What is solar DC string fusing?

With all electrical circuits some form of protection is required and solar DC cable is no exception.

Solar DC string fusing is protection against reverse currents if there is a fault condition and is designed to make sure the cabling is not damaged or compromised.

Fusing is required when paralleling multiple strings of solar panels and can be in the form of circuit breakers and DIN rail mounted slow blow fuses. This fusing is sometimes contained inside some large three phase inverters as an option.

Fuses

Fuses have three unique characteristics: 

• They are safe. Modern fuses have extremely high breaking capacities 
• Can withstand very high fault currents without rupturing.
• Fuses provide optimum protection by keeping fault currents to a low value

They are said to be current limiting, are a cost effective protective device for solar panels and the most common form of solar DC string fusing.

Where is string fusing located?

String fusing can be located at various points of the solar array. For example, string overcurrent protection where string cables join a sub array or array cables in a string combiner box, for sub array overcurrent protection where the sub array cables join the array combiner box and for array overcurrent protection where the array cables join the PCE (Inverter).

For more info on sub array/array configurations see AS/NZS5033, clause 3.3.6, figures 2.2 - 2.4.

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How do I calculate when string fusing is needed?

Ascertaining whether fusing is required involves the following process:

1. Find the short circuit current of the panel in question,
2. Find the panel’s Max fuse rating in Amps, reverse current rating.

Apply the following formula, ((SA - 1) x Isc MOD) > IMODMAX OCPR where:

• SA is the number of parallel strings,
• Isc MOD = Short circuit current of the module in question,
• IMODMAX OCPR, Max fuse rating in Amps/ reverse current rating.

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String fusing example

In our example we have:

• Panel Short-Circuit Current (Isc)  = 9.79A
• This panel has a max series fuse rating of 15A
• We are looking to parallel 3 x strings

So the calculation is  ((3 - 1) x 9.79 = 19.58 >15A

Yes to DC string fusing!

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Now what? 

The next step is to size the fuse correctly. This is where the designer/installer has to drag out one of the Australian electrical standards (in this case AS5033) and look at Clause 3.3.5.1:  PV string overcurrent protection.

Where "In" is the rating of the fuse or circuit breaker:

• In >  1.5 X ISC MOD and In <  2.4 X ISC MOD and In <= IMOD MAX OCPR.

A lot to digest so here’s the example: 

We have 3 x parallel strings, Isc of panel 9.79A so 9.79A x 1.5 = 14.68A. 

Meaning we can select a 15A fuse as 9.79 x 2.4 = 23.5.  The 15A fuse is < 23.5!!

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Conclusion

Solar DC fusing is protection against reverse currents if there is a fault condition and is located where multiple strings converge and connect. To calculate, determine the number of parallel strings, subtract one and multiply by panel Isc.  If this exceeds the panel's max series fuse rating, string fusing is required and then you need to determine the size of fuse. As always, arm yourself with the relevant Australian Standards, methodically go through the process and there you are.