Electronics > Beginners
power input for small transformer, good practice
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Cerebus:
What people are forgetting here is that fuses aren't there to protect the equipment, they are there to protect the local supply wiring between your equipment and the upstream fusing. So when choosing a fuse's breaking capacity you don't consider the load, you consider the supply. If you're lucky enough to live in a civilised country that mandates high rupture current fuses in all mains plugs then you can happily stick glass fuses in your equipment. If you don't, or your equipment might be used in countries that don't, then you would be wise to specify high breaking current fuses (i.e. not glass ones) in your equipment.

And whoever it was that said HRC fuses are expensive is talking out of their hat: 1A 250V 20mm x 5mm fuses, RS UK pricing - cheapest glass fuse £0.111 each, breaking capacity 35A, cheapest HRC fuse £0.142 each, breaking capacity 1.5kA. One third more expensive, 43 times the breaking capacity.
Yansi:
Fuses protect not upstream, but downstream wiring.

But it comes down to the definition of direction: Downstream means from the supply, to the load. So, your breaker panel protects the downstream wiring to the wall socket. And your small glass fuse does in fact protect against fire of the load - i.e. your small 10VA transformer, that could otherwise catch fire when severely overloaded.

The glass fuse in your appliance have nothing in common of protecting the wiring from the socket to it.  That is the job of the breaker panel.*

*Some freaked out countries even put another fuse in their crazy looking plugs, to compensate for the simple and dangerous design of their plugs.  ^-^
Cerebus:

--- Quote from: Yansi on July 01, 2018, 11:41:02 am ---Fuses protect not upstream, but downstream wiring.

But it comes down to the definition of direction: Downstream means from the supply, to the load. So, your breaker panel protects the downstream wiring to the wall socket. And your small glass fuse does in fact protect against fire of the load - i.e. your small 10VA transformer, that could otherwise catch fire when severely overloaded.

The glass fuse in your appliance have nothing in common of protecting the wiring from the socket to it.  That is the job of the breaker panel.*

*Some freaked out countries even put another fuse in their crazy looking plugs, to compensate for the simple and dangerous design of their plugs.  ^-^

--- End quote ---

It's a circuit, not a river. If the fuse at the appliance end opens, then current stops flowing without the inconvenience of a branch circuit breaker or fuse opening and taking everything on that branch down with it.

Upstream/downstream is generally defined from where you're standing, not from the head end of the river/supply, otherwise everything is downstream. But, arguing about definitions of upstream/downstream is pointless when the issue is that one should consider the supply capacity not the load in determining fuse breaking capacity.

Oh, and breaker panel fusing usually 15-30A in 220V countries, typical IEC cable rating 6-10A. So the breaker will open when the cable from the wall is overstretched will it?
exe:

--- Quote from: Cerebus on July 01, 2018, 11:28:48 am ---cheapest HRC fuse £0.142 each, breaking capacity 1.5kA. One third more expensive, 43 times the breaking capacity.

--- End quote ---

Concerning the price, it seems I was wrong. I took the price for fuses in DMM, but they have higher voltage rating. Indeed, ceramic fuses are not much more expensive than glass ones. I  didn't knew they are also HRC. Do they have sand inside?
Cerebus:

--- Quote from: exe on July 01, 2018, 02:02:02 pm ---
--- Quote from: Cerebus on July 01, 2018, 11:28:48 am ---cheapest HRC fuse £0.142 each, breaking capacity 1.5kA. One third more expensive, 43 times the breaking capacity.

--- End quote ---

Concerning the price, it seems I was wrong. I took the price for fuses in DMM, but they have higher voltage rating. Indeed, ceramic fuses are not much more expensive than glass ones. I  didn't knew they are also HRC. Do they have sand inside?

--- End quote ---

All the ones I've seen cracked open do have silica inside. Yes, good meter fuses are chuffing expensive.
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