Hello,
Can someone explain to me why some fuses are double barreled and how do they work?
Thanks

They are used at medium voltage to obtain a higher current rating. For example, 2 - 300E fuses in parallel (double barrel) have a rating of 540E The 540E comes from (2 x 300) x 90%

If it's what I'm thinking of, you use these when you need to achieve a current rating higher than the largest available fuse. So for instance for medium voltage cutout fuses the ANSI standard sizes end at 200 A. You can get up to 400 A by putting in two of them in parallel because the current will split APPROXIMATELY equally between the two fuses.

There are problems with this. First, the fuse accuracy rates add because worst case, you might end up with 2 fuses that are slower (or faster) than the rated value. Second, you have to assume that the fuse holders and all connections to those holders are extremely good because if they are not, then you get more load on one fuse than the other and this results in a lower trip point than expected. Third, if you have to install one fuse at a time such as in a cutout switch, the single fuse will see the load when you energize it. In the case of using them as primary protection on large transformers, the inherent issue is whether or not the transformer magnetization current is large enough to trip the fuse when you try to close in one but not the other.

I'm speaking from experience here. Had an issue after a hurricane came through. A portable mine substation, 10 MVA, 22.9 kV:7.2 kV L-L with a single output circuit driving a dragline, had a very old transformer on it and a metal enclosed fused switch. Water penetrated the fused switch and destroyed it. Not sure exactly what happened to the transformer but suffice to say that it was also severely damaged and probably beyond repair. We had a spare transformer but the only fuses I could get for 25 kV rating were ANSI-style cutout fuses which are only available up to 200 A and interchangeable between manufacturers but the transformer full load current was around 250 A. My other choices would have been S&C SM-4's, or SM-5's, or find a pole mounted breaker or recloser and figure out what it would take to put it in service instead of cutout fuses. SMD-40's (what I would recommend for this system) were not available. There might have been some other manufacturers (although I tried without success) but I'm not aware of any. I used what I had and put two 150 A rated fuses in parallel. I checked this design out multiple times including discussion with the vendor and came to the conclusion that it is acceptable. In fact within the S&C SMD-40 product line, they actually sell a special double-barrel cutout fuseholder to do this but delivery times on the entire SMD-40 product line are pretty long (10+ weeks for everything) for some reason.

Worked like a champ. Worked so well that we got capital approved and bought an entire new substation and then retrofitted the old spare transformer and refurbished the old substation so that now we have a complete spare portable substation in the fleet. As I write this right now, it's currently operating while one of the other units is offline for PM's.

All comments below are directed at cartridge style current-limiting fuses.

1. The fuse can be 2 or 3 parallel barrels.

2. In most cases you see multiple barrels in the medium voltage sector. However, semi-conductor protection fuses also come in multiple barrels.

3. It is not so much the current rating that cannot be achieved, the associated interrupting rating becomes limited when certain fuses have too high of a continuous current rating.

4. In all cases that I know of multiple barrel fuses are soldered together, therefore, they will see equal currents. In other words, the fuse clip or holder cannot alter the sharing. In addition, both MV fuses and semi-conductor current-limiting fuses are typically short circuit devices, they are not made to open overloads, therefore, the sharing (again) will be very good.

Usually MV fuses do not have ampere ratings, they have "E" rating. That is an ampere rating, but, it is skewed by the standard The E-Rating standard/test is as follows:

A. 100E or less must melt in 300 seconds at 200% to 240% of their E rating.

B. Over 100E a fuse must melt in 600 seconds at 220% to 264% of their E rating.

As I said previously, these are short circuit devices. On the other hand, here is a good question about a device that does have overload capabilities, when a 600A/480V circuit breaker is tested at UL under a load of 200% of the rating (1200A). what is the maximum opening time UL allows?

The fuses I was working with are boric acid expulsion fuses. Definitely short circuit only devices. The industry standard goes by the name SMU-20 for S&C and has an ANSI standard and goes up to 200E, although you can get different characteristics. Above that point unless it is one of the spring loaded fuse types or an indoor type, the only larger ones are SMU-40's that go up to 400E. Above that S&C makes a double fuse holder that lets you go to I think 600E. This is also specifically for 23 KV. Your fuse choices get extremely limited above 15 KV. The biggest problem with multibarrel fuses that I see since they are not factory matched is that with say +/- 10% accuracy you have to assume either worst case in which case the combination may be +/-20%, or assume some other estimate about the accuracy of the combined rating.

One thing that I have never understood about expulsion fuses is the maximum interrupting capacity. With other fuse types, I can see the issue because the fuse may rupture under excessive current, but an expulsion fuse is meant to rupture in the first place.