusing for that motor alone will blow, leaving power for the rest of your controls (possibly an alarm light to let you know that there is a problem with your motor. If the fuse ratings are cascaded right, you should only lose one piece of equipment instead of putting a spike on all components.