Fast blow fuses

[RandomGrinch]

Can 1A produce that much of an explosive power, or is the breakage expected from an external trauma?

Most likely external - the fuse will break when there is a fault current over 1A (plus a bit).
If that fault current was say, over 35A, with a glass fuse (unlikely), then arcing across the fuse may sustain that fault current.
In the case of the ceramic fuse, it would still protect the circuit up to a fault current of around 100A.

 

[DiyNutJob]

Ok, thanks all. Interweb says glass fuse can shatter when placed in high temperatures due to its low thermal stability.

 

[plugwash]

Because of this, in an over current situation, a glass fuse may be rated for a break capacity of 35A, a ceramic may be rated at 100A.

When I look at 20mm 1A fuses on Farnell, it seems most of the glass ones have a breaking capacity of 35A, and most of the ceramic ones have a breaking capacity of 1.5kA.

I can't seem to find a definitive guide to selecting fuse breaking capacities for appliances, but it seems to me that in an appliance powered by domestic mains, 1.5kA would be enough that in most cases the fuse would successfully interrupt a short circuit, and if it didn't then the current involved would be comfortablly above that needed to trip the next protective device in-line.

Whereas a fuse with a 35A breaking capacity seems like it would be about as useful as a chocolate teapot for protecting against short circuits in a domestic mains application.

 

[DiyNutJob]

Whereas a fuse with a 35A breaking capacity seems like it would be about as useful as a chocolate teapot for protecting against short circuits in a domestic mains application.

Where does 35A come from? A domestic appliance, at most, would have a 13A plug on it. On an appliance that hasn't a plug, the biggest fuse at the consumer unit is 30A. Are we scaring ourselves silly over nothing?

I think the most likely reason to not use glass fuse is because of potential heat damage. Even then, it's not clear at what temperature a glass fuse would break. Heat resistant cables fail at 90C. So the practical temperature risk to a fuse can't be high. If the rest of the appliance is already burning down, do we care the fuse will break?

 

[bernardgreen]

"Choosing the right fuse can be very confusing"

A slow blow fuse may be necessary if the device it is protecting has a high inrush current when power is applied but then protection of the components in the device may be compromised. Solid State switches need fast fuses if they are to be protected from damage by excessive current. One fuse may not provide the necessary protection.

A supply fuse ( slow blow ) and individual fuses for each of the output switching devices ( fast blow ) may be required ( cost permitting )

A fuse on the mains input may have to break well over 300 Amps, vapourised fuse wire inside an otherwise empty glass tube creates the ideal conditions for a plasma arc to form and be sustained until the glass eventually shatters or melts. Doesn't happen very often. The sand in a mains or other high rupture current fuse quenches the arc before it can damage the tube.

 

[DiyNutJob]

A fuse on the mains input may have to break well over 300 Amps

Without knowing anything about it, I am willing bet my lunch money that the speed of the fuse blow will reduce as the current increase. In other words, an extra slow blow 30A fuse would blow at 31A in, say, 1 second. The same fuse would blow in 0.01 second if the current is 300A. The risk of damage from over-current-than-spec is likely marginal if arcing is prevented.

 

[333rocky333]

When I look at 20mm 1A fuses on Farnell, it seems most of the glass ones have a breaking capacity of 35A, and most of the ceramic ones have a breaking capacity of 1.5kA.

I can't seem to find a definitive guide to selecting fuse breaking capacities for appliances, but it seems to me that in an appliance powered by domestic mains, 1.5kA would be enough that in most cases the fuse would successfully interrupt a short circuit, and if it didn't then the current involved would be comfortablly above that needed to trip the next protective device in-line.

Whereas a fuse with a 35A breaking capacity seems like it would be about as useful as a chocolate teapot for protecting against short circuits in a domestic mains application.

From memory using Rs components years ago for 12V and 24V panel fuses, barely any glass fuses were rated at more than about 50 volts, so i quess 35 amp was ample, I assume due to modern technology it seems a lot of the glass ones are now rated at 250volt, but there breaking capacity is still low, i suppose because there glass.

 

[AdrianUK]

Where does 35A come from? A domestic appliance, at most, would have a 13A plug on it. On an appliance that hasn't a plug, the biggest fuse at the consumer unit is 30A

I think you are missing something here. Just because the fuse in the plug is 13A or the MCB/fuse in the consumer unit is 30A does not mean that this is the biggest current that can flow. It simply means that this is the point at which the fuse/MCB will "wake up" and start to do its thing.

If we consider, for a moment, what happens if we apply a short circuit across the mains. The current which will flow varies in inverse proportion to the impedance at the point of the fault. We deliberately try to keep the impedance low since a side effect of higher impedance is that it will increase the system losses. The current that will flow is termed the Prospective Short Circuit Current (PSCC). This is one of the quantities that the circuit designer should have calculated to ensure that a protective device will clear a fault quickly.

One figure that is often banded around is the max Earth Fault Loop impedance. For a PME system this is 0.35 Ohms. For PME the EFLI is, to all intents & purposes, the same as the Phase to neutral impedance. So, working with that, the minimum current which would flow, for a short circuit at the DNO service head is 230/0.35 ~ 650A. We can allow for some reduction due to the internal house wiring to the point of the fault, but its very clear that the initial fault current is going to be very much greater than 13 or even 30A.

How long this fault current persists for & how much damage it will do is a function of the protective device that opens to disconnect this fault, but its not correct to believe that a 30A MCB will limit the fault current to 30A.

 

[EFLImpudence]

Without knowing anything about it, I am willing bet my lunch money that the speed of the fuse blow will reduce as the current increase. In other words, an extra slow blow 30A fuse would blow at 31A in, say, 1 second. The same fuse would blow in 0.01 second if the current is 300A. The risk of damage from over-current-than-spec is likely marginal if arcing is prevented.

You still don't appear to have read the link I posted (I am assuming it is correct).

You will see that the relevant current is ten times the rating of the fuse so the various speeds of the different types of fuses are quicker/slower for the same current.
At lower currents the fuses will take longer than those times in relation to the times stated.

No 30A fuse will blow at 31A.
That would mean it was alright at 30A then vapourises at 31A - it doesn't work like that.

Have a look at these graphs:
https://components101.com/articles/choosing-between-slow-blow-and-fast-blow-fuse-for-power-protection#:~:text=If the fuse is changed,the startup of the circuit.&text=If a circuit has a,needs to be immediately disconnected.

Not sure if fuses are exactly the same, but:
B type MCBs - e.g. 32A - will trip 'instantaneously' at 3 to 5 x 32A; will never trip up to 1.13 x 32A (36.16A) and at 1.45 x 32A (46.4A) will take an hour to trip - with currents in between relatively slower. 1.45 is the "fusing factor".

So, if the starting inrush current is more than that 3 to 5 a "slower" MCB will be required.
C type MCBs - e.g. 32A - will trip 'instantaneously' at 5 to 10 x 32A.
D type MCBs - e.g. 32A - will trip 'instantaneously' at 10 to 20 x 32A - not used in domestic premises.

The fusing factor of old-fashioned rewireable fuses is 2.

 

[AndyPRK]

The ones in plugs are filled with sand I think. So if they were glass, you wouldn't see fuse wire.

 

[DiyNutJob]

You will see that the relevant current is ten times the rating of the fuse

Are you saying a 13A plug fuse will blow at 13 x 10 = 130A? That makes no sense to me because the appliance would be on fire.

 

[JohnD]

Are you saying a 13A plug fuse will blow at 13 x 10 = 130A? That makes no sense to me because the appliance would be on fire.

not if it was quick.

It will blow on a short circuit, for example

or on an overload

 

[RandomGrinch]

Are you saying a 13A plug fuse will blow at 13 x 10 = 130A? That makes no sense to me because the appliance would be on fire.

As from some of your previous posts, you seem to believe research on the internet, over what some of us try to explain on here; this might be a very worthwhile video to watch! (some words about breaking capacity about 5 mins in)

 

[DiyNutJob]

Here's a fuse that didn't take 35A to blow. 35A would have blown it instantly. One of the commenters said the fuse would blow at the rated current when given time. I am of this sentiment. Going beyond the rated current will merely speed up the blow.

 

[333rocky333]

The video shows a 0.6 amp fuse, obviously it will blow at a lot less than 35amp,
A fuse with a 35 amp breaking capacity is the maximum current it can handle safely
Every fuse has a Bs standard and pretty sure that dictates that a fuse must handle its rated current, running at lower rated current may extend the life of a fuse for other reasons.