Dc breaker vs dc fuse

[Pink7]

So i planning for 2 x 16x280a lifepo4 batteries with 2P DC MCB 600V 125A Circuit breaker on each battery. The cables will be paralleled between breaker and Growatt inverters. Should i add any fuse to the plan? Its no fuse mention in the Growatt manual.

 

Pink

Edited January 7, 2023 by Pink7

[Crossy]

That's pretty close to what we have.

 

I would add a suitably rated Class-T fuse at the battery end of the cables as a last-ditch protection. Rate it higher than the MCB and it should never actually open, until it really has to.

[Muhendis]

I think you might have difficulty to finding a supplier of 125A fuses. So not needed.

Make sure the Circuit breaker is rated for DC use.

[Pink7]

1 minute ago, Crossy said:

That's pretty close to what we have.

 

I would add a suitably rated Class-T fuse at the battery end of the cables as a last-ditch protection. Rate it higher than the MCB and it should never actually open, until it really has to.

150A? Can you suggest any Class-T fuse

 

Pink

[Pink7]

5 minutes ago, Muhendis said:

I think you might have difficulty to finding a supplier of 125A fuses. So not needed.

Make sure the Circuit breaker is rated for DC use.

I have 2 x Tomz 2P DC MCB 600V 125A  Circuit breaker FOR PV System

 

Pink

Edited January 7, 2023 by Pink7

[OneMoreFarang]

This supplier has almost everything electrical - for a price.

 

Fuses | Cartridge Fuses | Car Fuses | RS (rs-online.com)

 

[Crossy]

Do ensure that you get the polarity correct on that MCB.

 

For your fuses make sure they are DC rated at a voltage above your maximum charging voltage, something like this: -

 

https://www.aliexpress.com/item/4000548444228.html

 

 

 

[OneMoreFarang]

49 minutes ago, Crossy said:

Do ensure that you get the polarity correct on that MCB.

 

For your fuses make sure they are DC rated at a voltage above your maximum charging voltage, something like this: -

 

https://www.aliexpress.com/item/4000548444228.html

 

 

 

@Crossy would you buy something like that from AliExpress? Or maybe: Would you buy it from any shop on that site or only from selected shops?

Obviously we all don't want to spend more money than necessary and the supplier which I mentioned is probably 5 times as expensive. But then, with a 100A fuse, the most important thing is that it actually works and not that it is cheap.

What is your opinion about where you buy things like that? 

 

[Pink7]

1 hour ago, Crossy said:

That's pretty close to what we have.

 

I would add a suitably rated Class-T fuse at the battery end of the cables as a last-ditch protection. Rate it higher than the MCB and it should never actually open, until it really has to.

Instead of bolting up some busbar on the wall i was thinking of doing it this way: There is 2 red cables from DC breaker and 2 red cables going to inverter so my plan was to bolt the 2 cables from DC breaker in the one end of  copper bar then bolting the  2 cables to inverter in the other side of the bar then shrink tubing around it so its just a part of the wire from A to B. Same for black. Any issues or problem with this solution? 

 

Pink

[Crossy]

RS are about double the price for a similarly rated fuse https://th.rs-online.com/web/p/tag-fuses/1884604 (only 150V DC but would do the trick).

 

Our fuses (Bussmann) came from my brother in Oz who does solar installations professionally after I asked if ANL (fork truck) fuses were OK at 60V. He classed them as "marginal" but many "48V" solar systems are using them.

 

Would I get them from Alixxx? Very good question. If I was doing again I'd probably get them from RS or from a UK specialist.

 

EDIT The more observant will note that my photos of our battery pack show ANL fuses. These have been replaced courtesy of my younger bruvver ???? 

 

 

 

 

[Crossy]

46 minutes ago, Pink7 said:

Instead of bolting up some busbar on the wall i was thinking of doing it this way: There is 2 red cables from DC breaker and 2 red cables going to inverter so my plan was to bolt the 2 cables from DC breaker in the one end of  copper bar then bolting the  2 cables to inverter in the other side of the bar then shrink tubing around it so its just a part of the wire from A to B. Same for black. Any issues or problem with this solution? 

 

Pink

 

I like any bolted connections to be accessible for maintenance, but I don't see any real issues with your proposal.

[TronxII]

There must be a fuse. These circuit breakers cannot be trusted.

For the DC fuse it is important to be filled with sand to extinguish the arc.  The linked datasheet does not mention the sand, but to break 50k A as they claim, there should be sand inside.

[Crossy]

7 hours ago, TronxII said:

These circuit breakers cannot be trusted.

 

What makes you say this?

 

MCBs have been used for decades on both AC and DC and provided you use a recognised brand are very reliable.

 

A correctly rated fuse is vital insurance of course to back up the protection from the BMS electronics and the MCB.

[Pink7]

What's best practice regarding cable length. Equal length from breakers to batteries and /or equal length cables from breakers to busbar? or to inverters? I'm my plan i expected to be equal on cables within each battery via breaker and to busbar. Then different length from busbar to inverters. but i got confused when watch this video with Will

 

 

Pink

Edited January 8, 2023 by Pink7

[Crossy]

12 minutes ago, Pink7 said:

What's best practice regarding cable length.

 

Generally, you want cables of the same function to be about the same length.

 

So: -

 

Battery +ve's to bus bar = same length

Battery -ve's to bus bar = same length (but doesn't have to be the same as the +ve's)

 

Similarly, for your inverters.

 

Unless you are actually paralleling cables the length matching isn't super-duper critical, say about 5%.

[Pink7]

14 minutes ago, Crossy said:

 

Generally, you want cables of the same function to be about the same length.

 

So: -

 

Battery +ve's to bus bar = same length

Battery -ve's to bus bar = same length (but doesn't have to be the same as the +ve's)

 

Similarly, for your inverters.

 

Unless you are actually paralleling cables the length matching isn't super-duper critical, say about 5%.

Thanks for the clarification Crossy.

 

Pink

[TronxII]

18 hours ago, Crossy said:

What makes you say this?

 

MCBs have been used for decades on both AC and DC and provided you use a recognised brand are very reliable.

Most of this stuff is not anymore what it was 30 years ago. Especially what you can buy in Thailand, is all from China with different quality, mostly in the lower range.

 

Once I lived in the UK, and the house had a power cut from time to time. Usually the main breaker would switch off. Probably we drew too much power. But it also happened when nearly nothing was on.

On one occasion, we had the power cut again, but the main breaker was on.  Electricity company came, measured, no power. Next day, they started to dig a hole in the street, took them a day, they didn't find anything.

In the end, the electricity guy switched the main breaker off and on again, and instantly the power came back. We looked at each other like idiots.

Neither me nor them have tried to switch the main breaker off, because it was on. That breaker was not a RCD.

Since then, I do not trust these breakers very much.

I don't have a picture of the breaker, but I documented the emergency measures, and I was very proud in these days to provide 230V out of the car battery with a UPS.

[Muhendis]

6 hours ago, TronxII said:

Most of this stuff is not anymore what it was 30 years ago. Especially what you can buy in Thailand, is all from China with different quality, mostly in the lower range.

 

Once I lived in the UK, and the house had a power cut from time to time. Usually the main breaker would switch off. Probably we drew too much power. But it also happened when nearly nothing was on.

On one occasion, we had the power cut again, but the main breaker was on.  Electricity company came, measured, no power. Next day, they started to dig a hole in the street, took them a day, they didn't find anything.

In the end, the electricity guy switched the main breaker off and on again, and instantly the power came back. We looked at each other like idiots.

Neither me nor them have tried to switch the main breaker off, because it was on. That breaker was not a RCD.

Since then, I do not trust these breakers very much.

I don't have a picture of the breaker, but I documented the emergency measures, and I was very proud in these days to provide 230V out of the car battery with a UPS.

Well yes. Equipment does fail from time to time but I note yours failed "safe" which is normally the default. 

I expect, when this happens again, you will check your isolator/CB before having companies dig holes to look for faults.

With the exception of a Safe T Cut, I have had a few MCB devices fail but always they have failed open circuit which, whilst a nuisance, is not dangerous. 

[TronxII]

9 hours ago, Muhendis said:

always they have failed open circuit which, whilst a nuisance, is not dangerous. 

Well, if they always fail open then this is fine. Trusting these mechanical devices from China for a battery which can deliver 1kA , well ... yeah....

For me, a sand filled fuse very close to the battery is a must, so I recommend it to the newbies.

[Crossy]

33 minutes ago, TronxII said:

For me, a sand filled fuse very close to the battery is a must, so I recommend it to the newbies.

 

As last-ditch protection after the BMS and then the MCB have failed to do their job. Definitely.

 

In normal operation (or most abnormal operation) the fuse should never open so it should be a one-off cost.

[Pink7]

Is there any reason for not parallel my 2 batteries by just parallel them at the battery breaker with wires between the breakers?

 

Pink

[Muhendis]

29 minutes ago, Pink7 said:

Is there any reason for not parallel my 2 batteries by just parallel them at the battery breaker with wires between the breakers?

 

Pink

That would be quite normal.

What I did was to place a breaker/isolator in series with each battery before the parallel connection.

This is to ensure that if a battery goes faulty, ie short circuit, then current from the other battery will trip the breaker. 

My batteries are Gel not lithium.

With lithium there should be a BMS for each of the parallel batteries which should protect the batteries from each other.

It's always useful to include a breaker/isolator for maintenance for each battery. 

[Pink7]

I will parallel the breakers at the inverter side ( bottom part of breaker) So the cables between the 2 breakers will be a busbar replacement.

 

Pink

[Crossy]

We have: -

The BMS

Fuses at the battery

Individual per-battery MCBs in a box with the bus-bars

 

A separate (larger) MCB in the cable feeding the inverter from the bus-bars. If you have multiple inverters use one MCB per inverter (allows you to isolate one unit if it goes faulty).

[Pink7]

My "easy" plan to parallel the breakers with just wire between will not work, the battery cable kind of expand when strip it so there is no chance to get 2 of them in the breaker. I also see that its quite a challenge to get 35mm2 cable in to 35mm2 lugs, i might need to return the 35mm2 lugs and get 50mm2 lugs.

 

Pink

[Pink7]

I might can use this terminals 2 pieces back to back to get wire between the breakers. 

 

Pink

[BritManToo]

On 1/26/2023 at 9:16 AM, Pink7 said:

Is there any reason for not parallel my 2 batteries by just parallel them at the battery breaker with wires between the breakers?

 

Pink

I using LifePo4 or Lithium just make sure the voltages are exactly the same (within 0.1V) before connecting them together.

[sezze]

47 minutes ago, BritManToo said:

I using LifePo4 or Lithium just make sure the voltages are exactly the same (within 0.1V) before connecting them together.

That's for serial , not for parallel use . You can parallel different banks together , but each serial bank need its own BMS as 1 might/will die sooner then the other .

[BritManToo]

2 minutes ago, sezze said:

That's for serial , not for parallel use . You can parallel different banks together , but each serial bank need its own BMS as 1 might/will die sooner then the other .

You're wrong.

[Crossy]

1 hour ago, Pink7 said:

I might can use this terminals 2 pieces back to back to get wire between the breakers. 

 

Aren't those intended to interface Aluminium cables to copper rated terminals?