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Need an inverter recommendation: Best of the worst?


Tasemu

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1 hour ago, IanD said:

Because if the inverter goes wrong (e.g. both output stage switches on at the same time, or a MOSFET failure) it doesn't appear as a dead short but a small resistance -- in this case you want the fuse to blow as quickly as possible...

No you don’t. That’s what the device’s own fusing is for. The Megafuse is to protect the cable, nothing else. 

1 hour ago, Boater Sam said:

A dead short on a cable with a resistance of 1/100 ohm will pass 1200 amps at 12V.

Yup, which would blow a 400A fuse prettty smartish. 

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32 minutes ago, WotEver said:

No you don’t. That’s what the device’s own fusing is for. The Megafuse is to protect the cable, nothing else. 

Yup, which would blow a 400A fuse prettty smartish. 

You're assuming that a cheap possibly shoddily-built inverter has an internal fuse...

 

Yes a "zero-ohm" dead short will blow a 400A fuse pretty quickly (or a 150A one), but not all faults are dead shorts, especially if the return current path for the short isn't through the heavy-current cable. Drawing 1000A or so for tens of seconds through a 400A fuse isn't going to do the cables or the batteries any good.

 

There's still no good reason to put a 2x rated fuse in -- no advantages, only disadvantages. Why do you think it's a good idea?

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1 hour ago, WotEver said:

Yup, which would blow a 400A fuse prettty smartish. 

 

Yes, between one second and five seconds according to the chart IanD linked, and assuming its a Class T fuse. Plenty of time to glow red and set the insulation alight I reckon!

Edited by Mike the Boilerman
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50 minutes ago, IanD said:

You're assuming that a cheap possibly shoddily-built inverter has an internal fuse...

 

Yes a "zero-ohm" dead short will blow a 400A fuse pretty quickly (or a 150A one), but not all faults are dead shorts, especially if the return current path for the short isn't through the heavy-current cable. Drawing 1000A or so for tens of seconds through a 400A fuse isn't going to do the cables or the batteries any good.

 

There's still no good reason to put a 2x rated fuse in -- no advantages, only disadvantages. Why do you think it's a good idea?

You’re still mixing up the two issues. The fuse is to protect the cable not the device. 

 

Let’s use some real figures. 

 

The cable will (should) be chosen to minimuse voltage drop, which can easily result in a cable rated at around 4 times or more than the current of the device being powered, depending on how long the cable run is. The fuse is chosen to protect that cable. So let’s take an instance of a 70mm2 cable powering a device which will draw 100A maximum, like a 1kW inverter. That cable would be good for a 300A load, even though it should never see that current. So as long as your fuse is rated no higher than 300A then the cable will be protected. Fitting a 100A fuse (the rating of the device) would result in that fuse regularly blowing due to ageing effects, so it makes sense to fit a 150A or even 200A fuse. That’s well within the capabilities of the cable even though it’s double the rating of the device so your scenario of “Drawing 1000A or so for tens of seconds through a 400A fuse” simply isn’t going to happen. That takes care of the cable protection. 

 

If the device is so poorly built that it doesn’t have any internal fusing then more fool you for purchasing such a piece of junk in the first place. 

35 minutes ago, Mike the Boilerman said:

 

Yes, between one second and five seconds according to the chart IanD linked, and assuming its a Class T fuse. Plenty of time to glow red and set the insulation alight I reckon!

What cable do you buy that catches fire when it’s loaded less than its current rating?

By that rationale nobody should manufacture 400A fuses because they’ll set everything alight if they blow. 

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1 minute ago, TheBiscuits said:

That's why they have a cover :D

 

Indeed. Plus their design means that they don’t ‘glow red hot’. There’s a lot of BS in this thread from folk who don’t know what they’re talking about. 

 

A little info about Megafuses:

https://www.shop.solar-wind.co.uk/acatalog/megafuse_specifying.PDF

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2 hours ago, WotEver said:

You’re still mixing up the two issues. The fuse is to protect the cable not the device. 

 

Let’s use some real figures. 

 

The cable will (should) be chosen to minimuse voltage drop, which can easily result in a cable rated at around 4 times or more than the current of the device being powered, depending on how long the cable run is. The fuse is chosen to protect that cable. So let’s take an instance of a 70mm2 cable powering a device which will draw 100A maximum, like a 1kW inverter. That cable would be good for a 300A load, even though it should never see that current. So as long as your fuse is rated no higher than 300A then the cable will be protected. Fitting a 100A fuse (the rating of the device) would result in that fuse regularly blowing due to ageing effects, so it makes sense to fit a 150A or even 200A fuse. That’s well within the capabilities of the cable even though it’s double the rating of the device so your scenario of “Drawing 1000A or so for tens of seconds through a 400A fuse” simply isn’t going to happen. That takes care of the cable protection. 

 

If the device is so poorly built that it doesn’t have any internal fusing then more fool you for purchasing such a piece of junk in the first place. 

What cable do you buy that catches fire when it’s loaded less than its current rating?

By that rationale nobody should manufacture 400A fuses because they’ll set everything alight if they blow. 

No (good quality) fuse with a given rating should blow due to aging effects; the whole point of the rating is that the fuse should be able carry it for the lifetime of the device, even at the maximum rated ambient temperature -- this is what the fuse rating means.

 

What it's intended to protect (battery, cable, switch, device, ar*e on fire) doesn't matter, because not all failures are dead shorts. The general principle is that a fuse should be correctly sized to carry the maximum intended current, then it will do the best job of protecting the circuit and what connects to it no matter what goes wrong. It's why 13A plugs (for 3kW 230V loads) have 13A fuses in them, not 26 A ones. If the fuse is *only* supplying the inverter -- which is what the OP seemed to be suggesting -- then it should be sized to suit this load, which means 150A for a 1.5kW inverter. If it's also supplying other high-current loads then it should be bigger.

 

Plenty of equipment -- especially low-cost stuff from China -- which should have internal fusing doesn't, because this saves money. And it's not much consolation for them telling somebody they shouldn't have bought that cheap Chinese inverter as they watch the fire brigade putting out the last dying embers of their boat, even if it makes you feel superior...

Edited by IanD
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42 minutes ago, cuthound said:

 

I know I've designed installations using these, have you?

No. But I've seen what happens when a puck thyristor carrying kiloamps and kilovolts fails, which is *much* more spectacular (and noisy)...

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2 minutes ago, IanD said:

No. But I've seen what happens when a puck thyristor carrying kiloamps and kilovolts fails, which is *much* more spectacular (and noisy)...

 

All the high power equipment using thyristors that I  have seen (older UPS's and DC Power Plant) have had the thyristors protected by inline fuses, where apart from alarm lights there hasn't been much to see. I guess it comes down to the internal design of the equipment.

 

Now large electrolytic capacitors failing, that can be spectacular. ?

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5 minutes ago, Tasemu said:

Right... SO... I'm just gonna go with... 200a fuse for my 1500w sunshine inverter and hopefully that side of this argument was correct lol. :)

The larger rated fuses will fit in the holder when it blows.  Your Sunshine should have internal fusing - mine does!

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1 minute ago, TheBiscuits said:

The larger rated fuses will fit in the holder when it blows.  Your Sunshine should have internal fusing - mine does!

Ah right, scratch that, 350 then. the original suggestion.

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1 minute ago, cuthound said:

 

All the high power equipment using thyristors that I  have seen (older UPS's and DC Power Plant) have had the thyristors protected by inline fuses, where apart from alarm lights there hasn't been much to see. I guess it comes down to the internal design of the equipment.

 

Now large electrolytic capacitors failing, that can be spectacular. ?

These were the kind of >1kV traction thyristors where one of them is a whole silicon wafer in a circular can, being tested in the lab at several thousand amps...

 

I agree, I've seen a 100,000uF electrolytic go bang and spew its guts all over the room ?

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8 minutes ago, Tasemu said:

Right... SO... I'm just gonna go with... 200a fuse for my 1500w sunshine inverter and hopefully that side of this argument was correct lol. :)

 

There is no right or wrong answer. As I said in post #46, the primary reason to fuse is to protect the downstream cable, but designers can choose use smaller fuses to protect downstream equipment if they choose to, for example to protect a poorly designed inverter which doesn't have internal fuses to protect it.

Edited by cuthound
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28 minutes ago, cuthound said:

 

There is no right or wrong answer. As I said in post #46, the primary reason to fuse is to protect the downstream cable, but designers can choose use smaller fuses to protect downstream equipment if they choose to, for example to protect a poorly designed inverter which doesn't have internal fuses to protect it.

Have a greenie for that :)

45 minutes ago, IanD said:

No (good quality) fuse with a given rating should blow due to aging effects; the whole point of the rating is that the fuse should be able carry it for the lifetime of the device, even at the maximum rated ambient temperature -- this is what the fuse rating means.

Now I know for sure that you’re simply spouting theory and have no practical experience. Thanks for proving it. 

46 minutes ago, IanD said:

If the fuse is *only* supplying the inverter -- which is what the OP seemed to be suggesting -- then it should be sized to suit this load, which means 150A for a 1.5kW inverter.

And your theory states that the occasional peak loads which will comfortably exceed 150A won’t cause any aging of the fuse. Good luck with that. 

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48 minutes ago, IanD said:

It's why 13A plugs (for 3kW 230V loads) have 13A fuses in them, not 26 A ones.

Firstly, we’re not talking about 13A loads, we’re talking about a varying load which can rise from the nominal 150A to 250A. Secondly, a 26A fuse would not protect the cable, and that’s its sole purpose. 

38 minutes ago, TheBiscuits said:

Your Sunshine should have internal fusing - mine does!

Only the cheapest junk doesn’t. We’ve already seen a YouTube link showing what happens to those units. 

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49 minutes ago, Tasemu said:

Right... SO... I'm just gonna go with... 200a fuse for my 1500w sunshine inverter and hopefully that side of this argument was correct lol. :)

So the next question you need a definitive answer to (allowing for the fact you've already revised the rating to protect your cable rather than the device) is: exactly where in the circuit should it  be fitted?

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4 minutes ago, Sea Dog said:

So the next question you need a definitive answer to (allowing for the fact you've already revised the rating to protect your cable rather than the device) is: exactly where in the circuit should it  be fitted?

Already answered by WotEver. "Yes. As close to the butteries as possible but outside the battery box. "

 

Though i must confess, I am not sure what butteries are.

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4 minutes ago, rusty69 said:

Already answered by WotEver. "Yes. As close to the butteries as possible but outside the battery box. "

 

Though i must confess, I am not sure what butteries are.

Ah, sorry missed that. I thought it important to cover it but I didn't want to chip in with a answer when the thread already has all the skilled input it needs. :)

 

(I can't believe it's not butteries) ;)

 

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41 minutes ago, WotEver said:

Have a greenie for that :)

Now I know for sure that you’re simply spouting theory and have no practical experience. Thanks for proving it. 

And your theory states that the occasional peak loads which will comfortably exceed 150A won’t cause any aging of the fuse. Good luck with that. 

Same approach as Brexit -- apply insults instead of having a meaningful discussion which might help the OP. So I won't bother rising to your bait again -- go and engage in online keyboard-to-keyboard combat with somebody else.

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