ANL fuses are labelled wrong

[sparklehunt]

I know class T is preferred for it's 10kA+ AIC blah blah...

but I was just curious about ANL fuses and looking at the Eaton Bussman (reputable brand) datasheet for their fuses. See anything weird?

The labelled nominal fuse Amperage doesn't trip at that nominal!

 

[Hedges]

Not in only 100 seconds, maybe eventually.
You can also get fuses that are much faster.

The Square-D QO thermal-magnetic breakers I use trip in about 10 minutes at 150% of load.

In most cases, OCP should be selected 125% of continuous load.

 

[Bud Martin]

I know class T is preferred for it's 10kA+ AIC blah blah...

but I was just curious about ANL fuses and looking at the Eaton Bussman (reputable brand) datasheet for their fuses. See anything weird?

The labelled nominal fuse Amperage doesn't trip at that nominal!

View attachment 112745

Fuse or breaker rating as printed is for the holding current at 25c without tripping so it should not trip at the printed rate.
Fuse and breaker have trip curve. The fuse/breaker are selected for the application you need, I.E. if the load has high in-rush current you will not choose fast blow fuse for it.

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solution-center/bus-ele-tech-lib-time-current-curves-how-to-read.pdf

 

[sparklehunt]

I'm understanding that the fuse rating is for not tripping but for normal operation, and one must exceed it by a safety margin to blow. Then why do normally we add that 1.25 or 1/0.8 nuisance factor if the fuses account for that already?

 

[Hedges]

The fuses typically have a minimum trip current of 100% of their rating, at 25 degrees C.
Manufacturing tolerances may mean maximum trip current is 125% or even a bit higher, depending on type/quality.
Time to trip at that current could even be hours.

Adding 125% to continuous operating current of load also accounts for elevated ambient temperature, and heating of wire and fuse inside box.

Because magnetic/hydraulic breakers don't have a thermal component, I've seen them suggested for 100% of rating (in the case of one where minimum trip was 105% of rating.) Others I see minimum trip 100%, suggesting you might get away with operating at 95% of rating.

Hmm, thermal devices are affected by current squared, so non-sinusoidal current deliver less power but operate closer to tripping. Wonder if magnetic trip is linear with current?

 

[sparklehunt]

The fuses typically have a minimum trip current of 100% of their rating, at 25 degrees C.
Manufacturing tolerances may mean maximum trip current is 125% or even a bit higher, depending on type/quality.
Time to trip at that current could even be hours.

This seems like a long time, but I need some experience with how fast wire get hot get when used at different multiples of their rating.

Adding 125% to continuous operating current of load also accounts for elevated ambient temperature, and heating of wire and fuse inside box.

Because magnetic/hydraulic breakers don't have a thermal component, I've seen them suggested for 100% of rating (in the case of one where minimum trip was 105% of rating.) Others I see minimum trip 100%, suggesting you might get away with operating at 95% of rating.

Hmm, thermal devices are affected by current squared, so non-sinusoidal current deliver less power but operate closer to tripping. Wonder if magnetic trip is linear with current?

Non-sinusoidal, as in DC?

 

[Hedges]

Non-sinusoidal as in PWM, or peaks of current drawn from AC line by VFD or SMPS.
DC is simple, if a constant. And sine wave, RMS value same power as DC of the same number. "RMS" means Root Mean Squared, so the Squared term weights higher values more heavily. Distorted sine waves typically cause power power dissipation in wires and fuses for the same power delivered.

Wires take a long time to heat significantly, except at high multiples of rated current.
I've tested breakers, e.g. 20A rating with 30A worth of space heaters. That took 10 or 20 minutes. Depends on ambient temperature and air circulation, as well.
Wire insulation does get cooked eventually with such overload. But the rules of 15A breaker for 14 awg, 20A for 12 awg, 30A for 10 awg, which are overly conservative, probably prevent that. Good for outlets consumers can overload, as opposed to dedicated circuits.