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
Best Answer
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
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
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:
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!.