Fuse maximum voltage versus mains voltage - Page 1
Forum section: electronics / beginners

author: pope
author_rank: Frequent Contributor
timestamp: March 10, 2025, 11:23:40 am
content: AFAIK, in Europe the mains voltage regulation is 230VAC +/-10%. So, theoretically you can have mains as high as 253VAC.However, most slo-blo fuses I'm aware of have an upper limit of 250VAC.Does it compute?

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2025, 11:32:52 am
content: It will be fine. 3V extra is just over 1% of 250V. The rating of 250V also dates back to a time when the mains in some areas was 250VAC and could even be a bit higher than that.

author: Gyro
author_rank: Super Contributor
timestamp: March 10, 2025, 02:34:41 pm
content: It's nice to see someone actually considering worst case tolerances. As Zero says, in this case you are fine with a nominal 250V fuse. Things get more tricky when considering DC ratings where there are no zero crossing to extinguish an arc.On a related subject, it's worth considering worst case interrupt current rating for mains fuses. If you're talking a standard 20mm fuse, then a glass fuse is typically rated at around 35A maximum interrupt rating. I have seen equipment where an internal mains short has caused the inside of the glass to become mirror coated with evaporated metal or outright shattered. Far better to use ceramic fuses for mains, where a 20mm fuse is rated to interrupt typically 1500A. Obviously this isn't going to make a difference for something like a transformer shorted turn but for a direct short, it becomes important.

author: pope
author_rank: Frequent Contributor
timestamp: March 10, 2025, 07:44:33 pm
content: Thank you gents.Gyro, are you referring to the ceramic fuses that you can find in the UK kettle leads or to the actual slo-blo fuses that are in line with the Live and the primary of a transformer?If the latter, I believe almost all manufacturers use the glass L (low breaking capacity) type fuses. Do you recommend some H ceramic fuses instead?

author: Gyro
author_rank: Super Contributor
timestamp: March 10, 2025, 10:17:01 pm
content: I'm referring to the fuse that you might find on the primary side inside the equipment. UK plug fuses are 25mm (actually 1"), larger diameter and have a breaking capacity of 6kA. Yes, a lot of manufacturers tend to use glass LBC fuses and work on the basis that a direct internal short will never occur, thisisless likely with something like a traditional mains transformer - you're much more likely to find a ceramic fuse on the input of an SMPS in things like main stream consumer equipment though.Personally, I think it's good practice on the primary side of any mains connected equipment - especially in countries where there is no plug fuse between the equipment and the main breaker. It's a lot of extra protection for little cost.

author: pope
author_rank: Frequent Contributor
timestamp: March 10, 2025, 10:22:10 pm
content: I see. Thank you for clarifying.

author: golden_labels
author_rank: Super Contributor
timestamp: March 10, 2025, 11:29:35 pm
content: Not only 253 V. This is RMS voltage. Peak instantenous voltage exceeds 356 V, and may be higher due to inductances! But don’t worry: this isn’t a problem. For multiple reasons.If you check in the datasheets, the 250 V rating is used as a condition for breaking current specs. This current is hundreds to thousands amps range. What it says, is that the fuse is not guaranteed to break the full advertised 1.5 kA, if voltage is 253 VAC. It’s a matter of breaking capacity derating, not fuse not being able to withstand the voltage.In virtually all cases you’ll not reach the limits. But even in a direct short scenario, wiring in the walls limits the current and also drops the voltage. But if this happens: this is why you have circuit breaker on the entire installation. Should the fuse fail, the breaker cuts off power half a second later.Finally, fuses are inherently probabilistic devices. They give best effort protection, but not 100% coverage. So you we may as well throw your scenario into the basket with unfortunate fails.You may see how that voltage is used in LittleFuse’sFuseologydocument, page 5 (archived).

author: calzap
author_rank: Frequent Contributor
timestamp: March 11, 2025, 12:57:07 am
content: golden_labels thanks for the useful link to Littelfuse fuseology docs. The section on voltage rating is:“Voltage RatingThe voltage rating indicates that the fuse can be relied upon to safely interrupt its rated short circuit current in a circuit where the voltage is equal to or less than its rated voltage.”I assume unsafe outcomes would be the fuse exploding or internal arcing after the element melts. Are AC voltage ratings always based on 50-60 Hz? Higher frequencies can jump greater distances at any voltage.Mike

author: CaptDon
author_rank: Super Contributor
timestamp: March 11, 2025, 01:15:37 pm
content: Is there and equivalent D.C. derating for 125VAC and 250VAC fuses? I seem to recall the 125VAC being derated to 32VDC and I also seem to recall car fuses had 32V stamped on them beside the ampacity. The old car fuses I had looked like a butterfly bandaid inside. Very thick with a short thin section. I was told this was to help survive vibration. We had a lot of trouble with D.C. sustained arc destruction on locomotives. The low voltage system was 74VDC nominal and the breakers carried a 75VDC rating stamped right on them.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 11, 2025, 03:00:39 pm
content: Quote from: pope on March 10, 2025, 11:23:40 amIt's very simple: just consider rated voltages 230VAC and 250 VAC. 230 < 250.(Fuse doesn't stop working at 251 VAC).