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[♪ Music ♪]
You open your solar kit and pull out the isolator, how do you know if it is suitable for the
installation you are about to do?
Hello my name is Mick and I'm going to run you through how to check whether a DC isolator
is suitable for a particular PV installation.
Just because it comes in the kit, doesn't necessarily mean it's the right one for this
job.
A lot depends on how you connect your PV modules.
There are some basic steps you will need to follow to ensure that the DC isolator you
are about to install complies with 5033 and is suitable for the solar system design.
You should also know that 5033 states that DC isolators shall have a utilization category
of at least DC21B.
What does this mean?
They are not tested for rapid on/off operation. A cooling down time of 30 seconds is required
between operations, otherwise you will destroy the contacts and the isolator may catch fire
under full load. Before you begin installing, take a few minutes to check the isolator rating
for your own piece of mind.
So now let's get started.
You have un-packed the kit, got the isolator out, now lets check and see if it is suitable.
First off, check that the isolator is non-polarised.
You can no longer use polarised switchgear.
If that is OK then does it comply with the International Standard IEC60947?
If it is yes to both of these then it may be OK to use subject to the voltage and current
ratings required.
So the next step is to work out the string configuration so you can calculate the system
voltage and current.
When calculating your string voltage and current, remember to de-rate for the lowest expected
operating temperature. This gives you the maximum module voltage. There is a formula
provided in 5033 that allows you to calculate PV array maximum voltages using manufacturer's
specifications.
You also need to determine the current rating.
This is a matter of multiplying the ISC of the array by 1.25.
Remember that 5033 has examples of different array configurations; you need to select the
one that matches.
Now, remember that the type of inverter you are installing with the system, will determine
the current rating required. That is, whether it is transformer or transformer-less, or
more correctly separated or non-separated.
For a separated inverter, both poles of the DC isolator will share the array voltage and
current. Whereas for a non-separated inverter, each pole must be capable of breaking the
array voltage and current.
For ease, and in case the inverter is changed at a later date, we recommend the isolator
be selected based on the non-separated inverter and each pole is capable of breaking the whole
array voltage and current under load.
With your calculated voltage and current, you can now check the manufacturer's specification
sheet.
If your calculated values are less than the manufacturer's values, then the isolator is
OK to use.
If it's NOT suitable, don't use it, take it back.
Just to re-cap, to make sure you are using correctly rated DC isolators:
Work out your array configuration.
Check whether the inverter is separated or non-separated.
Calculate the voltage and current ratings at the coldest temperature.
Check the manufacturer's spec sheet for the DC isolator for suitability
and remember to let the isolator cool down between operations.