Fuse breaking capacity vs overvoltage / Surge protection  - Page 1
Forum section: electronics / beginners

author: Miyuki
author_rank: Frequent Contributor
timestamp: November 20, 2017, 10:04:51 pm
content: HiIf I have at distribution board fuses rated at 50A 500V breaking capacity 100kA and then is surge protection type 1+2 (as norm specify it)What will happen when ligthning stikes ?Surge protector clamp at 1.5kV and fuses meltBUT will they really break arc when there will be few to many kV peak at line ?What will fuses do at indirect ligthning strike voltage peak ? Fuses have breaking capacity at rated voltage, but I cannot find any specification to use with surge protection and huge voltage peak.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 20, 2017, 10:20:49 pm
content: If there is an arc under 100 ka then the voltage across that arc will be under 500 volts so the arc will be broken by the fuse.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 20, 2017, 10:47:52 pm
content: The fuse only opens if melting current has been applied to it. An indirect surge may not be sufficient -- indeed, shouldn't be, I would think.Perhaps if the surge protector has been damaged by the surge, and then draws fault current. But that's consequential, not the direct effect of the surge on the fuse itself.If a direct strike delivers enough melting current, then the fuse will open. As you suppose, the fuse will not be able to open during an extreme overvoltage. In that case, it will remain conducting (arcing internally), as long as there is enough excess voltage and current to keep it going. It will break the circuit when the surge dissipates.How long the fuse continues to arc for, and how much voltage is necessary to maintain that arc -- and therefore, how much of the surge is cut off by the fuse -- depends on the fuse design. You'd have to ask the manufacturer.If the protection device is rated for direct surge, it will keep the voltage clamped throughout the surge, whether the fuse melts and arcs, or not.If the surge exceeds 100kA, the fuse may fail in an undesirable manner, such as vaporization and explosion. There should be some upstream means to limit this -- for example, lightning itself is rarely much more than this, and the generator, transmission line, and transformer impedance usually suffices to limit current to about this level.Tim

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 20, 2017, 11:07:23 pm
content: Quote from: T3sl4co1l on November 20, 2017, 10:47:52 pmShould also mention that if the current exceeds 100ka the fuse will not break the circuit and current will continue to flow. Once that 100ka is exceeded the behaviour of the fuse is unknown and it may be that the arc will continue all the way down to zero amps before it stops due to having no voltage.

author: Gregg
author_rank: Super Contributor
timestamp: November 21, 2017, 12:29:27 am
content: You might want to look at triggered spark gaps:http://www.lightningman.com.au/surge_technologies.htmlEnclosed are a couple of pictures of lightning damage at a US house with 100 amp 120/240V service entrance. The utility inlet is at the top, next is the meter socket contacts (no meter, obviously) then the two pole main breaker. The wire is aluminum due to cheap construction and may have had something to do with one of the wires melting completely at the breaker. The pole across the street was burned significantly and a 8ga copper ground wire running down the pole had melted with balled up ends.