# 3.15A and 4A fuses - can I swap?

I have an old oscilloscope which doesn't work. I tried swapping the 3.15A/250VAC fuse with a 4A/250VAC fuse. The fuse immediately blew. My questions are: Did the fuse blow because it was the wrong fuse? Or did it blow because there's likely something else wrong with the oscilloscope? Must the fuse be exactly as specified? Thank you.

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• The amperage rating of a fuse is the maximum. I would say something is wrong with the scope. (Physics lab instrumentation repair question ;) ) – Bill N Oct 14 '15 at 20:30
• Thank you @Qmechanic and Bill N. Yes, it looks like a problem with a blown capacitor and possibly some shorting. There may have been a previous attempt to fix it. – Amygdala Oct 14 '15 at 21:02
• The rating on the fuse is the current at which the fuse will break. Therefore, if you put in a 4A fuse and it blew, then there was at least 4A flowing through that fuse. Fuses are always (supposed to be) low resistance, so if you had put in a 3A fuse, the current flow would have been the same and the 3A fuse would have blown too. Something else is wrong with the scope. – DanielSank Oct 22 '15 at 4:57
• Many thanks, @DanielSank. I think there might be a short somewhere. Do you know what would happen if there was a short to ground in the circuit? Would that cause the fuse to blow? – Amygdala Oct 22 '15 at 9:22

First, replacing a fuse with one of a higher current rating is a bad idea. In this case, assuming the two fuses were the same type (fast or slow blow), then there is definitely something wrong with your scope. Since the scope is not meant to exceed 3.15 A, but actually exceeded 4 A, there is clearly something wrong. By making it take more current before the power was shut off, you may have even damaged it further.

This is all assuming the two fuses were of the same type. If the original was a slow blow, and the new a fast blow, then what you observed could still happen in normal operation. Look at the old and new fuses carefully. This isn't universal, but usually a fast blow fuse is just the melting link suspended between the ends. A slow blow usually has the melting wire wrapped around a ceramic rod.

If the original was fast blow and you replaced it with a slow blow, then there is really something wrong with the scope.

• I thought it was universal that 'slow-blow' fuses could be identified because they are marked with a T either before or after the current rating, on one of the end caps - e.g. T1A or 3.15A T? – nekomatic Nov 13 '15 at 0:00
• @nekomatic The T designation for time delay is very common but not universal and may not be standard in countries where the latin characters are not used. – KalleMP Mar 25 at 16:39

It is also possible that you have different types of fuse between the 3.15 A and the 4A. Fuse comes in two types: Slow-blow and regular. Slow blow fuses are used when there can be a short-term current surge in the system, like when you power on your oscilloscope. If the old 3.15A is a slow-blow and you put in a 4A regular, it will blow right when you turn it on, which is what you are seeing. I'd suggest looking at the 3.15A to see if it's a slow-blow kind or if it's a regular. Then you need to match the 4A to it. I'd bet that it's a slow-blow.

• Hello Lost in Knowledge, yes, I used a time-delay 4A fuse. One difference is that the 4A fuse is glass, the 3.15A is ceramic. Would that make a difference? – Amygdala Oct 14 '15 at 21:03
• Theoretically, there is a difference between a glass fuse and a ceramic fuse. A ceramic fuse typically withstands more abuse when it comes to surge current. Having said that, I have yet to see substantial differences in practice. In your case, your best bet is to get the same fuse as the original one and try it out with your scope. – Lost in Knowledge Oct 14 '15 at 21:48
• In truth there are 3 types of fuse. Ultra-fast, Fast (the most common) and Slow. Ultra-fast are recommended for sensitive circuits (like CMOS devices) and when you`re designing you should pick a fuse with I2t lesser than the minimum I2t of your circuit. Fast are the ordinary ones, recommended for general use. Slow ones can withstand for a few seconds much larger currents than its specification, so then are recommended for inductive circuits, like motors. When picking a fuse, you should not look at only its nominal current, but the right type is equally important and sometimes maybe even more. – Pedro Quadros Oct 22 '15 at 11:26

By changing the fuse to a higher current, you are changing the power which the oscilloscope can draw from the wall. The power is $$P = IV$$ for current $I$, voltage $V$. So by moving from a 3A to a 4A fuse you are increasing the power available inside the scope by about 30%.

You're unlikely to start a fire by switching to a 4A fuse, but I wouldn't go any higher without debugging the actual problem.

• Ok, thanks Rob. So in your opinion, is it likely that the fuse would still blow even if I used a 3.15A? – Amygdala Oct 14 '15 at 17:19
• If the 4A fuse blows, then the device is drawing more than 4A and the 3A fuse will also blow. – rob Oct 14 '15 at 17:24

Just a comment.

If this is a T3 fuse, (small, glass or ceramic tube, measuring 20mm length x 5mm diameter) you are likely mis-reading the etched info on the silver end-cap.

Are reading is T3.15A250V, yes? The period/dot may be a separator that indicates you have a T3 fuse, with a 15A, 250V rating. (I'm looking at a ceramic one I just removed from a power supply right now as I type this ... it's definitely a 15A fuse ... and is marked as such on the fuse holder in the power supply as well.) Amps did not used to be marked with periods, per the old standard,

So that could explain why you are instantly blowing a 4A fuse. You probably need to draw somewhere near 5-10 Amps through that part of your device (likely the electron gun of the CRT on the oscilloscope). Beware though - we haven't eliminated the possibility of a fault. Something blew the original fuse ... be physical weakening due to age / impact or a true fault.

Long story short, read your manual (or Google for it) and verify what you need ... especially if you picked up the device second-hand. You never know what the previous owner installed correctly or incorrectly. Trust, but verify. ;>

PS: T shapes come in multiple sizes (eg: 1/4"d x 1-1/4"l) and in both glass (with metal link) and ceramic. In something like an oscilloscope (or anything with a CRT) I wouldn't be surprised to see a ceramic fuse to prevent the possibility of arcing in a very high current-flow failure.

Edited to correct something ... there are 3.15 Amp fuses are marked in A and not mAh ... something I had not previously seen. Thanks to Sam, for pointing that out.

• Yeah, verify. A 15A fuse is robust enough for 1.6kW in the USA and 3.2kW in 220V regions. This does not sound like a suitable fuse for an oscilloscope. – KalleMP Mar 25 at 16:44
• Welcome John, but -1 from me; 3.15A 20x5mm fuses do exist, marked with 3.15A. I'm not disputing that you believe you have a 15A fuse ((I'm not there to see it) but as an example, here is a page from RS Components of a 3.15A fuse, and you can see the end cap making in the photo is 3.15A. (Just FYI, don't include your signature in an answer or question, it's against site rules. Thanks!) – SamGibson Mar 25 at 17:21
• This question is from 2015. – mkeith Mar 26 at 2:53