# What happens when you blow a fuse really hard?

Ambiguous is the definition of 'hard'. What is 'hard' on a fuse is voltage that a fuse must interrupt. All fuses have a maximum voltage. If that voltage is too high, then arcing problems, discussed by John Rye, means that fuse may not protect from fire.
For example two cylinder type fuses appear same. The 3AG fuse once used in autos is only rated to interrupt 32 volts. A standard line fuse for appliance operation (same physical dimensions) must be rated to interrupt 250 volts. Using that old style 32 volt automobile fuse in an appliance may not properly interrupt AC mains voltage for reasons cited by John Rye. IOW interrupting 120 VAC with a 32 volt automotive fuse could be 'too hard' on that fuse.
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I would add a couple of more points.
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.
daestrom
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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.
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| |>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. |> | | 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.
I'd still feel a bit safer if I had protection on the main entrance by both (with appropriate or excessive interruption rating, of course). The question I've pondered is, if it is better to have the main breaker first followed by the fuses, or have the fuses first followed by the breaker. I suspect the former might be a bit safer than the latter as it gives you an extra disconnect to completely de-energize the box when replacing a fuse.
| 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.
Don't people get shock out of that?
I do know my dad has inserted the wrong size fuse, before.
| 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.
the only failure I've ever experienced of an OCPD is a breaker, not a fuse. But my experiences are not statistically significant.
FYI, I do make sure all the breakers where I live right now (my dad's house) get flipped off and back on at least once a year. The power goes out often enough for that to be no additional inconvenience.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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Breakers are not necessarily intrinsically safer than fuses... This brings to mind a somewhat infamous example of slipshod manufacturing and safety testing.
There was a whole series of one brand of breakers (Federal Pacific, I think - Google to check it out) that, more often than not, simply failed to open during an overload...
Here is one link detailing a similar problem: http://www.inspect-ny.com/fpe/fpepanel.htm
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Beachcomber wrote:

Not just slipshod testing. FPE sent fraudulent test information to UL. This was discovered after FPE was sold to Reliance. Reliance informed UL and most of the FPE line was delisted. Reliance sued the company that sold FPE and got a lot of money to cover liability.
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bud--

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daestrom wrote:

The issue, for the questions of the OP, is not the current or voltage rating of the fuse. It is the fuse rating for the available fault current of the supply, as daestrom wrote.
Fuses (and circuit breakers) meant for use in power circuits [US] will have a rating for available fault current. Fuses are readily available [US] that can be used on circuits with an available fault current of 200,000A. FRN fuses from Buss are one type (time delay, relatively inexpensive, widely available). As I wrote previously, these fuses will open far before the first current peak approaches 200,000A. Fuses that open that fast are called "current limiting". Circuits are designed so the the current peak that can get through the fuse is safe for what is connected downstream.
If you have a point in the power system that has an available fault current of 50,000A and install a fuse rated for 5,000A, with a heavy fault the fuse and a considerable amount of surrounding equipment may disappear. That is not a failure of the fuse. It is a failure of the design or maintenance that allowed a fuse with inappropriate ratings to be used.
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bud--

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daestrom wrote:

[snip]
Now, try to teach this to a utility that serves its customers from a networked secondary system or with primary service. Back when a customer originally requested service, the fault current available on a 480V service might have been around 50 kA. Their service equipment was sized to interrupt this level of fault current.
As the years go by and more capacity is added, the fault current available can go up to 100 or 150 kA. Does anyone bother to notify the customers? Not as far as I've seen.
The problem is either caught when someone does a major remodel, requiring the service equipment design to be revisited, or when the service equipment (and sometimes the building) fails catastrophically.
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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| The problem is either caught when someone does a major remodel, | requiring the service equipment design to be revisited, or when the | service equipment (and sometimes the building) fails catastrophically.
And the lawyers get rich.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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Paul Hovnanian P.E. wrote:

I suspect this is a much bigger problem than people realize. Even some residential areas have increased as more & bigger homes are added. Your house might be on the same pole pig, but the primary side could have much more available SCC than it used to. Even if the utility informed customers, how many companies or homeowners are going to replace entire service panels/switchboards? As you said, it will only get done when it is upgraded or redesigned for some other reason or when it fails.
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Benjamin D Miller, PE
www.bmillerengineering.com
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| Paul Hovnanian P.E. wrote: | |> |> Now, try to teach this to a utility that serves its customers from a |> networked secondary system or with primary service. Back when a |> customer originally requested service, the fault current available on |> a 480V service might have been around 50 kA. Their service equipment |> was sized to interrupt this level of fault current. |> |> As the years go by and more capacity is added, the fault current |> available can go up to 100 or 150 kA. Does anyone bother to notify the |> customers? Not as far as I've seen. |> |> The problem is either caught when someone does a major remodel, |> requiring the service equipment design to be revisited, or when the |> service equipment (and sometimes the building) fails catastrophically. | | I suspect this is a much bigger problem than people realize. Even some | residential areas have increased as more & bigger homes are added. Your | house might be on the same pole pig, but the primary side could have much | more available SCC than it used to. Even if the utility informed customers, | how many companies or homeowners are going to replace entire service | panels/switchboards? As you said, it will only get done when it is upgraded | or redesigned for some other reason or when it fails.
And whose responsibility is it to pay for that upgrade "out of the blue" when the utility decides to save their own money by increasing the size of the transformer somewhere along the line, rather than splitting up the services among smaller transformers? Or, another way, when it turns out a home burns down because the OCPD failed due to the now higher available fault current, but was correctly sized when installed, and this is due to the fact the owner could not afford the upgrade right away even though he was notified by the utility. What if someone dies in that event?
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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Ben Miller wrote:

Back when I was a distribution engineer, my standard practice was to calculate the secondary fault current available based on a distribution transformer located just outside our largest standard substation. Even if the customer was 20 miles down a farm road. One never knows if the next big substation will go in just across the street from him.
But this wasn't standard practice across the company. And for larger commercial/industrial customers, they need more information than just a maximum fault current level. Any significant change to the system needs to be looked at. But when I'd call a customer about a system upgrade (a 12 kV to 34.5 kV cutover for example) and suggest they forward it on to an engineer, I was usually met with a blank stare. Even some primary service customers, who are supposed to have "qualified" personnel operate and maintain their systems had no one on call. Some of our linemen made good money on the side, as they were the only people in town with the tools and know-how to do H.V. maintenance.

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Paul Hovnanian snipped-for-privacy@hovnanian.com
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One of those MBA type people, had they known what you did, might have been able to accuse you of not saving the company as much money as you could. Never mind the liabilities you avoided ... or even the higher future costs.
As for H.V. maintenance ... I watched a substation near here being upgraded for a couple months period along my daily drive route to work. It appears all the work was contracted out. Only a couple times did I see AEP trucks there. The rest of the time was one or more of about 3 different private contracting companies. I suppose the industrial customers could have used them as well.
I sure don't want the hassles of having 12 kV to 34.5 kV coming in to my house :-) I'll stick with the "under 600 volt" service as long as the available fault current stays under 22kA (the rating on my panel main). I'm on a 100 kVA pad with 2 or 3 other houses (it's how to tell how the two pads in the neighborhood are exactly devided up without calling in the guys that come spray paint red lines all over the yards). It would have to be under 2% to exceed my ratings, assuming infinite distribution fault current and superconductor underground service. I do get noticeable light dimming all over the house when my A/C compressor starts, lately, so I've probably got plenty of impedance.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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Hello Paul

If these figures are correct there appears to be a big difference between Uk & US practice. The vast majority of domestic/small commercial supplies in the UK will be 230V phase to neutral from a 3 phase transformer with a maximum size of 1 MVA, and an impedance of about 4.5%. This gives a maximum potential short-circuit current of 33 kA, which will be reduced by HV supply impedance, and the cable between the transformer and the customer.
At the end of the utility cable supplying the customer will be a fuse box containing current limiting fuses rated for this short-circuit current. This does not rule out the possibility that the customers switchgear is inadequate, but it does provide a back stop in that if the customers's switchgear fails the utilities fuses will usually clear the fault.
John
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John Rye
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John Rye wrote:

Is it normal UK practice to install fuses at the source end of the utility cable? "Current limiting" fuses should also reduce the available fault current downstream.
A replacement service downtown [US]was a mere 800A. Right the other side of the basement wall was a utility transformer vault for part of a downtown block. The utility said the available fault current was 200,000A. All the service cable connections in the vault were "cable limiters" - a fuse crimped to the wire on one end and a lug bolted to the vault bussbars on the other end. I have never seen information on the cable limiters - probably "current limiting".
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bud--

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bud-- wrote:

Just think of them as big fuses designed for short-circuit protection only. They are generally used on parallel sets of lines, so a fault on one line will open its limiter, removing that line from the system. This prevents more catastrophic damage. The service remains on, but that phase is then running with one less conductor.
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Benjamin D Miller, PE
www.bmillerengineering.com
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Ben Miller wrote:

But for a parallel run the wire is still backfed from the load end (unless there are cable limiters on both ends). The utility is protected but I would think the customer would still have a failure.
And opening one wire puts the load on the other parallel runs (but what better option).
Not obvious to me why there is more advantage on parallel runs than single unless because the wires are usually larger.
----------------- The building where the service was replaced had burned down so only the basement and 1st floor slab remained. That was used for parking. The original 208/120 service in the back corner remained - 6 parallel runs. Someone had the bright idea of storing snow melting salt on top of the switchgear. In the resulting burndown some of the wires burned back into the service conduits. At least a couple of the wires welded to the conduit - the utility broke a come-along trying to pull one. Some of the limiters didn't blow - must have burned free. But the utility was protected.
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bud--

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bud-- wrote:

Yes, you need them at both ends in order to isolate the cable.

Exactly. Of course, without some type of fault detector, nobody is aware that a fault occured or that the service is not running on the full conductor ampacity, so if it is heavily loaded it is likely to fail over the long term anyhow.

They protect the cable in either case. However, with parallel conductors they add the function of isolating the one faulted conductor, and allowing continued operation. I have never seen them on a single service conductor.
Here is the Bussman info: http://www.bussmann.com/pdf/1042.pdf
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Benjamin D Miller, PE
www.bmillerengineering.com
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bud-- wrote:

Larger? The individual conductors of a parallel run would be smaller than a single conductor.
That is one reason to use cable limiters. For a single conductor (per phase), one can protect them with a transformer primary side fuse. On the other hand, each conductor of a paralleled group is significantly smaller than the total service rating.
In the event than one of the group opens, the remaining conductors will carry more load current, even though the upstream protection will 'see' nothing unusual. Once the load current exceeds the cable limiters rating, it will open. As the load current shifts to the remaining cables, their limiters will open as well.
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Paul Hovnanian snipped-for-privacy@hovnanian.com
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Paul Hovnanian P.E. wrote:

I did not clearly state what I mean. The wire for parallel runs is likely to be larger than a different run that was just single. The idea was parallel runs are near the max size wire that is economical, but a single run is not because the loads vary from small to large.

If a transformer feeds only one set of wires, the transformer primary fuse may provide short circuit protection. Utility transformers often feed multiple services.
The one place I saw cable limiters was in a downtown utility transformer vault. The 800A service I was working on was almost trivial compared to the transformer rating. But if there was a fault on my wires why couldn't it propagate into the vault and cause some real excitement? Why wouldn't cable limiters (utility end only) be useful on my single set of conductors?

Ben, and the Buss data sheets he provided [thanks], say cable limiters are for short circuit protection. The curves on the data sheets do not go out to a long enough time to determine what kind of overload protection the cable limiters provide.
(The data sheets show those limiters are current limiting.)
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bud--