10A and 2A fuses in parallel=12A?

[Zippy]

I thought blowout coils were mainly used to mitigate premature degradation of the contacts...not necessarily to prevent a constant arc situation?

That is certainly part of the concern.

The systems that I was running had a large component of inductive load, so, as I learned at the time, arc interruption was a real necessity. Uninterrupted, the sustaining arc would melt contacts - and sometimes did anyhow.

I had one incident where 3" bus bars were melted through even though the breakers tripped. How did that happen? I was paralleling a diesel generator with the turbines (3-phase, 450VAC) and the automatic breaker was slow to shut. Of course, once the control signal to "Shut" was locked in, there was no way to cancel the operation. All I could do was watch and think "This is going to be bad." It was - much melted switchgear, but no casualties.

[tictac]

Why would you add the expense of two fuseholders and two fuses when you can simply but a 12A fuse, put it in, and be done with it?

The only overcurrent devices I've even seen are designed to operate in series with the current flow; I can't see how a repeatable, predictable, protection circuit could be devised using parallel fusing....



TT

[Structo]

Thanks for the information guys.
I don't retain knowledge as well as I once did....

I have heard of glass/ element slo blo fuses arcing after they blow, leaving a metal deposit that will still conduct.
True?

Not sure if this is a valid concern for a guitar amp voltages but the story is out there.

I have read that the white ceramic slo blo fuses are a better choice since they won't arc over.

My other question is on my 100w ODS (4x 6L6GC) amp, the CE chassis is labeled as Slo Blo 5A.

Is that too large of a capacity for this amp.
Seems I recall that a 3A fuse was a safer/ adequate fuse for this amp?

Also I read somewhere before that Slo Blo fuses can wear out due to the current surge at startup.
In other words, after so many switch flips, the fuse can blow without a fault or short circuit type situation.
Is that true?

[Reeltarded]

Seems 3.5-4A fuse would be safer than a 5A.

I know that fuses just pop on startup some of the time. Not often. Maybe it's cycling. Everything wears out. It's a lightless bulb anyhow, at least visible light.

[Tillydog]

Also I read somewhere before that Slo Blo fuses can wear out due to the current surge at startup.
In other words, after so many switch flips, the fuse can blow without a fault or short circuit type situation.
Is that true?

Yup, they all can, not just slo-blo. It's called fuse fatigue, and happens if the fuse sees surges of current (like at switch on) - the fuse wire warms up, expands then cools down and contracts again. Over time this mechanical movement cracks the fuse wire somewhere.

This is worst where the selected fuse rating is a bit too low for the application.

[gingertube]

Why would anyone want a 12A fuse.

A fuse is put into a circuit where it can sacrafice itself to protect everything else, IN THE PRESENCE OF A FAULT CURRENT. Its exact rating is not too important. If a 10 Amp is not high enough rating then use a 16 or 20 Amp. They will give at least as good a protection as a 12 Amp fuse would.

Of-course I've seen it done badly too, one circuit with fuse of too high a value and the MOSFET in the circuit used to blow to protect the fuse.

Cheers,
Ian

[Leo_Gnardo]

Why would anyone want a 12A fuse.

Got 12A line fuses in my Carver 1200W sand-amps.

Remember, the (1960's) definition of a transistor is a device that blows up to protect fuses.

What is our OP up to with the need for 12A fuses? Inquiring minds want to know...