Types of equipment fuses-Please educate me - Page 1
Forum section: electronics / projects

author: djsb
author_rank: Super Contributor
timestamp: October 13, 2012, 05:21:54 pm
content: Ok, I'm confused as to the different types of equipment fuses used in power supplies etc. What are the different types of fuses and when should they be used.This topic has sprung from a scenario I hope to never encounter at work, power supplies constantly blowing a fuse when turned on.Thanks.David.

author: FenderBender
author_rank: Super Contributor
timestamp: October 13, 2012, 07:56:03 pm
content: There are lots of different characteristics of fuses. Here are two important ones:Reaction time:Fast Acting: These are very sensitive to over-current conditions. If your circuit exceeds the rated current, even for a very short duration of time, the fuse will blow. These may be used when you have very sensitive electronics. If you put a FA fuse in a place where a Slow Blow was intended, the FA will almost 100% blow.Slow Blow: These are less sensitive to over-current conditions. They are designed to allow current greater than the rated current pass for a short period of time. Don't be fooled. These are not "worse" than fast acting fuses. They just have a different purpose. Slow Blow fuses are used when you have a circuit that might have a high inrush current, such as a large transformer or inductor, or if you have a circuit that draws transient spikes of current. The fuse will not blow. However, if too much current passes for a long time interval, the fuse will blow.Medium Acting: These are some where in between slow and fast. Best to look at the datasheet when looking for these.Materials:Glass: A glass body fuse is the cheapest and most common type in electronics. You can visibly see if you have blown a fuse because of the transparent body. However, glass fuses have a fairly low interrupt rating which means that you should really only use a glass fuse in a circuit that doesn't draw a huge amount of current (<10A is a good rule of thumb). They also can be less safe in some conditions. Since the only thing in a glass fuse is the current sensitive wire and air, arcing can occur under fault conditions which poses a whole bunch of dangers. Glass fuses also can explode if there is too much current, which can destroy components in its surroundings. Look for a # followed by AG. The number describes the size and the AG means it's a tubular glass fuse.Ceramic: These are generally used in higher current circuits, >10A because they have a better interrupt capacity than glass fuses. Ceramic fuses are constructed with a current sensitive wire and encapsulated in ceramic sand material. In this way, if there is too much current, the fuse blows peacefully. It does not shatter or burn things in its surroundings. It also will greatly reduce the chances of arcing as the ceramic material inhibits that. The disadvantage is that they are more expensive than glass fuses and you cannot visually see if the fuse has blow. You'll need a ohm/continuity meter. Look for a number followed by AB. Same as before, the number is the size of the fuse, but now AB means it's a ceramic type fuse.If you've ever watched any of Dave's multimeter teardowns, you'll observe that he always comments on the presence or lack of "HRC fuses". HRC stands for High Rupture Capacity. They are a type of ceramic fuse but have higher interrupt ratings (maximum current that it can stop). So while a 10A ceramic fuse might have a 100A interrupt rating, a 10A HRC ceramic fuse might have a 1,000A interrupt rating. They are just intrinsically safer.