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Fuse and Quick disconnect between Pylontech battery and Inverter


georgelza
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Hi guys

So for those using Pylontech, supplied cables to connect the Pylontech battery to the inverter is 25m2's...

So connection options are, connect battery/MPPT directly to Terminal lugs on Inverter or connect Battery/MPPT and Inverter to a Buzz bar... ok this is not the question though.

Between my panel combiner box and the MPPT my sparky asked that we install a dual pole isolator,

now ... the question... hehehe

are you connecting the batteries directly to the Inverter or Buzz bar, or are you installing a inline fuze and quick disconnect ?

G

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I don't know where the 250A recommendation came from but if it was from my post then I want to apologize for it not being clear. The statement was made that Mersen is not rated for DC, I merely commented that my Mersen is rated for 220V and 250A DC, I did not say that you need a 250A fuse in there. Sanity should still prevail as Plonkster pointed out with the 125A/150A fuse suggestion.

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4 minutes ago, plonkster said:

I agree. If you can add a measure of protection by not oversizing the fuses, why not 🙂

No, that is way too soon. The datasheet shows a recommended maximum CONTINUOUS discharge value of roughly C/2, but it also shows a peak of 100A (2C) for 15 seconds. This is the rating you want to protect against, so I would size the fuse at 100A or even at 125A. That is half of the 250A recommendation of earlier, so I am still agreeing with you broadly 🙂

 

The problem is slightly more complex than most people realise. Unfortunately it isn't as easy as saying "pick a fuse that will blow at X amps".

Here is a picture of the current-time characteristics from one of the Jean Muller DC fuses:

Capture.thumb.JPG.bc68192eae3407c31a6dcfee30b2ba94.JPG

So if we agree that 100A for 15s is the limit that we don't want to exceed, then we need to choose from the fuse values that fit inside the box I drew (time on the Y-axis, current on the X-axis, graph is logarithmic). But we need to be careful that the fuse we choose don't blow too quickly.

For example, if we operate at the recommended currents of 37A (for the US3000), we can't use the 20A fuse, since it will blow at around 11s. The 100A fuse might last indefinitely at 37A, and even last 10 000s at 100A, so that might not work. The 40A or 50A fuse might be a good compromise. 

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3 hours ago, CarlT said:

For example, if we operate at the recommended currents of 37A (for the US3000), we can't use the 20A fuse, since it will blow at around 11s. The 100A fuse might last indefinitely at 37A, and even last 10 000s at 100A, so that might not work. The 40A or 50A fuse might be a good compromise. 

You went one step further and looked at the actual chart, which I didn't do, so you're absolutely correct. Nice graph too!

I still think that I would rather let the BMS make the call for currents that are technically within the capability of the battery, for example even a 4C current (200A) for a second is not really a reason to pop a fuse, which would mean the 80A fuse is the one to go for.

But broadly we are in agreement. You really can get away with a much smaller fuse, and it adds safety.

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15 hours ago, plonkster said:

You went one step further and looked at the actual chart, which I didn't do, so you're absolutely correct. Nice graph too!

I still think that I would rather let the BMS make the call for currents that are technically within the capability of the battery, for example even a 4C current (200A) for a second is not really a reason to pop a fuse, which would mean the 80A fuse is the one to go for.

But broadly we are in agreement. You really can get away with a much smaller fuse, and it adds safety.

The Jean Muller fuses are nice, but it seems the local agents don't import them. Everyone uses the AC fuses!

I am cautious by nature, so in my list of priorities, first is the safety of my family, followed by the protection of the equipment.

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7 minutes ago, CarlT said:

The Jean Muller fuses are nice, but it seems the local agents don't import them. Everyone uses the AC fuses!

I can't say I've had this experience. I got the Mersen holder and fuse right in the beginning. It has a DC rating. Sure, I couldn't just buy it at the local AC/DC, I had to actually order it from Rubicon... 🙂

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32 minutes ago, FixAMess said:

So, is the recommended fuse size for  3.5KW  Pylontechs 50A fuses? I just bought  125A fuses....😞

I'm running 2 x US3000s on 125A Jean Muller fuses, I'd be interested in the answer to this question as well. 

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So just for clarity on this. With the Lithiums out there (Pylon/Dyness) which have their own switches and also BMS to handle any surge issues. 

Does one still need, by regulations, a battery disconnect and fuse? Agreed it is good practice, BUT is it law in terms of SANS and Insurance? 

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You should always think about failure modes, belts and suspenders. What happens if just one of the big old FETs (which many use now) is damaged and the battery does NOT disconnect in a timeous fashion? Then you want a safe part of the connection to burn out rather than an unsafe part, that is to say, you want the fuse to blow and not the copper cable.

I have a similar situation inside my coffee maker. It has a combination thermal fuse and thermal switch (aka klixon) device which costs like 12 Euros. One could argue, why have the fuse? The Klixon part already interrupts the current if something bad happens. And the answer would be, because the klixon could fail, and then you want a backup.

Of course my beef with this particular Siemens coffee maker is that the thermal fuse blows before the klixon interrupts things whenever the water pump delivery is low... which is just terrible design. The fuse should blow as a last resort... not the other way round!

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Thanks and agreed 100%, but again my question is more directed to the legality of an install. Specifically how complete the install is as supplied by the installer. 

If the cable melts and burns down the house, will the insurance say, no game, you didn't have your fuse installed as determined by the regulations. 

If a fuse is all it takes, then just install a mega fuse and holder for instance. Btw this is for 3x 2.4 Dyness and would be looking at 125A fuse.  That or the Keto with 125A. 

 

 

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23 minutes ago, Tsa said:

If the cable melts and burns down the house, will the insurance say, no game, you didn't have your fuse installed as determined by the regulations.  

Why would you take the chance? But okay, you're right, I'm arguing the sensibility and you're arguing the legality... so here goes...

Selection_644.png.3e24dac7db257457eba2570b2339ccd6.png

That's from a draft copy of SANS-10142-1 that has the new DC bits added to it. It says you MUST have an overcurrent device in place. Does not have to be a fuse, can be a breaker, but it must be able to interrupt the full short circuit current of the the battery (which would be a very hefty breaker).

The insurance company will smack you on the letter of the regulations. Put the damn fuse in 🙂

What is also interesting here is how ground-faults are avoided. The cable between the battery and the protection device (fuse) must be kept as short as possible, and the positive and negative cabling up to the fuse must be kept separate. They are clearly trying to avoid fault conditions on the wrong side of the fuse. When you do this, you need a battery fuse only in the positive line, since ground faults are now unlikely (it would have to happen in the very short cable to the fuse on the positive side).

Edit: Also note "NOTE 3", which already refers to the battery protective device, aka the BMS.

 

Edited by plonkster
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That then makes it 100% clear, thanks @plonkster. So in terms of legality and issuing of CoC on such a system, this SHOULD have been in place. 

Will do some further reading on SANS -10142-1 on what else "should" have been in place. Next step is decide on and source 125A fuse + holder or go the Keto route. 

Thanks again

 

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7 minutes ago, Tsa said:

To add to this. Would this be sufficient protection, or would a normal fuse then be a safer option?

https://lithiumbatteriessa.co.za/collections/frontpage/products/1p-dc250v-125a-dc-circuit-breaker

The critical part is that the breaker must be able to break the full short circuit current of the battery, and for that you need the kA (kilo-Amp) rating of the breaker. It is usually written on the breaker, and is usually something like 3kA, 6kA, or similar. The short circuit current of an LFP battery can exceed 30C (30 times the capacity). On a 100Ah battery, a 3kA breaker would not be enough for example. The fuse is generally a better call in my opinion. It is faster too. A breaker needs 5 times the rated current before it trips instantly.

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