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Disconnector fuse question


JimboJambo

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Currently receiving my loadshedding gear and the AC kit just arrived. For preface I have a Luxpower SNA5000 and a Dyness BX51100. The AC kit I bought was supposed to ship with 80A fuses for the battery disconnect but found there to be 160A fuses in the box.

Are these fuses ok to use for my setup or should I be requesting smaller fuses to be sent to me?

Edited by JimboJambo
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7 hours ago, JimboJambo said:

Currently receiving my loadshedding gear and the AC kit just arrived. For preface I have a Luxpower SNA5000 and a Dyness BX51100. The AC kit I bought was supposed to ship with 80A fuses for the battery disconnect but found there to be 160A fuses in the box.

Are these fuses ok to use for my setup or should I be requesting smaller fuses to be sent to me?

That battery has a maximum charge/discharge of 75A. The normal levels are 50A.

Thus you would rather blow a fuse than exceed 75A.

If your inverter has settings for these maximums I would use the 80A only but set the inverter on say 70A.

If the inverter does not cater for these settings rather use a 63A fuse to stay save. 

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@JimboJambo  Fuses are there to protect the battery cable from melting. What is the max rated discharge current of your battery cable? If it is less than the fuse then get bigger cable or a smaller fuse. It has nothing to do with the battery nor inverter draw

Edited by iiznh
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25 minutes ago, JimboJambo said:

the battery cable is 35mm2, would that be sufficient? sorry im very new to this so all help is appreciated 🙏

35mm² battery cable is rated at 105 amps ensure equal lengths(+-) and short as possible

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3 minutes ago, TaliaB said:

35mm² battery cable is rated at 105 amps ensure equal lengths(+-) and short as possible

the battery will be directly next to the inverter, so nothing too far. In this case 100A fuses should be sufficient? or would lower be preferable?

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23 minutes ago, JimboJambo said:

the battery will be directly next to the inverter, so nothing too far. In this case 100A fuses should be sufficient? or would lower be preferable?

I would use 80 amp dc fuses as the battery would be the limiting factor your inverter would be able to deliver 3300 watts and your battery current would be 75 amps, maximum for your battery. 160 amp fuses are too high get them changed.

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Always very interesting to read how the solar fraternity choose their cables based on what someone posted and read. Generally your thermal overload for things like motors or transformers are set at about 1.25 x the continuous full load current. Cable protection fuses are at 2 x the continuous current. 

The problem with solar is we see we use fuses based on the cable size. When thicker cable are used to prevent volt drop we will never choose a fuse sized on the cable size. If the continuous load is say 12A but the volt drop calculation indicates to use 16 sq mm cable one will not use a 63A fuse/MCB but rather a 16A. For inverters we take the maximum current the battery can supply per spec and add the 25%. 

The example of a battery that can supply 100A for 15sec is not used as one cannot ensure the fuse will blow within 15sec and you NEVER want to rely on the BMS to switch off the system. We see BMSes failing and one should never think oh it will not happen with mine. Relying on the weakest module in the system adds a lot of risk. 

By the way cables can withstand an easy 10 x rated current before the heat up enough to be a problem. By this time the fuses would have done their job. 

Just my 2c as applied over decades on many projects. Feel free to find someone who agrees with your way of thinking but it could be risky. 

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Unless a device is designed with shunt protection, a fuse can not protect it.

Any current flowing through the fuse is already flowing through the device.  The electronic protection within the device will react far faster and far more accurately than any fuse can.  If damaging currents are flowing through the device, then the device is already broken.  The fuse is just there to prevent the fire once the device can no longer protect itself.

Using an undersized fuse though can cause significant damage - especially if it is on a battery cable under charge, in which case it will generate a massive surge on the charger output.

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1 hour ago, JustinSchoeman said:

Unless a device is designed with shunt protection, a fuse can not protect it.

Any current flowing through the fuse is already flowing through the device.  The electronic protection within the device will react far faster and far more accurately than any fuse can.  If damaging currents are flowing through the device, then the device is already broken.  The fuse is just there to prevent the fire once the device can no longer protect itself.

Using an undersized fuse though can cause significant damage - especially if it is on a battery cable under charge, in which case it will generate a massive surge on the charger output.

Very well articulated. I will still stick to using fuses. There are times that you have to investigate why did it blow and not just replace or double it it up. 

I see a couple of strands of wire wrapped around glass fuses on a daily basis. The interesting thing it's never the client or installer who did it. 3 times more reverse polarity than wire wrapped around a fuse. Luckily the PCBs tend to make it without serious damage but a lot of PCBs with tracks burnt off that could gave been prevented. 

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