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Class T fuse and breaker?

Roswell Bob

Solar Enthusiast
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Warner, NH
Has anyone given any thought to using a large Class T fuse and a breaker? I am considering using a breaker to protect #2 wire to inverter and a fast Class T or Semiconductor fuse to protect against short circuit events. The fuse would open and clear large fault current before breaker would respond.
 
Yes, this would be one approach. I believe the ABYC E-11 (marine standard) mentions this approach as one possible option.

You could also just use a class T fuse + mechanical switch. Seems this would accomplish the same functions (OCP and manual on/off). Is there another reason you are thinking a breaker might be better suited?
 
This is the excerpt I was thinking of:
11.10.1.6.2.3
be capable of an interrupting capacity according to TABLE 4A and E-11.10.1.2, and remain operable after the fault, and
EXCEPTION: Integral overcurrent protection in electrical devices.
NOTES:
1. A fuse in series with, and ahead of the circuit breaker, may be used to comply with TABLE 4A.
2. Consult the circuit breaker manufacturer to determine the fuse size and the type of fuse.
 
Yes, this would be one approach. I believe the ABYC E-11 (marine standard) mentions this approach as one possible option.

You could also just use a class T fuse + mechanical switch. Seems this would accomplish the same functions (OCP and manual on/off). Is there another reason you are thinking a breaker might be better suited?
I might choose a large fuse (250A) with a higher I^2t rating to allow the inverter bus to charge without melting fuse. Then the breaker might be near 100A to protect the #2 wiring to inverter. In the event of a direct short circuit the fuse would clear before the breaker so that the breaker might not need so high an interruption rating.
 
I might choose a large fuse (250A) with a higher I^2t rating to allow the inverter bus to charge without melting fuse. Then the breaker might be near 100A to protect the #2 wiring to inverter. In the event of a direct short circuit the fuse would clear before the breaker so that the breaker might not need so high an interruption rating.
I think I see, the theory is that the fuse is there for the high AIC, not sized to protect the wire in all situations, but sized to handle a catastrophic failure/short, whereas the breaker would be sized to protect the wire, but lacking the AIC to handle a high current short. Makes sense I think.
 
I think it all depends on then fault curves of both the brk and fuse, which one do you a want to operate first and which one can operate quickly for high fault current.

OCP coordination is a tricky thing to balance and even though it may look good on paper may not work 100% as designed in real world.

Either way quality brk and fuses should be used to reduce voltage drop and losses under normal operations.

Also is there a reason why you’re sticking to #2 as the limiting element?
 
I think it all depends on then fault curves of both the brk and fuse, which one do you a want to operate first and which one can operate quickly for high fault current.

OCP coordination is a tricky thing to balance and even though it may look good on paper may not work 100% as designed in real world.

Either way quality brk and fuses should be used to reduce voltage drop and losses under normal operations.

Also is there a reason why you’re sticking to #2 as the limiting element?
I have a bunch of #2 welding wire on hand. We are off grid and our power needs are very modest. All appliances are LP (so not so far off the grid) and we heat solar passive home with home grown firewood. We use about 6kWh/day with a 6kW inverter. I intend to use brands that I know and trust for fuses/breakers/disconnects. I don't believe it is wise to look for bargains when buying protection devices.
 
with a 6kW inverter
What voltage are your batteries configured? I assume 24V

My observation is since 2ga is only about 120A on the ABYC chart for 12V and a 6000W inverter is a potential of 250A at 24V that’s one thing to consider. Unless I missed where you addressed that?

Not criticizing- just thinking through. 6kW inverter just seems huge to me if you only burn 6kWh/day.
 
It is a 48v inverter, so 120A should get me to near 5kW considering efficiency. We have well pump that takes near 8kW for 20 cycles to get up. I was surprised it is go stubborn. It is about 6A FLA so 36A on start is not too wild of a number. I believe motor inrush current is in-phase, so VA is also Watts. There are other high load devices, microwave, hair dryer and so forth that I have to account for of course they are very low duty cycle tho. So 6kW is not too too large.
 
It is a 48v inverter, so 120A should get me to near 5kW considering efficiency. We have well pump that takes near 8kW for 20 cycles to get up. I was surprised it is go stubborn. It is about 6A FLA so 36A on start is not too wild of a number. I believe motor inrush current is in-phase, so VA is also Watts. There are other high load devices, microwave, hair dryer and so forth that I have to account for of course they are very low duty cycle tho. So 6kW is not too too large.
I have been studying 6Kv and 8kv 48 volt inverters for over a month and wanted to go with the more efficient 48 volt system. I have a base knowledge of electrical systems but not a pro. I just happened to find that a T-Class (very expensive) fuse is required on 48 volts system on Wills mobile site under All-In One 12/24/48 packages at https://www.mobile-solarpower.com/all-in-one-122448v-packages.html and the LV6548 kit.

All of the hours of videos and technical data I have watched and read from the manufactures users manuals has anyone stated or mentioned this. I guess they print "WARNING! All wiring must be performed by a qualified person" is in there for a reason. There is a little more to this than slapping a AIO unit on the wall and connecting wires.
 
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I suppose through code.

No, it's because many other fuses such as ANL just aren't rated for this voltage. ANL is only rated to 32V.
There are breakers, but the good ones are much more expensive than a Class T fuse. Bite the bullet, better safe than sorry. For reference:

 
As a kid I had a race between a 15a screw in fuse and the circuit breaker. Flipped on the lamp with a fuse screwed into it and got nothing... the breaker tripped first.

I don't consider Class-T to be a fast fuse. Blue Sea trip graph shows it will pass 2x rating for close to a minute.
 
As a kid I had a race between a 15a screw in fuse and the circuit breaker. Flipped on the lamp with a fuse screwed into it and got nothing... the breaker tripped first.

I don't consider Class-T to be a fast fuse. Blue Sea trip graph shows it will pass 2x rating for close to a minute.
In the grand scheme of fuses it is fast. It is fast enough to protect diodes or SCRs looking into a dead short. Not quite as fast as a semiconductor fuse, but will keep up with the KTK series. The speed is generally considered to be in the sub cycle range (<16ms or so) rather than in the seconds. I had to design a brake for a 20hp motor drive. If the brake transistor shorted it would catch on fire and create all kinds of problems. I was able to use a small SCR and clear 60A class T line fuses as well as discharge a 2,000uF capacitor bank from 800v to zero. I recall near 10,000A into the SCR. So yes, they are rather fast.
 
No, it's because many other fuses such as ANL just aren't rated for this voltage. ANL is only rated to 32V.
There are breakers, but the good ones are much more expensive than a Class T fuse. Bite the bullet, better safe than sorry. For reference:

I went to your web page and followed your link to Alibaba and Must products page. 5 yesr warranty on AIO inverter? for below $600? Too good to be true? Kind of wary...
 
I went to your web page and followed your link to Alibaba and Must products page. 5 yesr warranty on AIO inverter? for below $600? Too good to be true? Kind of wary...

It's not an all in one though, it's a 6kW low frequency inverter/charger that I link to. They do have the same model that also includes solar charge controller. I've been running it for almost two years now without issues. I did contact them to see if I could buy spare parts: $50 or so for the control board and $50 for the power board, shipped for $80 by courier and delivered within two weeks. I think that's pretty good service and I don't think they would not honor the warranty when e.g. the power board blows up.
 
Is there any downside to this, aside from wasting money? I have the same type of system setup now with breaker/T Fuse combo and figure it can't hurt.
 
It's not an all in one though, it's a 6kW low frequency inverter/charger that I link to. They do have the same model that also includes solar charge controller. I've been running it for almost two years now without issues. I did contact them to see if I could buy spare parts: $50 or so for the control board and $50 for the power board, shipped for $80 by courier and delivered within two weeks. I think that's pretty good service and I don't think they would not honor the warranty when e.g. the power board blows up.
I looked on YT for any info on Must inverters and found the majority of the 7 or so videos by Must Energy Group. There are many replies on the video asking questions but none were answered by the "Group". That kind of sends up a flag to me.
And it is the units are not well defined for use in the North American market or European/Asian.
 
No, they mostly have their market in Africa. There are tons of online market places there where you can get spare parts, etc. It's one of the reasons I got interested in them: having a spare parts market like that is like owning a Toyota in Africa, where people make those things run forever. They're almost unknown anywhere else, have an SEO problem due to the name, etc. All that made me even more curious...

As I said, I'm running their inverters and their charge controllers. No issues. There are maybe two or three people I know that also run them in the US and/or Europe (one of them was inspired by my system write-up) and I've searched a lot when I was doing my research.

Edit: by the way, all of this is just my opinion. I'm not endorsing anything. I just share my personal experience.
 
No, they mostly have their market in Africa. There are tons of online market places there where you can get spare parts, etc. It's one of the reasons I got interested in them: having a spare parts market like that is like owning a Toyota in Africa, where people make those things run forever. They're almost unknown anywhere else, have an SEO problem due to the name, etc. All that made me even more curious...

As I said, I'm running their inverters and their charge controllers. No issues. There are maybe two or three people I know that also run them in the US and/or Europe (one of them was inspired by my system write-up) and I've searched a lot when I was doing my research.

Edit: by the way, all of this is just my opinion. I'm not endorsing anything. I just share my personal experience.
I appreciate your input and understand your are just conveying your experience. I just went down the rabbit hole of curiosity with your selection of Must Equipment. As yes, as a retired Auto shop owner of 38 years (50 years in the industry), I am always amazed how the people in the 3rd worlds keep those things running. Just being cautious and doing do diligence.
 
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