We get this question from customers fairly often:
"The voltage and current are the same. Why can't I use a normal DC fuse?"
Sometimes you can find a DC fuse with exactly the same numbers as a PV fuse.
For example:
1500V DC / 20A
The marking looks right.
But we would not approve a replacement based on those two numbers alone.
A PV fuse is selected for the way a photovoltaic circuit behaves, not just for the voltage and current shown on the label.
Look at the Circuit First
A conventional DC fuse can be used in many types of equipment.
It may protect a battery circuit, industrial power supply, control circuit or other DC load.
A PV string is different.
The current available from a solar module is relatively close to its normal operating current. A PV array can also contain many strings connected in parallel.
That creates a different fault condition.
For this reason, when we check a PV fuse application, we start with the circuit rather than the fuse catalogue.
The Problem With Parallel Strings
Take a simple combiner box with ten PV strings.
Each string is connected to the same DC bus.
If one string develops a fault, the other nine strings do not simply stop working.
They can continue to feed current into the faulted string.
That current can flow in the reverse direction through the affected string.
The string fuse may therefore need to interrupt current supplied by the other strings.
This is one of the main reasons PV string protection needs to be considered separately from a general DC circuit.
"But the Fuse Is Rated 1500V DC"
That still doesn't settle the question.
Suppose a standard DC fuse is marked:
20A / 1500V DC
and the PV project requires:
20A / 1500V DC
It looks like a direct match.
Before using it, we would still want to know:
What application was the fuse designed for?
What standard was it tested against?
What is its rated breaking capacity?
Has it been tested under the required PV conditions?
What are its temperature limits?
Is the fuse suitable for the maximum PV system voltage?
Does the fuse holder have the same level of suitability?
Without this information, we would not treat it as a direct substitute.
The PV String Voltage Also Needs Checking
Another point that gets missed is the actual maximum string voltage.
A module's open-circuit voltage, Voc, changes with temperature.
When the temperature drops, Voc increases.
So if a customer tells us:
"The system voltage is 1500V."
we may still ask for the module Voc, the number of modules in the string and the minimum site temperature.
For example:
Voc = 49V
28 modules per string
Basic calculation:
49 × 28 = 1,372V
But 1,372V is not necessarily the maximum voltage that the fuse will see.
If the module temperature falls, the string voltage can rise.
The equipment therefore needs to be selected using the appropriate maximum string voltage, not simply the nominal operating value.
Breaking Capacity Matters
The next question is what happens when the fuse actually opens.
A PV fuse needs to interrupt the available DC fault current safely at the specified voltage.
This is not the same thing as saying:
"The fuse will melt at 20A."
The melting behaviour is only part of the job.
After the fuse element opens, the circuit still has to be interrupted safely.
There is an arc.
The fuse has to control and extinguish that arc under the specified DC conditions.
This is where the fuse's construction, voltage rating and breaking capacity become important.
A Higher Voltage Rating Doesn't Automatically Make a Fuse Better
We sometimes see customers choosing a higher-rated fuse simply because they think it gives them more safety margin.
That isn't how we normally approach it.
If the system requires 1000V DC, a properly specified 1000V PV fuse may be exactly what is needed.
If the system is designed for 1500V DC, then a suitable 1500V PV fuse is required.
There is no need to choose the highest number in the catalogue just for the sake of it.
The rating should correspond to the actual system.
Don't Forget Temperature
Temperature comes into the calculation in two different ways.
At low temperatures, the module Voc increases, which can increase the maximum string voltage.
At high temperatures, the fuse and fuse holder operate under different thermal conditions.
This matters in outdoor combiner boxes.
A box installed in direct sunlight can become much hotter than the surrounding air, particularly in places with a hot climate.
For a project, we may therefore ask about both the minimum and maximum operating temperature.
These two numbers tell us different things.
The Fuse Holder Is Part of the Protection System
There is little point in selecting a suitable PV fuse and then putting it into a holder that is not rated for the same application.
The holder needs to be checked for its voltage, current, insulation, creepage and clearance, dimensions and temperature characteristics.
This is particularly important with 1500V DC systems.
For this reason, when we quote a PV fuse for a new project, we usually prefer to confirm the holder at the same time.
What About UL or TÜV Certification?
Certification needs to be checked carefully.
A DC fuse may carry a UL or other certification and still not be the right fuse for a particular PV application.
The important question is not simply:
"Does the manufacturer have UL?"
It is:
"Does this particular fuse model have the certification and ratings required for this application?"
For project work, we normally check the actual model, rating and corresponding documentation.
That avoids problems later when the fuse has to be included in the customer's technical file.
So When Can a DC Fuse Be Used?
A standard DC fuse is not a bad product.
There are plenty of applications where it is exactly the right choice.
If you are protecting an industrial DC circuit, battery circuit or another DC load, a conventional DC fuse may be appropriate.
The problem is using it in a PV string simply because the voltage and current happen to look the same.
For a photovoltaic application, the fuse needs to meet the requirements of that particular circuit.
If a standard DC fuse has been specifically tested and rated for the required PV application, then it can be evaluated.
But that needs to be confirmed from the product documentation.
How We Check This at Chifeng
When a customer sends us a standard DC fuse and asks:
"Can we use this instead of your PV fuse?"
we normally don't answer from the label.
We ask for the datasheet and check it against the PV system.
Usually we need:
Maximum system voltage
Module Voc
Module Isc
Number of modules per string
Number of parallel strings
Maximum series fuse rating
Expected fault current
Minimum and maximum temperature
Fuse holder
Required standard
Certification requirement
For a straightforward project, these details are usually enough to make the comparison.
For a larger system, a single-line diagram is useful as well.
Our Approach at Chifeng
Chifeng has been manufacturing fuses since 1988.
Over the years, the applications have changed, but the basic engineering question has not:
What does the circuit need the fuse to do when something goes wrong?
For PV applications, our product range includes fuses for 1000V and 1500V DC systems, with different current ratings and configurations.
We also supply matching fuse holders and can provide the relevant technical and certification documents according to the product and project requirements.
For an OEM customer, we prefer to confirm these details before the equipment design is finalized.
It is much easier to get the protection specification right at the beginning than to change it after the combiner box has already been produced.
So, Can You Use a Standard DC Fuse?
Not just because the current and voltage ratings are the same.
Before replacing a PV fuse with a standard DC fuse, check the actual product against the PV application.
At a minimum, look at:
Maximum system voltage
Maximum string voltage
Current rating
Breaking capacity
PV application requirements
Temperature
Fuse holder
Applicable standards
Certification
If the standard DC fuse has the required ratings and has been properly tested for the application, then it can be considered.
If not, we would stay with a PV-specific fuse.
The fuse is a relatively small part of a solar installation.
Saving a small amount on the fuse is usually not worth taking a chance with the protection of the whole string.
At Chifeng, this is how we approach the question:
We don't start by asking which fuse is cheaper. We start by asking what the PV circuit needs the fuse to do.
