125V 1A fuse versus 250V 1A fuse - Page 1
Forum section: electronics / projects

author: Peacefrog
author_rank: Contributor
timestamp: May 02, 2022, 02:19:20 pm
content: At the weekend, I courted disaster by accidentally plugging an old sampler (Akai S1000) which needed 110V into my 220V transformer. Seemingly, all that was damaged is a blown fuse ahead of a transformer - after replacing it temporarily, it works fine.However, the fuse in question is seemingly a little non-standard - it's listed as a 'TSC 125V 1.00A' in the service manual, which I believe is a slow-blow fuse made by a Japanese manufacturer. Most slow-blow fuses, though, seem to be rated at 250V. In this case, I've found a 125V 1A medium-blow fuse at Digikey that should do the job, but I'm wondering - it's normally the amperage I'd care about with a fuse, but did the 125V rating save me in this case? Thanks!

author: alm
author_rank: Super Contributor
timestamp: May 02, 2022, 02:37:01 pm
content: Fuses respond to current, more specifically the I^2 R dissipation in the fuse that heats up the wire. The voltage rating is only to indicate it should not arc over when up to that voltage is applied across it. A fuse with a higher voltage rating but same current and response curve should be a safe replacement. Not the other way around, obviously.

author: Peacefrog
author_rank: Contributor
timestamp: May 02, 2022, 02:41:51 pm
content: Thanks! That's what I figured. However, the service manual specifically calls out a 125V fuse for 100V/110V regions and a 250V fuse for 220/240V. Was that a mistake (or a cost saving, not that I can see how?) on the manufacturer's part?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2022, 02:45:30 pm
content: A higher voltage rating is fine. Lower is not.The rating is what the fuse is guaranteed to function at, without undesirable behavior. What could happen at higher voltages, is the element continues to arc, creating a fire hazard either by not clearing the fault in time (does not meet specified clearing time) or by itself breaking and emitting sparks/arcing.Fuses are also rated with a maximum fault current, beyond which, similar things might be expected to happen: typical commercial equipment / residential fuses are rated a few kA, more than you'd expect to get from a residential circuit. Industrial fuses (say 400/480V circuits) are rated 100kA and up, which is enough voltage and current not only to maintain an arc, but propel a quite severe ball of plasma -- arc flash. The concussive force of which can blow open panels, and the heat can not just melt but literally vaporize whole components, connectors, etc.So, arc flash hazard of residential sorts of circuits is fairly low for the most part, and modest sorts of fuses are adequate to protect it.There's no downside from using a 250V fuse at 120V, but it may malfunction if a 125V fuse is used at 240V or more.As for time curve, clearing speed -- this mainly affects the low current range, and is usually selected based on inrush current. Which, transformers and older power supplies can draw fair amounts of, so slow-blow fuses were often used in such. The fault clearing time I think isn't much longer (but I haven't checked datasheets in a while, maybe it is significantly worse?), mind we're talking fault currents of 10-100x rated current, or more -- fuses typically clear such a fault in 10s of ms, i.e. a few cycles at mains frequency. Whereas melting at lower currents (2-10x rated) might take seconds, minutes even.Tim

author: Peacefrog
author_rank: Contributor
timestamp: May 02, 2022, 03:18:51 pm
content: My summary here is that the voltage of the 125V fuse didn’t save me per se when the 220V was applied, and it could have been actively dangerous as the given rating was exceeded.I’ve replaced with a 250V 1A slow blow I had to hand, so I’ll assume this is good to go. Thanks all!

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: May 04, 2022, 02:04:25 am
content: TSC series fuses are slow blow.Jeff

author: james_s
author_rank: Super Contributor
timestamp: May 04, 2022, 02:56:10 am
content: Quote from: Peacefrog on May 02, 2022, 03:18:51 pmIt's unlikely that a 125V fuse will have problems on 240V but it's not guaranteed not to have problems. Worst case situation is a direct short that causes the fuse to blow violently.