When To Fuse, When not to Fuse?

Determining when fuses are required at PV string outputs and when they are not is often not as clearly defined.  Here I will show how to determine this easily, to help avoid a potential problem, and to save on unnecessary or redundant components in a system.

A common question that we encounter with our customers is whether or not their string inverter comes with string fuses.  The answer to this, depends on the manufacturer.  Some of those include strings fuses whether you will be using them or not and others just provide placeholders for them if you choose to install them or not.  I will explain the reason for this inconsistency.

We all know that the purpose of having fuses, primarily, are to protect the wire and, secondarily, to protect the electrical equipment.  It would seem to be a no brainer to fuse every PV source regardless, but that may be redundant in smaller single or two string systems.  The reason for this is that module manufactures are required to have integrated over-current protection devices (OCPD) of a minimum of 1.56 times their short-circuit current (ISC).

Therefore, in a single string system there is no other OCPD needed at the string output since it is already protected by its internal fusing and no other source of current would be present to backfeed through this circuit

Let’s consider a two string system.  We know that each string in this condition could produce current that is 1.25 times that of their Isc rating.  If a short were to occur the un-faulted string of modules could force its current through the faulted string.  In this scenario we know that the faulted string can already handle 1.56 times its Isc rating so it is still protected.  No fuse necessary.

Now let’s evaluate a system where there are three strings of modules connected in parallel to determine whether or not fusing would be a requirement.  If a fault were to present itself in one string the other two healthy strings could force their currents through the faulted string.  This implies that 2.5 times the Isc rating of the modules could be forced through the faulty string.  In many cases the integrated OCPD of the faulted string would be exceeded, requiring that there be fuses present on the output of each string to protect the wiring and equipment from damage.

That covers the topic of when to fuse and when you may opt not to use them.  Let’s, for a moment, look at two examples where we should sharpen our pencils and evaluate a system to determine if we should need additional fuses or if we can safely avoid them.

Example one:

We are using this module below in a three string system:

It has an ISC rating of 8.87 Amps and a fuse rating of 15 Amps.

If a fault occurs in one string would there be a need for each string in the system to be fused in order to protect the circuit?

8.87A x 2 strings x 1.25 = 22.18A > 15A (FAIL)

So in this scenario we would be required to have fuses on the output of each string to protect for a fault if this ever were to occur in the system.

Example two:

In this system we are using the module below in a three string system:

It has an ISC rating of 9.44 Amps and a fuse rating of 25 Amps.

Now, were a fault to occur in one string in this system do you think we need to have fuses on each strings output?  Let’s discover the answer.

9.44A x 2 strings x 1.25 = 23.6 < 25A (PASS)

As you can see in this example you could omit fuses on the output of each string, all thanks to the rating of the OCPD of the modules.  I chose to show this example because it illustrates that careful evaluation of the modules in the system can help to not only determine when to fuse but also that in certain situations where you may think it is necessary it turns out that it may not.  Considering these variables helps you to ensure that your circuits are indeed protected and in turn could save you from having to add fuses to the circuit outputs when it is redundant or unnecessary.