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In this critique of an ELV PSU design, it is suggested that a "Live-Neutral" fault could result in a fault current of thousands of amps.

It says:

Fire – (d) the plastic had a UL94-HB rating ( which limited its use to a decorative enclosure); (e) the fuse rating would not protect the transformer from fire; (f) a glass fuse will have a typical maximum “breaking” current of only about 30 Amps. This means that a Live Neutral fault could create a fault current in excess of 2,000 Amps – causing the fuse to explode and create a plasma that would sustain the fault current – perhaps for several seconds.

I'm sceptical. Please could someone explain how this could be? I can't imagine any circumstance where such a 100 mA, 240 - 15 V transformer could produce such a current. And if there is a live-neutral fault, would the fuse not blow, as they say, at about 30 Amps (at the most)? I thought the mains wiring and associated resistances*, capacitances and inductances would limit a current surge such that the 1 A fuse would clear before exceeding 30 Amps. Isn't that, after all, what fuses are for? To repeat my question, how is the figure of 2000 Amps arrived at? Thanks

Edit: Such as the resistance of the fuse. 0.25 Ohms for an average 1A 250v fuse

try-catch-finally
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Jodes
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  • If you search prior material on stack exchange EE you'll find a substantial amount of material on this. The figures you cite are in the order of right. Fuse blowing current and fault clearing current are different. HRC (high rupture capacity) fuses exist to deal with this difference. A glass fuse may blow but sustain an arc of 100's of amps long enough to kill you. | Id your pole fuse is 100A and your neighbours is 100A and ... what is the street cct able to supply. | If you draw 50A from your home mains supply and it sags 1%, what current would you expect it to supply if you hard shorted it. – Russell McMahon May 11 '13 at 16:38
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    At 50 Hz, 230 VAC, what inductance do you need to add say 1 Ohm reactance to your house feeder circuit. What inductance do you think the feeder has? | A friend has an electrician (stupidly) reverse phase and neutral when wiring up their house . Steam came out of the cold taps (really) and worms crawled out of the ground (really). What current do you think flowed :-) :-( ? – Russell McMahon May 11 '13 at 16:40
  • Thanks. Considering resistances alone, a figure of 2000 amps at would mean the resistance of the circuit would have to be 0.1 ohms. Is that reasonable? Especially with contact resistances, I'd have thought it's a magnitude out – Jodes May 11 '13 at 16:41
  • @RussellMcMahon That "worms crawled out" verbal visual totally made my day. – Anindo Ghosh May 11 '13 at 16:43
  • I don't suppose you have any links to the other material on SE EE? Or perhaps you suggest search terms? Thanks – Jodes May 11 '13 at 16:44
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    The resistance of a plasma arc across the gap where a glass fuse body used to be will be "somewhat lower" than that of the prior fuse . – Russell McMahon May 11 '13 at 18:32
  • Refs / Notes added – Russell McMahon May 11 '13 at 18:45

1 Answers1

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4:40am
Rushing.

If you search prior material on Stack Exchange EE you'll find a substantial amount of material on this.
The figures you cite are in the order of right.
Fuse blowing current and fault clearing current are different.
HRC (high rupture capacity) fuses exist to deal with this difference. The ceramic bodied fuses you see in better equipment are HRC.
A glass fuse may blow but sustain an arc of 100's of amps long enough to kill you.

If your pole fuse is 100A and your neighbours is 100A and ... what is the street cct able to supply?

If you draw 50A from your home mains supply and it sags 1%, what current would you expect it to supply if you hard shorted it?

At 50 Hz, 230 VAC, what inductance do you need to add say 1 Ohm reactance to your house feeder circuit. What inductance do you think the feeder has?

A friend had an electrician (stupidly) reverse phase and neutral when wiring up their house.
Steam came out of the cold taps due to electrical heating in the grounded copper "cold" water pipes as current flowed from mains phase via switchboard ground to copper pipes and thence to ground. (really)
and worms crawled out of the ground (really)
and they tell me that the house made groaning sounds.
I imagine that that was probably from water boiling in the cold water pipes.
What current do you think flowed :-) :-( ?

HRC fuses - there will be somje ueful links there.

Wikipedia - fuses

  • The breaking capacity is the maximum current that can safely be interrupted by the fuse. Generally, this should be higher than the prospective short circuit current. Miniature fuses may have an interrupting rating only 10 times their rated current.
    Some fuses are designated High Rupture Capacity (HRC) and are usually filled with sand or a similar material. Fuses for small, low-voltage, usually residential, wiring systems are commonly rated, in North American practice, to interrupt 10,000 amperes.

Wikipedia - breaking capacity

  • Miniature circuit breakers and fuses may be rated to interrupt as little as 75 amperes and are intended for supplementary protection of equipment, not the primary protection of a building wiring system. In North American practice, approved general-purpose low-voltage fuses must interrupt at least 10,000 amperes and certain types useful for large commercial and industrial low-voltage distribution systems are rated to safely interrupt 200,000 amperes..

ADDED

Stack Exchange:

Similar material.

Fuses: What are the practical differences between Ceramic and Glass cartridge fuses

What is the Thévenin equivalent of the mains power supply? - 1st approximation - a piece of copper busbar :-)

The Impact of Mains Impedance on Power Quality

Useful. See fig 6.
Note transformer impedances specified as a % - this is the % drop in output voltage at rated load.

Added 2:

Thanks for the clarification of breaking capacity and highlighting reactance. I still think 2000 amps is over the top. 200 amps I could understand.

I'd guesstimate that 2000A would probably be getting on the high side in a residential situation. But 200A is far too low.
Far far too low.

If you can get 50A intended current at your home's distribution board and your neighbour's lights do not flicker, what would you get if you shorted it?

People have died from mains arc discharge that was improperly interrupted.

Standards typically allow a 5% V drop at the farthest outlet from the distribution board in a home at rated load.
At 20A rate that implies available current is ~+ 20A/0.05 = 400A.
And that's worst case on house wiring!.

Russell McMahon
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  • +1 Thanks for the clarification of breaking capacity and highlighting reactance. I still think 2000 amps is over the top. 200 amps I could understand. I suppose I need to do some number crunching for the worst case scenario – Jodes May 11 '13 at 17:05
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    @Jodes - I'd guesstimate that 2000A would probably be getting on the high side in a residential situation. But 200A is far too low. Far far too low. If you can get 50A intended current at your home's distribution board and your neighbour's lights do not flicker, what would you get if you shorted it. People have died from mains arc discharge that was improperly interrupted. || Standards typically allow a 5% V drop at the farthest outlet from the distribution board in a home at rated load. At 20A rate that implies available current is ~+ 20A/0.05 = 400A. And that's worst case on house wiring!. – Russell McMahon May 11 '13 at 23:55
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    @vMMF I left your minor alterations - capitali[s|z]ation of "Stack Exchange" and a typo space in another word. And/But, I corrected the incorrect corrections to the spelling of neighbours, which is how it's done where the Queen's English she is spoke, and corrected the miscorrection of thence to then, as the miscorrection changed the sense and intention of the sentence, albeit subtly, and was entirely good Engrish as she was initially written. FWIW :-). – Russell McMahon Oct 23 '18 at 23:02