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Fuse blowing

Kelly Marie Angel
So just now I was adding an extra light over my day bed  it's a vintage 60 watt bulb looks like an old radio valve it even has the little glass pip on top. Anyhow as I was wiring it up two whiskas of wire touched there was barely any sound but it popped the 5 amp fuse in the plug adaptor and tripped 16 amp MCB how is this even possible even under partial short circuit conditions surely a 5 amp fuse should clear before a 16 amp  MCB?
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    I have no idea what sort of impedance my fault had it was just a few strands of copper wire which accidentally contacted the opposite wire. Maybe it serves me right for working live I guess. Ancient mariner there was no other load on that MCB as it serves only a double socket in the disco room which is where the day bed is. I have read on another forum that there is an arc form when the bulb filaments break and acording to some measurements made currents of several hundred amps can flow for a few cycles so not surprised it trips the MCB. Why ever did they stop putting Ballotini fuses in bulbs. Incidentley  does anyone know was Ballotini the name of the Italian  who invented them? That name sounds Italian so guessing he was. Are those LED lights you have fitted with switch mode supply's or simple capacitive dropper ?
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    a rather nice article showing plain and ballotini fuses in lamp caps and explaining why.

    This is the safest and most reliable fuse type - but also the most expensive. The fuse wire is encapsulated in a small glass tube filled with tiny glass beads called Ballotini, named after Potters Ballotini, the Italian producer. Should a secondary arc form between the broken pieces of fuse wire, its heat will melt the glass beads to form an electrically insulating barrier which contains and rapidly quenches the arc. It is so efficient that only one fuse is required, but on account of their higher price they are generally employed only in the more expensive special lamps




    In a mains fuse, the sand does a similar thing.


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    Chris


    I was not watching but nearby when I heard the bang and all the lights went out. Not one of my merry men, not my job and I was not supervising, but I can see the picture in my mind of your description.
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    AncientMariner:

    [snip]  Seems to suggest that as the filament failed it arced across, but is that the reason?  From Tungsten went to compact GU10 florescents and now LEDs. But the LEDs do not have the expected life, nor did the compact florescents. Their electronics should have handled the over-voltage (we are not far from the sub-station, whilst within limits, generally around 243 volts. So perhaps spikes?


     




    Indeed, we were in the same position with GU10's. My hypothesis is, that the low Zs value due to being close to the substation (it's just across the road) and the higher than normal voltage (240-245 here depending on time of day) causes the GU10 capsules to arc over internally, given their small size, the electrodes are closer together than in a typical GLS lamp. I caught the violet flare from one before the mcb tripped, which is what confirmed arcing as the cause for me.  I replaced them with LEDs, never looked back.

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    is this even possible even under partial short circuit conditions surely a 5 amp fuse should clear before a 16 amp  MCB? 



    Almost all over-current devices have(at least) two stages to their operation - firstly the "commit" to opening but the actual stopping of the current happens some time later. Fuses first melt a small gap in the fusewire but the current arcs over that gap and so still flows in the circuit - only when the gaps gets large enough (or we get to the zero point in the a.c. cycle) does the arc fail and the current really get interrupted. MCBs have an even longer sequence - first 'de-latching' then physically moving the contacts apart before starting on the arcing phase before finally properly breaking the circuit. In each case once the first stage is reached, the device will eventually be left in an open state, even if some other device in the circuit gets to the final stage first.


    MCBs are notoriously bad at discriminating - they de-latch relatively quickly (usually faster than a fuse will completely open) but take much longer to interrput the current - so allowing other devices plenty of time to see and start responding to the same fault current. MCBs pretty much don't discriminate at all with other MCBs (excepting where the fault current is too low to trip the larger rated MCB at all), and are pretty poor with downstream fuses. Fuses upstream of MCBs have a much better chance - but usually have to be significantly larger rated (say 60A+) even for small MCB ratings.


       - Andy.
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    To go slightly off topic our local substation has a cable which runs most of the length of my street down to a link box then from the link box back up the street to feed the houses and a shop at the top of the road. When our nearest sub had a fault a short circuit in the LV that goes to the link box we were back fed from another substation the point I'm trying to make is just because your near the tranny don't assume you are on a short length of cable from it. My appoligies if I'm stating the obvious  especially to our more seasoned members
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    Kelly Marie:

    To go slightly off topic our local substation has a cable which runs most of the length of my street down to a link box then from the link box back up the street to feed the houses and a shop at the top of the road. When our nearest sub had a fault a short circuit in the LV that goes to the link box we were back fed from another substation the point I'm trying to make is just because your near the tranny don't assume you are on a short length of cable from it. My appoligies if I'm stating the obvious  especially to our more seasoned members




    The nearest tranny to us is about three doors down on the other side of the road. Ze is 0.21Ω which seems a bit high except that we are on a corner and the cable goes all the way past the house, round the corner, and back in.


    Kelly does prompt an interesting question. Suppose that you normally have a low Ze and despite that one or more circuits have a marginal Zs. If the DNO alters it's configuration, ADS may not be quite as fast as it should be.

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    John Peckham:


    I just did a quick selectivity study and the fuse should have beaten  the MCB as in theory full selectivity is achieved. But that is only theoretical and in realatity, as you have proved, theory is based on variables not all of which can be quantified. I have seen recently a sparks drilling a hole in trunking hit a cable inside and blow the MCB on the board and the MCCB back at the main panel. He also needed a new drill bit.




    We used to hold the cables away from the drill site with some cardboard, wood or a small length of plastic mini trunking to prevent drill damage.


    Z.

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    Chris Pearson:

    Kelly does prompt an interesting question. Suppose that you normally have a low Ze and despite that one or more circuits have a marginal Zs. If the DNO alters it's configuration, ADS may not be quite as fast as it should be.




    Indeed, and that is the reason for using the DNO’s maximum value for your design, rather than the figure you measured. 


    Regards,


    Alan. 

  • Former Community Member
    0 Former Community Member
    Although, to be fair the DNO pretty universally trumpet a value of 0.35 Ohms


    I was told this categorically on a recent application for a 630A service  - and no amount of me pointing out that wasn't credible based on the ASC and their obligations under ESQCR in terms of voltage was going to make them change their mind, no siree, Bob


    Some DNO's are better than others though - or at least with some it's easier to fight your way through to the technical guys to get some sense


    Regards


    OMS

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