On 2018/12/08 at 8:40 PM, phil.g00 said:

1. You need to rate your fuses for VA not for watts.

Thanks.

I've never really understood why marketers provide both a VA and a Watt rating. That almost always relies on assuming some kind of power factor (often 0.8), and as pointed out, on the DC side it is the same thing anyway.

I have a 160A fuse on my 3kva inverter... also because that is what I had. At 24V, that's only around 3.8kw, but as @Javi Martínez points out, it will handle a bit more for several seconds, and the Multi can only do overload conditions for a matter of seconds (I've done 3.2kw for several seconds without a shutdown, but I know 200% shuts it down in about a second).

But... it really should be 200A... but 160A works just fine in my case.

I also have a 160A fuse on a 48V 5kVA unit, no issues (so far).

As @Javi Martínez's curves illustrate it is unlikely that someone will operate the inverter at full tilt continuously long enough to blow a 160A fuse even on your 8kVA unit.

However fuses suffer from "fuse fatigue", so over time I think you'd get there.

The issue is when you blow a fuse you want to know there is an problem, and not be guessing. I think that's where Victron's conservative rating is coming from.

@Jaco de Jongh I don't think I go below 225/ 250A on a permanent basis. That'll give you the faster clearance times, but probably anything under that an you're well into nuisance fuse blowing territory with your size unit.

 

 

 

Edited December 9, 20187 yr by phil.g00

 

1 hour ago, plonkster said:

I've never really understood why marketers provide both a VA and a Watt rating. That almost always relies on assuming some kind of power factor (often 0.8), and as pointed out, on the DC side it is the same thing anyway.

@plonkster Sorry I may have misled you with my general reply to @Jaco de Jongh the difference between VA and Watts mentioning power factor.

In this instance, I believe the culprit is "crest factor", where some loads, lets say switch in and take a current peak of more that the normal 1.414 of the sinusoidal wave.

But the power output can still be the same or greater. It is an unusual rating that stems from UPS ratings, and the fact that UPS's are generally used for computers. And that computer power supplies present this kind of load to the UPS, hence it had to be rated for it. Maybe a pic will help.

Edited December 9, 20187 yr by phil.g00

On 2018/12/08 at 6:31 PM, phil.g00 said:

Negating inversion losses, on the battery side:

8000VA/48V =167A  possible sustainable full load draw from the batteries. 

I think a 160A fuse for 8kVa is too low.

That's not exact at all. If we suppose no inverting loses we Will have 6500w/ cut off Voltage. You have to consider that if we have unclumped loads, when current and Voltage are opposite, the inverter works as rectifier. How? Current pass trough the antiparallel diodes of IGBT, so, the rise of loses are related to the rise of loses un IGBT, which are small.

For example, we can imagine a geyser of 2 kW and an inverter which has an efficiency of 95% and battery Voltage is 48 V. Then current will be 2000/(0.95x48).

Now if we have a motor of 2 kW and 4 KVA. Efficiency Will be round 93-94%, we could stimate, so, current woul be 2000/(0.93x48) and no 4000/(0.95x48).

You can test It.

Edited December 9, 20187 yr by Javi Martínez

I know not taking losses into account is not exact.

That is precisely why I stated that caveat from the outset, so I didn't have to go into the minutiae of things, so as not to complicate things. 

It was an unnecessary detail for my argument and would only serve to indicate that because off efficiency losses the DC input was in fact higher than the AC output anyway.

I was making an in principle case, not arguing the toss between the difference between 95% and 93-94%.

"Voltage and current opposite": this is a DC fuse it will blow on current magnitude regardless of direction, it must be rated to carry the larger of the currents in either direction.

I don't think I am making any statement that is contradictory to yours.

I do think what you have said is accurate, but I think you may be missing my point.

But back to business a 160A fuse is too small for an 8kVA unit. It 'll work for a while, but fuse fatigue will kick in and it will blow in service given time.

 

Thanks for all the fuse lessons above, but for now I will rather stick to the Victron recommendation of 300 Amp. I am sure they took all the above into consideration when they designed their equipment... 

I dont agree at all...Victron doesnt know the size of your bank and shortcircuit current...

Concerning fuses, the smaller you can install the safer It is. It is not easy to choose a fuse for a battery, because shortcircuit current is not always the same. When we install a breaker in a grid install is very easy because shortcircuit current is known. In a battery bank, when time goes by ir the buttery isnt fully charged, Isc goes down, and the worst thing could happen is the fuse response time were higher, 0.5-1 s because those gap are an "eternity" for electronics

Edited December 10, 20187 yr by Javi Martínez

11 hours ago, phil.g00 said:

I also have a 160A fuse on a 48V 5kVA unit, no issues (so far).

As @Javi Martínez's curves illustrate it is unlikely that someone will operate the inverter at full tilt continuously long enough to blow a 160A fuse even on your 8kVA unit.

However fuses suffer from "fuse fatigue", so over time I think you'd get there.

The issue is when you blow a fuse you want to know there is an problem, and not be guessing. I think that's where Victron's conservative rating is coming from.

@Jaco de Jongh I don't think I go below 225/ 250A on a permanent basis. That'll give you the faster clearance times, but probably anything under that an you're well into nuisance fuse blowing territory with your size unit.

 

 

 

You can solve It installing a bigger frame. For example, you can install a 160 A fuse in BUC NH0, or NH00 even NH1...

1 hour ago, Javi Martínez said:

I dont agree at all...

Javi, will you feel better if i tell you I still have a 250Amp breaker in place.... 

So i have the 200% instant cutout of the Inverter, a 250Amp breaker as well as a 300Amp fuse.... 

 

1 hour ago, Jaco de Jongh said:

Javi, will you feel better if i tell you I still have a 250Amp breaker in place.... 

 

 

...yes...

 

1 minute ago, Javi Martínez said:

...yes...

Nice, problem solved then....

4 hours ago, Javi Martínez said:

Victron doesnt know the size of your bank and shortcircuit current...

@plonkster, the R&D team should look into this, I want to install a few Victrons this month and cant sit with fuse related issues... 

Edit: Just teasing, I trust their recommendations.

I use a "fast rule". Fuses current shouldn´t be bigger than a half of Ah battery bank.

For you, 400 Ah, you shouldný install a fuse bigger than 200 A.

If you had a 1000 Ah battery bank, i would have said nothing, because Isc of those kind of banks use to be 10.000 A.

More kan julle my dalk help met victron inverter parte

8 hours ago, jaco2306 said:

More kan julle my dalk help met victron inverter parte

Hallo Jaco, wat het jy nodig?