What happens if one were to overload a fuse so strongly the fuse's maximum safe interrupting rating was exceeded? My guess is that it would start arcing enough to allow high current through the device it is protecting for a (relatively) protracted period of time, therefore potentially causing damage to it (and hence defeating the purpose of the fuse, which is to _protect_ the device from damage). Not to mention the arc itself might be damaging as well (due to the high temperature.). Is this right?
Is there a video or something showing what happens?
If you think about it the interrupting rating is likey due to the voltage of the fused circuit rather than the current rating; A highly inductive circuit will look like a transient high voltage source and possibly arc over the fuse and sustain a plasma arc. BTW, a fuse is there to prevent the source wiring from burning up, not keep the load device from getting damaged; the load is already damaged if it is going to pull enough current if one assumes the fuse was sized correctly in the first place.
On Fri, 21 Mar 2008 15:17:59 +0000 (GMT) John Rye wrote: | Hello Mike | | In article | , | mike3 wrote: |> Hi. | |> What happens if one were to overload a fuse so strongly the fuse's |> maximum safe interrupting rating was exceeded? My guess is that it |> would start arcing enough to allow high current through the device it |> is protecting for a (relatively) protracted period of time, therefore |> potentially causing damage to it (and hence defeating the purpose of |> the fuse, which is to _protect_ the device from damage). Not to |> mention the arc itself might be damaging as well (due to the high |> temperature.). Is this right? | |> Is there a video or something showing what happens? | | There are substantial textbooks written about fuses and how they work. (For | example "Electric Fuses" by A Wright & P G Newbury ISBN 0 906048 78 8) | | Operation is different on alternating current and direct current. | | To try and keep it simple on alternating current in correct operation :- | | (1) If the current is high enough for long enough it melts a short bit of | metal which is thinner than the rest of the conductor and leaves a gap. | | (2) An arc forms across the gap. | | (3) The material surrounding the arc cools the arc, and when the current | drops to zero the arc goes out. | | (4) Providing the resistance across the gap is big enough the arc does not | restrike and the circuit is broken. | | If the fault current is higher than the fuse is designed to handle too much | of the element will be melted before a current zero occurs and the arc will | restrike. | | If the fuse is used on a higher voltage system than it is designed for the | thinner part of the element that is intended to melt will not produce a big | enough gap and the arc will restrike. | | If the arc restrikes in the fuse and keeps burning the heat will destroy the | body of the fuse, and the arc will then continue burning outside of the fuse. | | In either case the result is usually considerable damage at the site of the | fuse, and also at the site of the problem which caused the fuse to operate in | the first place.
If the current of the arc is sufficiently high, the damage can also occur in the wiring supplying the fuse (for a main fuse or single circuit) and in the wiring between the fuse and the faulty load. That damage could be as extensive as complete building destruction due to fire and the death of a great many people inside depending on the lack of escape opportunities (e.g. a fire at night in a home full of small children).
IMHO, fuse and breaker interruption ratings are one thing that should never be grandfatherable. In cases where the utility increased the fault current availability, they should pay the upgrade costs for all customers involved. Or they should split up LV circuits when they upgrade to transformers to deal with increased power demands.
. John Rye has a nice answer. A high current can produce an arc larger that the fuse was designed to extinguish destroying the fuse (and a lot more).
One of the ways to make a fuse that is safe with very high available fault currents is for the fuse to open before the first current peak so the fuse does not have to interrupt the full available fault current. .
. Fuses primarily protect load side wiring and distribution equipment. They will in many cases protect load side equipment - like a stalled motor (usually protected by means other that a fuse).
| So what happens if the current got so high the fuse was totally | destroyed? | Would the motor still have been protected or not?
The arc is still a high current channel. It is dissipating some of that power in the form of a voltage drop over the arc channel times the current flowing through the arc.
If there is a fault in the motor that results in the high current flow, the exact scenario depends on the nature of that fault. If there is an arc in the motor itself, such as in the windings, that could get very destructive. If there is a bolted fault that is not producing an arc, then the damage would be from the dissipation of the fault current in the wiring. That will be less until it reaches the point where wiring fails and an arc likely begins at that point. Or maybe that arc will cool sufficiently and extinguish at the next zero cross.
| So then my guess was right, it fails to stop the current (therefore | defeating | the purpose of the fuse), not to mention causing direct damage due to | the | arc itself.
It would be a failure of the fuse. It would be a failure of the process to select the proper fuse and supporting equipment to ensure that the existing potential conditions (voltage and available fault current) can be dealt with. But this by no means justifies omitting any kind of current interruption capability.
The interrupting rating needed is *not* a function of the load connected, but of the *source*. If the source is a hi power supply that can theoretically supply several thousand amps (i.e. it has a low internal impedance), then protective devices such as fuses and breakers must have a higher interrupting rating. Good design assumes that a bolted, zero-ohm fault occurs right at the wiring leaving the fuse box and calculates the maximum current that could occur (based on the source impedance and voltage). Then size protective devices to interrupt that current.
Some fuses are 'slo-blow' meaning they have a larger thermal mass so it takes more heating to melt them. These are for service where the normal load can occasionally draw higher than fuse rating current. Such as motor starting.
Some fuses I've used were filled with sand to increase the interrupting rating. The idea is that when the fuse melts, bits of sand would 'fall' in between the melted ends and help interrupt the arc. If the arc got hot enough, the grains of sand would melt together into a crude form of glass. A small, 10A fuse of this type that measures just 2 1/2 inches long is rated for interrupting 10 kA.
All this talky-talk has convinced you, somehow, that fuses don't work... because the mains have low impedance, or something. I just gotta say, what a crock. The fuse melts when the current through it exceeds the fuse's current rating. Doesn't take long. Now you have a gap, which interrupts the current. Here we get to the misleading part. Somebody comes along and tells you'll have an unstoppable arc, because the mains can deliver hundreds of amps, or thousands, or whatever. Or you came up with the idea on your own, I don't know. Whatever.
It ain't gonna happen unless you have the VOLTAGE to make an arc jump that gap. And if you have the right fuse installed it has a current rating which -- wait for it -- exceeds the voltage available from the mains! What a concept!
Install a fuse with the proper ratings, and it will work.
Years ago, organizations such as UL and the National Fire Protection Association did studies on thousands of homes with fuses vs. circuits breakers. The bottom line was that the houses with fuse boxes were more likely to burn down in an electrical fire vs. the homes with circuit breaker panels.
Now that could be because the wiring was older in the fusebox homes, or the owners were so stupid that they were more likely to insert objects such as coins when they ran short of replacement fuses, etc.
But, for the most part, if the circumstances are not right, it is possible for even the standard Edison-base safety fuses to explode during a severe short or overload and melt their holders. If the arc cannot be contained, wires can melt and damage can extend beyond the fuse box.
Fortunately, this is rare... but it is also among the reasons that circuit breakers are considered the more modern and safe choice.
way down the page is a video clip of a fuse failing to meet BS1362 for fuses in BS1363 / IS401 (British, Irish, etc.) plugs. (Warning: There are now lots of (mainly Chinese-made) travel adaptors/adapters on sale worldwide to fit these sockets that don't even have a fuse!) Martin.
On Fri, 21 Mar 2008 16:32:25 -0700 (PDT) mike3 wrote: | On Mar 21, 2:02?pm, firstname.lastname@example.org wrote: |> On Fri, 21 Mar 2008 11:53:40 -0700 (PDT) mike3 wrote: |>
|> | So what happens if the current got so high the fuse was totally |> | destroyed? |> | Would the motor still have been protected or not? |>
|> The arc is still a high current channel. ?It is dissipating some of that |> power in the form of a voltage drop over the arc channel times the current |> flowing through the arc. |>
| | So then I was right, the fuse fails to do it's job of stopping the | current.
Indeed. It can happen. FYI, I have personally seen a circuit breaker fail to trip on solid short circuit. After the short itself opened a couple seconds later, the lights came back up (so I know it was not a case of tripping and arcing).