Electrical Safety

It is the amount of current which passes through your body which determines the lethality and shock effects, not the voltage or power. You can “feel” an electric shock when you get about 1mA passing through you and you will die at about 60mA as your heart stops beating. This current can come from both AC or DC sources, although AC is generally considered more dangerous as it can cause your heart to loose coordination at a lower current (60mA). From our previous section you should now understand that current can be calculated using the equation I = V / R.

The resistance of human skin varies greatly depending on if it is dry, wet, or broken (a wound). According to https://en.wikipedia.org/wiki/Electric_shock it can vary between 1000Ω and 2125Ω from one hand to the other (the most common way that people get zapped) and as high as 100,000Ω depending on the path taken by the electricity through the body. If you assume worst case scenario and you touch a 240VAC connection with one hand whilst the other is “grounded” then the current flow is I = 240 / 1000 = 0.24A or 240mA which is more than enough to kill you in a fraction of a second.

Working in reverse, what voltage is actually enough? Well if we take V = IR then V = 60mA x 1000Ω which is V = 0.0060 x 1000 = 60V. This is handy to know as it tells you that working with 12V, 24V or 48V battery systems is the voltage is unlikely to kill you… however your solar panels are connected in “strings” which increases their voltage to typically around 150V or more, so they most definitely could kill you.

Fire Risk

Now one thing that you have to get right is the wiring of your solar array back to the charge controller (see here) or you can risk burning down your home by causing an electrical fire.

Water and electricity just do not mix. You have to take extra, extra care to ensure that all of your connections are both water proof and fused.

When purchasing enclosures that are going to be exposed to rain (such as on the roof with your solar panels), you must ensure that they are rated at least IP55 or better. The IP rating system is used by all manufactures to class their products against solar particles and water. You can read more about IP ratings here.

Glands

When building enclosures / junction boxes / etc for your solar system, you will need to purchase what are known as glands. These are threaded parts that either screw into pre-threaded holes on the junction boxes or into holes that you have drilled yourself. Typically they are 25mm for most applications, however you can get smaller and larger if required.

A 25mm Gland Fitting

When you are feeding solar cables into the glands (making a solar junction box) you will need to protect again water ingress along the cable. This is done by adding cable gland inserts into the gland through which cables pass. When the gland is then tightened, the rubber gland insert compresses against the cable making it water tight. They come in one, two, four and six hole versions.

A Cable Gland Insert (2 hole)

DC Isolation Switches

On the roof where your solar panels are located, you will need to fit a DC Isolation Switch (about $40 each) to turn the supply of energy from your solar array off when you need to do so (for example when replacing the charge controller).

Solar rated DC Isolation Switches are readily available and will accept up to 1200V input voltage at typically a maximum of 32A. They are water proof as long as you ensure the correct glands and inserts are used.

DC Solar Isolation Switch

Australian Standards make it a requirement that the DC Isolation Switch is fitted on the roof on the same rails that mount your solar panels. To protect the switch from the rain and sun, you must also fit a solar isolation switch cover.

Solar Isolation Switch Cover

Fusing

To ensure that your installation does not catch fire, all components in your system must be correctly fused to prevent overload of the wiring.

You will require fusing in place for the strings of solar panels (so called combiner fuses) as well as fuses between the charge controller and battery array and between the battery array and the inverter. As a rule, each fuse should be rated at 125% of the current you expect to flow through a specific part of your system.

There are two types of fuses in use in solar installations: DC Circuit Breakers and NH style fuses.

DC Circuit Breakers

DC Circuit Breakers are typically employed for fusing the solar panel array. They have the advantage that once “tripped” they can be simply turned back on and if they trip again, well you know you have a problem.

Wiring of DC Circuit Breakers must be done correctly. There is an excellent article here that provides information on the correct way of doing this.

Double-pole DC Circuit Breaker

The main thing to realise is that when connecting the circuit breaker up you are connecting through both the positive and negative (earth) lines from your solar panels to the charge controller. This is why when purchasing DC Circuit Breakers they come as a double-pole breaker (you can see in the image above that the switch part of the breaker is actually connected across both breakers making a single double-pole unit).

To figure out what capacity DC circuit breaker you need to use, you need to know the short circuit (Isc) value for your solar panels. This is part of the specifications for any panel as given by the manufacturer.

The calculation is then simply the Isc of the panels multiplied by a 125% percent safety factor, then rounded up to the nearest breaker size.

For example an Isc current of 5.78A multiplied by a 125% percent safety factor equals 7.22A. The next standard breaker size is 10A, so you would purchase a 10A rated DC circuit breaker.

WARNING: Note that there is an assumption here that you are connecting your solar panels in series (each panel is plugged into the next, like a battery, in one long string). In series voltage is increasing but supplied current is not. If you connect your panels in parallel, your voltage will stay the same but the supplied current will increase by Isc for each additional panel. You must also not exceed the maximum voltage rating of the DC circuit breaker, typically this is so high that it is unlikely for you to exceed it, but you must be aware!

NH Fuses

NH fuses (also known as NH Knife Blade Fuses or DIN NH Blade Fuses) are general purpose fuses for the protection of conductors. NH fuses are one-time fuses, meaning once they have blown, they must be replaced with a new fuse with the same characteristics.

You will most probably use NH fuses to protect your charge controller and inverter as DC Circuit Breakers in the higher (80A+) range start getting very expensive and you may be unable to source them anyway.

NH Style Fuse Components

When it comes to protecting your batteries and inverter, you will generally go with NH style fuses as they are rated for much higher amperage than DC Circuit Breakers. When designing the fusing for your system, you need to allow 125% of the rated capacity of the device in question as the rating of the fuse. For example, a 60A solar charger must have a minimum 75A rated fuse on the charging / output side of the charger.

When purchasing NH style fuses, you will need both the fuse and a fuse switch disconnector. Double pole disconnectors are convenient for disconnecting both solar and battery conductors simultaneously. Most of the ones I have come across on eBay are 3 pole. The handle that you can see at the top of the disconnector below when pulled down extracts all the fuses and disconnects the power.

Example of a 3 pole NH style fuse disconnector

The NH fuses themselves are rated for different voltages and also the manner in which the fuse will blow or type. If you look on a NH fuse, you will see that it has the ratings on the fuse plus its type, on the sample below the type is “gG” (look just above and right of where it reads “~500V”) and the maximum voltage is 500V.

A sample NH style fuse

The table below gives the different types of NH fuses and what they are typically used for. For our purposes, protecting the inverter and charger, you would get gG type NH fuses.

ApplicationTypeNH Fuse SizeVoltage
General PurposegL / gG000 to 4a500VAC to 690VAC
Motor protectionaM000 to 4a500VAC to 690VAC
Semiconductor protectionaR / gR000 to 3690VAC
Semiconductor protectionaR / gR00 to 01000VAC
Solar PV protectiongPV1 to 31000VDC
Battery protectiongS000 to 3440VDC to 550VDC

In addition to all the above, NH fuses also come in a range of different sizes. For solar applications, size 00 is the most common used.

SizeBody TypeOverall
Length
Body
Length
Body
Width
NH 000Standard795321
NH 00CCompact795321
NH 00Standard795328
NH 0Standard1256828
NH 1CCompact1356828
NH 1Standard1357246
NH 2CCompact1507246
NH 2Standard1507254
NH 3CCompact1507254
NH 3Standard1507265
NH 4Standard20075100
NH 4aStandard2007595