fuses, CB, RCDs OK? (newb)

[Wick]

Hello all, wondering if you can advise on my selections. System specs in my signature.

 

One thing I'm wondering about is where to use CBs vs actual fuses - I am fairly sure fuse is needed on battery, but is it needed on PV as well? Or will CBs do for PV?

 

Battery-positive to EasySolar-II

 

100-125A DC Midi fuse for battery (Victron Wiring Unlimited claims slow blow for DC...?) + fuse holder - should be fast blow? (F or preferably FF)

Am using 35mm / AWG 2 with a rating of 105-120A (48V 240Ah battery). So an 80-100A fuse should work for battery @ ~80% of cable ampacity rating.  Better to go 100A as it is a very short cable (2m each for positive and negative)? - and actually EasySolar-II manual recommends 125A. However I doubt I will be drawing anything near as high (48V @ 100A = near 5kva, much higher than the inverter's 3kVA recommended for regular use).

 

PV to SmartSolar 250/70

 

15A DC blade fuse for each of 1-2 series of PV in parallel

OR

15A DC circuit breakers to save replacing fuses? or is a fast blow fuse safer?

 

30-80A 160-250VDC master fuse / CB between PV array's individual fuses and battery - ceramic 55mm Bussmann?

 

AC Distribution box

 

32A total (matches 48/3000/35-32 rating

16A heater, stove,  water heating, aircon

10A computer & digital equipment (should be 16A? swap?)

2A lights

30mA "high sensitivity" RCD / RCBO (63A standard? or 32A?)

 

 

Thanks for any advice you can offer. Let me know if any clarifications are needed.

[Jaco De Jongh]

30 minutes ago, Wick said:

100-125A DC Midi fuse for battery (Victron Wiring Unlimited claims slow blow for DC...?) + fuse holder - should be fast blow? (F or preferably FF)

Am using 35mm / AWG 2 with a rating of 105-120A (48V 240Ah battery). So an 80-100A fuse should work for battery @ ~80% of cable ampacity rating.  Better to go 100A as it is a very short cable (2m each for positive and negative)? - and actually EasySolar-II manual recommends 125A. However I doubt I will be drawing anything near as high (48V @ 100A = near 5kva, much higher than the inverter's 3kVA recommended for regular use).

 

I use 80 Amp fuses on the + as well as - cables when installing a 3kva. I also use 25mm cable feeding the inverter.

32 minutes ago, Wick said:

PV to SmartSolar 250/70

 

15A DC blade fuse for each of 1-2 series of PV in parallel

OR

15A DC circuit breakers to save replacing fuses? or is a fast blow fuse safer?

 

30-80A 160-250VDC master fuse / CB between PV array's individual fuses and battery - ceramic 55mm Bussmann?

12 amp fuses per sting ins fine. Again on the + and - wires. Installing a breaker in its place will just increase the cost without any value adding to the system. 

You dont need a fuse between the battery and the MPPT, the mppt clips its current on the 70 amp mark and can not supply more current than that. In the event of a short circuit the MPPT will switch off, so the current will never reach higher than the rated 70 AMps, Again the fuse will not have any function in the circuit. 

[___]

Fuses are to protect the cable. As long as the fuse burns before the cable does, you're fine, which is why Victron calls for slow-blow. That will blow long before anything else gets hot enough to start a fire.

You fuse both sides whenever you're worried that a single ground fault may go unnoticed. Since the PV wires run long distances, often inside steel conduit, and in hostile hot environments at times, there is a higher likelihood of that happening, so PV cabling definitely gets fused on both sides, using fuses rated for at least 500VDC (though most solar places sell 1000VDC, about R20-R30 each). Keep a few spares.

I prefer to have my MPPTs fused (or breaker-ed) as well, but it depends on how you have things laid out. If you have 25mm^2 cabling to the MPPT, then the fuses you installed battery side already protects it. If the cabling to the MPPTs are much thinner, you need to fuse smaller for that run. I like the ability to be able to disconnect my MPPTs individually as well, so I use DC-rated breakers here. But you don't have to, as Jaco said.

The reason you need to consider the cabling to the MPPT, is under a fault condition. If something goes wrong in the MPPT and let's say a dead short develops across the battery terminals, you want something to blow before that cable goes up in smoke. If properly designed, the battery fuses will do that for you already.

[Wick]

Good to get some ICTRON & general fusing advice from those in the know, first thing in the morning. Thank you both.

Quote

I also use 25mm cable feeding the inverter.

Useful to know one can get away with 25mm sq. I went with 35mm sq. on EasySolar manual's recommendation... can't hurt if I ever want to run a higher load.

80A + and -, got it @Jaco de Jongh. Wasn't sure if there was a benefit to fusing both ends. @plonkster's comment seems to confirm.

Quote

using fuses rated for at least 500VDC (though most solar places sell 1000VDC, about R20-R30 each). Keep a few spares.

I should get away with 250VDC if available, SmartSolar only rated up to that anyway? I shall keep several! (provided they don't cost R500 each like mega-fuses  )
 

Quote

PV cabling definitely gets fused on both sides,

Appreciated. I get this feeling from the many sources I've looked at, Victron and otherwise. Going to rather play safe than sorry, no offense @Jaco. A little extra resistance is not going to kill me, but fusing may save me. From what I've seen (e.g. this), breakers should work OK here? Would you opt for fuse or breaker, @plonkster?

Quote

12 amp fuses per sting ins fine. Again on the + and - wires.
If the cabling to the MPPTs are much thinner, you need to fuse smaller for that run

12A PV string fuses - OK - "in" = on the input side? Which is that? Doh. From what I read, amperage fluctuation from MPPT can be high... maybe not 5A though? Isc on panels is 10.16A so I agree 12A should be enough in general... the panel-attached cables are MC4 rated 15A, 4mm sq. core.

Overall cable losses should be low - 10-12m total for 3-4 series panels (including their own attachments).

Edited July 28, 2020 by Wick

[___]

3 minutes ago, Wick said:

I should get away with 250VDC if available, SmartSolar only rated up to that anyway? I shall keep several! (provided they don't cost R500 each like mega-fuses  )

I've never seen 250VDC fuses. I like the bussman style ceramic fuses, which is usually the style you need anyway (cause they are filled with silica to extinguish the arc), and I put them in DIN-mount fuse holders. Your options are then 500VAC or 1000VDC from the electrical wholesalers, at least in my experience.

Mega Fuses aren't that expensive. They should be no more than R200 or so. Maybe with the holder it's R500...

5 minutes ago, Wick said:

fuse or breaker

Fuse is cheaper. Breaker is nicer.

7 minutes ago, Wick said:

12A should be enough

20% over the maximum expected at least.

[Wick]

Re 500V - noted. Victron mega fuses are indeed that expensive - do a search and you'll see everyone trying to coin $30+ off you for these. Ridiculous.

However I'm guessing regular brand mega fuses should be roughly as good...? Those can be gotten a lot cheaper as you say.

Nice to know about the bussman ceramic cylinder fuses - they have some nice closeable holders + the 80-100A fuses at AC-DC in town. One less thing to ship.

Edited July 28, 2020 by Wick

[Wick]

Just to confirm re RCD / RCBO...

So 30mA is the rating to stop fibrillation in the body if something does happen. The secondary rating in 2-digit Amperes - is this just the same as any other CB? Just acts as a CB at that level of current? Any benefit for me, in this small mobile setup, to using RCBO?

Edited July 28, 2020 by Wick

[Louisvdw]

55 minutes ago, plonkster said:

Mega Fuses aren't that expensive. They should be no more than R200 or so

The Mega-Fuse that are 32V are cheap (R100), but the 58V versions tend to be R500-R600 

[___]

16 minutes ago, Louisvdw said:

The Mega-Fuse that are 32V are cheap (R100), but the 58V versions tend to be R500-R600 

I've seen them at R250 in a few places (just the fuse). The higher voltage one is indeed quite a bit more expensive. You can use any good brand (eg Littelfuse) fuse, either MegaFuse or ANL.

1 hour ago, Wick said:

nice closeable holders + the 80-100A fuses at AC-DC in town

Just watch out. At least one of the AC/DC units is rated 500VAC... I have one in my spares box. But I got 1000VDC holders at the local wholesaler, they are DF PMX 10x38 units.

1 hour ago, Wick said:

So 30mA is the rating to stop fibrillation in the body if something does happen. The secondary rating in 2-digit Amperes - is this just the same as any other CB? Just acts as a CB at that level of current? Any benefit for me, in this small mobile setup, to using RCBO?

There are a few ratings to take note of.

There is the kA rating, which is the maximum current it can handle under fault conditions. This will be something like 3kA or 6kA, literally kilo-ampere. In a typical single-phase house where the short circuit current is in the order of 1000A or less (this can actually be tested with the right equipment), a 3kA breaker is sufficient.

Then there is the maximum current the breaker can switch, but it does not necessarily imply that the breaker has overload protection built in. In other words, just because it can switch 60A (when operated by hand) does not mean it will trip if you exceed 60A.

Then there is the residual current rating. That's 30mA as you mentioned.

RCDs usually don't have overload protection. The ones that do are quite a bit more expensive, to the point that most installations use two separate breakers next to each other.

An RCBO is a breaker that has both overcurrent protection and residual current protection. It is most commonly used for a single pole switch (like a normal breaker) that has a short neutral and sometimes also earth cable dangling from it which it uses to measure for fault conditions. Like this:

RCBOs are a nice (but expensive) way to give each circuit its own residual current protection, so that you don't trip the entire house/phase each time.

[Tariq]

Yay, another Easysolar GX II member, I am also in the middle of my installation, using two 2.4 Pylontech batteries and eight 355 watt Canadian Solar panelsf

[Wick]

Nice one @Tariq - yes well worth it given the relative ease of installation.

In the end I have gone with 100A Victron midi fuses for 58V - R200 per fuse and R150 for the holder, through Livestainable.

I still feel a fuse is better than a CB between inverter / charger & LiFePO4 where energy density is high and the last thing you want is a battery to discharge too quickly.

As for fuses between cells / batteries... not sold on this aspect if they are all new, balanced cells as mine are. @plonkster have you ever done this?

Edited July 29, 2020 by Wick

[___]

31 minutes ago, Wick said:

fuses between cells / batteries

Between cells? No, why would you want to do that? You need one fuse in the path, and sometimes two (if there is concern for multiple ground faults). It depends also on what the battery maker requires. Traditionally, if you had multiple parallel strings of lead acid batteries, you'd fuse them individually, or at least I've heard that it is advocated, but with the Pylontech modules as above you don't do that cause the manufacturer doesn't say that you should

The battery maker will often put some limits as to how many you may put in parallel, and this is again not because the laws of nature  break down at some point, it has to do with what happens during a fault condition. If you have 10 batteries in parallel and one of them develops some sort of problem and it starts shunting current, then there are 9 others dumping power into the fault one. Under those conditions, you want something to stop that, lest you have a fire.

So... I guess my main point is: Follow the instructions. Fuse the main cabling from the battery. Done.

[Wick]

Thanks. The above makes sense; learnt something new today.

[Wick]

Addition for other folks who might browse this thread...

Use "high rupture current" (HRC) DC fuses between battery bank and inverter / charger / other devices, especially for high discharge rate / low internal resistance battery like LiFePO4.

Why? When a DC fuse breaks, there can still be an electrical arc powerful enough to "jump the gap" - which means your fuse has failed in its task and your goodies on the other end, like your inverter charger, will likely be fried. HRC prevents this. 3A fuse can have a rupture current of as high as 40 000A! Obviously special case, but illustrates the point.

This relates to @plonkster's mention of ceramic fuses - the highest rupture current fuses are nearly always ceramic. Locally, these include Bussman tubular and box fuses, and Littelfuse (also Pudenz) bolt/box fuses. So far, we have been talking about DC fuses sensitive only to current. 
...

Now, as a specific category of HRC, there are semiconductor DC fuses. MIDI and MEGA fuses should generally support high rupture current - but it's on you to check, based on your setup. Semiconductor fuses are made of semiconductor materials that are sensitive to  both current AND voltage. For example, you will see voltage ratings on Victron DC fuses but not on all Bussmann DC fuses - that is the difference in construction talking. 

Semiconductor DC fuses and ceramic fuses are typically different in construction, rupture time and current rating . 

If you're not sure - look on the fuse to see if a voltage rating is displayed. If not, best be careful and get something that is specifically voltage-rated to your application.

If you're not too worried, an AC circuit breaker generally does the same job - current AND voltage based trip - although depending on conditions, it may take longer to trip (higher current) or shorter (higher voltage) as compared with a simple fuse. This delay can mean the difference between your equipment being good or scrap.
 

 

Edited August 3, 2020 by Wick

[___]

1 hour ago, Wick said:

jump the gap

I always tell people to imagine what happens in slow motion. It's a bit like a dam wall that breaks. First there is just the tiniest bit of a gap, and the energy can still jump across. As long as it jumps across it continues to generate heat and more of the fuse material is melted away, increasing the gap. At the same time the air around the gap is ionized and this makes it easier to jump across. The metal itself can also be vaporised (this is of course much worse with copper!) which also improves the conductivity of the ionized air gap.

Now at some point the gap will become too large and the flame will be extinguished. There are some things that happen to speed this up too. With AC current, for example, the fact that the current goes to zero 100 times a second significantly reduces the distance it can jump. If you have some silica around the gap, that also helps to absorb the heat, and the motel silica impedes current flow. While the flame is being extinguished, you also want to keep it from setting other things on fire, and here the silica again helps, but also in some DC breakers there is a magnetic cage that keeps the flame inside the cage.

Of course a major factor in how far the energy can jump is the voltage. Generally 1mm for each 100V (a rule of thumb), but of course much further once the air is ionized or you have some vaporized metal in there.

You can draw a good 1cm flame from a 200V solar string. You should try it, it is quite the party trick. It's only about 10 amps, but do try not to set things on fire...

This is why the fuse has a voltage rating. It has to create a larger gap, and arrest the flame, to interrupt the current flow. And that's why the 60V one is so much more expensive too, compared to the 32V one.

1 hour ago, Wick said:

fuse has failed in its task and your goodies on the other end, like your inverter charger, will likely be fried

The fuse is there to protect the cable. Nothing more. The goodies at the other end has to protect themselves. Quite often, the very reason the fuse has to blow is because the goodie on the other end has already failed

 

[Wick]

Quote

The goodies at the other end has to protect themselves. Quite often, the very reason the fuse has to blow is because the goodie on the other end has already failed 

What is the difference between those fuses, and fuses on the greater circuit? They must be very fast blow, no? I notice Victron does not sell replacements - they have to service the device to replace these.

Why can a fast blow fuse on the circuit, not serve the same purpose?

Edited August 3, 2020 by Wick

[___]

12 minutes ago, Wick said:

What is the difference between those fuses, and fuses on the greater circuit? They must be very fast blow, no? I notice Victron does not sell replacements - they have to service the device to replace these.

I'm not sure what fuses you are referring to here. The smaller Multiplus units have a built-in fuse, which is just a Megafuse. You open it up and you replace it. from 3kVA up you have to fit external fuses, and Victron specifies fuses that are double what you might expect and slow-blow too. This is because the fuse is there to protect the cable.

The inverter itself will switch off to protect itself if there is an overload. The inverter can handle a dead short on the AC side. I've seen a 15kVA repeatedly dead-shorted in an endurance test, running for days on end. When testing overload faults on the smaller inverters, I literally have an AC breaker tied across live and neutral of a 3-point plug...

Fuses are largely reacťive rather than proactive protection. By the time they need to blow, something is already badly wrong...

[Wick]

11 minutes ago, plonkster said:

I'm not sure what fuses you are referring to here. The smaller Multiplus units have a built-in fuse, which is just a Megafuse. You open it up and you replace it.

Yes, referring to the internal fuse in the Victron EasySolar-II unit, should that blow. Mine is  3kva so may need external fusing too.
 

Quote

from 3kVA up you have to fit external fuses, and Victron specifies fuses that are double what you might expect and slow-blow too. This is because the fuse is there to protect the cable.

Hmm, I've been assuming I should use a 100-125A 58V fuse on battery positive. But if that's already what's inside the EasySolar...? I think you previously recommended 500V ceramic fuses? Do you still stand by this or should I get a voltage rating closer to the max current passing through (5500W peak @ 50+V = 100-110A)?

Still seeking 58V fuses at reasonable cost... Victron MIDIs were not available locally...

Edited August 3, 2020 by Wick

[___]

2 minutes ago, Wick said:

I think you previously recommended 500V ceramic fuses?

Oh hang on, I see the confusion. I forget that the context here is the EasySolar-II with the MPPT built in. I'm actually not that familiar with that model. I don't have one on my local bench and I always forget what is in it.

You usually have to fuse between the Multi and the battery. For this I recommend the big Mersen/Jean Muller fuses, but you can also use a Megafuse/ANL fuse of the correct voltage rating. The reason I prefer the Mersen/JM fuses is that they double as a disconnect. The inverter does not have its own fuses on this side, and you must add your own. If that big fuse blows, it is because something else has already broken elsewhere, and you would prefer to not turn the copper cabling into heating elements

But you also have to fuse between the PV modules and the MPPT (which in this inside the inverter), and here I usually use the ceramic 10x38 1000VDC fuses. Much cheaper than Megafuses/ANL. I don't know if there are already fuses inside the inverter, if there are, then I suppose you don't have to fuse another time. Refer to the manual I suppose. I suspect that the engineer will want to see external fuses when he signs off on it.

So yeah... sorry... I got confused between battery-side and PV-side fusing. Battery side needs a Mega or Mersen/JM large fuse. PV side can use the smaller ceramic cartridge ones. Battery side needs 60V or so. PV side needs whatever the PV modules put out, but the DC stuff you buy off the shelve is usually rated for 1000VDC.

[Wick]

1 hour ago, plonkster said:

For this I recommend the big Mersen/Jean Muller fuses

OK, sure, clarified. So here's my quibble with that. Those are all rated 400VAC / 250VDC and upward. Don't get me wrong - I'd love to - as they're available for collection here in P.E. with holders (58mm) and I could go buy them tomorrow and get started - but they're not at anywhere near a 60-100V rating. My EasySolar's rated inverter input is max 66VDC. I would rather fuse accordingly?

Edited August 3, 2020 by Wick

[___]

1 minute ago, Wick said:

OK, clarified. So here's my quibble with that. Those are all rated 400VDC and upward. Don't get me wrong - I'd love to - as they're available for collection here in P.E. with holders (58mm) and I could go buy them tomorrow and get started - but they're not at anywhere near a 60-100V rating. My EasySolar's rated inverter input is max 66VDC. I would rather fuse accordingly?

I'm confused. You are allowed to use a higher rating. Just not lower. Unless the cost is prohibitive of course, then you'll buy the least expensive one that still has a high enough rating.

As far as I am aware you should have a DC disconnect in the system. If you opt to go for a cheaper fuse (such as a 150A Mega Fuse) on the battery side, that usually means the additional cost of a DC switch to disconnect things. The nice thing about those Mersen fused disconnects is they do both, and the price is not exorbitant.

[Wick]

I'm confused too, because I don't get why one would use a 250VDC fuse on connection to a 66VDC (max, 58VDC nominal) input on the inverter. It seems to defeat over-voltage protection... unless there is something I'm not grokking here? I'm trying to protect the EasySolar's input port. Certainly if it were 100-120V (2x) protection even, I could swing that. But 250VDC is 4x higher than the rated voltage on that wire.

About the disconnect - which I presume you mean by physically decoupling the fuse from its holder, or swinging out the holder if it is the hinged sort - noted.

Edited August 3, 2020 by Wick

[Wick]

OK, I'm reading up and trying to understand this. A 250VDC fuse rating means that we can suppress arcing up to and include 250VDC - but not beyond? Is that correct? But if over that figure, we're in trouble?

So we then want the highest fuse voltage rating that we could get, so as to suppress arcing up to 400, 600VDC and so on, if we could get that at the same cost as 250VDC ?

Edited August 3, 2020 by Wick

[Louisvdw]

My 2c worth:

If I understand it correctly, then the fuses are more to protect against the Amps that is going through the cables. Too much amps will melt your cables and you want the fuse to melt first, saving the cable and not starting a fire.

The fuse is not really there to protect against too high a voltage. So long as the fuse is rated more that the voltage that you are using you will be fine. 

To protect against Voltage spikes like in the event of a lightning strike you will need a surge protection device, rather than a fuse.

[___]

1 hour ago, Wick said:

OK, I'm reading up and trying to understand this. A 250VDC fuse rating means that we can suppress arcing up to and include 250VDC - but not beyond? Is that correct? But if over that figure, we're in trouble?

Correct.

Same with capacitors, for example. They come in standard values, 10V, 16V, 25V, 35V, 63V, etc... you buy the next one up from what you need, so if you open a car radio you will see they use 16V caps inside. But sometimes you will see a 25V cap inside a 12V appliance... simply because that's what they had available, or perhaps the price was better. As long as you're rated higher than the expected operating voltage.

7 minutes ago, Louisvdw said:

fuses are more to protect against the Amps

Correct. But once the fuse melts, there will be a potential difference on either end of it, and that's when the voltage rating comes in.