Electricity from the sun is cheap, but don’t think that way about PV installation gear. Phil Kreveld tells how to get the best out of those essential bits and pieces.

Using the right installation equipment for a solar photovoltaic rooftop system ensures maximum operating life – and customer satisfaction.

When the solar installation industry was in its infancy there was some excuse, or at least an understanding, for very poor choices of cabling, switchgear, junction boxes and conduit.

There were even instances of AC-designed switchgear being used for DC carrying circuits. We hope that doesn’t happen these days, but there is still much room for improvement in durability and safety.

If there’s any unfamiliarity on the part of the installer it is likely to be on the DC circuit side. The difference is not so much in current ratings for cables (we are always dealing with rms AC current as equivalent to the same value DC current) but in the switchgear.

DC current is difficult to interrupt. There are no ‘swing through zero points’ for the arc, which can be drawn on opening a switch, to be extinguished.

For AC, load-make and load-break are about the same challenge, whereas for DC load-make is not a problem but load-break is. In practice the DC interrupters (load-break, since there is no way we can stop the sun from shining on the PV panels) are bulky compared with their AC current rating equivalents.

In AC switching, the interruption can occur anywhere in a 10 millisecond period. Even the possibility of drawing an arc on opening the circuit is minimised if that happens to be near a crossover point. The speed of opening contacts is therefore less critical in AC than in DC circuits.

In DC switching, the idea is to build up voltage across the switch rapidly. If this doesn’t happen an arc will strike and, because of its low impedance, current will continue to flow.

A well-established technique of dealing with the interruption problem is to use an arc-extinguishing method. To handle this in a compact device, rather than an air blast to lengthen the arc, a permanent magnet is used.

The left-hand rule (Figure 1) shows how it works. We are relying on the force F to blow the arc (current-carrying conductor) away from the contact points.

It’s a neat way of solving a problem, but the use of such ‘polarised’ devices can be problematic. If not connected properly (ie: current polarities not observed), or if current direction can reverse, the arc will be sucked in instead of being blown out.

In the case of compact switches, the heat generated as a result can provide all the conditions for a fire to start. For this reason, Australian Standards stipulate that polarised devices can no longer be used.

The multiple-contact ganged switch, although bulkier by virtue of three or four contact sets, is superior. Furthermore, both conductors in a DC circuit can be interrupted (this cannot be done conveniently in the permanent magnet device).

In most installations two isolators are required: on the roof and at the input to the inverter. For transformer isolating inverters you will need a DC breaker or isolator that is double pole (breaks negative and positive simultaneously).

Switches should be rated to break 1.25 times the short circuit current (Isc) rating of the solar PV array and 1.2 times the open circuit voltage (Voc) of the array. Look carefully at suppliers and the specs because this is a critical area of an installation.

If you are aware of polarised switches or breakers in existing installations, you must replace them. They are not allowed under AS/NZ 5033. Ganged switches suitable for solar installations come in current ratings of 8, 10, 16, 20, 25, and 32 amps, and voltages of 250, 440, 500, 800 and 1000V.

The rooftop installation is an example of unprotected consumer mains, though DC rather than AC. The regulations require DC conductors, where otherwise exposed, to be housed in heavy-duty conduit, obviously resistant to UV degradation and vermin.

Note that in some panel mounting rails, provision is made for routing cables. However, the installer needs to ensure that entry slits are too small for any creatures to enter. In general, it is better to err on the side of safety and enclose all conductors in conduit without over bunching, as this will lead to heat problems.

In terms of total system cost, DC conductors account for a small proportion (2-5%), but bad cabling is responsible for 7-10% of installation problems. In many instance this has caused a fire.

Cable ties, clips and other attachments should be properly used so that cable electrical properties are not compromised. A PV installation is in the open, and movement is something that installers have to factor into cable management.

Installers use different ties to attach cables to frames or other supporting features. Cable ties are usually UV-stabilised and available in a wide range of tensile strengths, bundle diameters and styles. However, if they are not UV-rated they will fall apart in due course, as they can’t withstand 20-25 years of sun (plus rain and curious animals with a penchant for plastic).

Note that the use of metal wire ties is discouraged, as they can cut into cables. In addition, AS/NZS 5033:2012 explicitly states that plastic cable ties are not to be used as a primary means of support.

Where landscape orientation of PV panels is called for because of roof topography, installers should look very carefully at the lead lengths, as these are often relatively short and designed for the usual portrait orientation.

This can lead to excessive time, as well as materials, to splice in extra conductors. To prevent kinking and compromising conductor life, the minimum bend radius of solar cables should be observed. Over-bending of a conductor can cause excessive heat at the bend and stress on the connection. This also increases conductor resistance.

The importance of good quality connectors cannot be overstated, particularly because cheap counterfeit versions of the well-known MC4 connection system are available. You indeed get what you pay for.

MC4 connectors (named for the original manufacturer, Multi-Contact USA) are single-contact items designed for DC connections. The MC4 system allows strings of panels to be easily constructed by pushing the connectors from adjacent panels together by hand. However, a tool is required to disconnect them to ensure they are not accidentally disconnected when the cables are pulled.

The MC4 system and compatible products are common in the solar market today, equipping almost all solar panels produced since about 2011.

In short, the use of the correct protection gear, conductors and conduit is essential for solar PV rooftop installations capable of withstanding the rigours of weather, sun, vermin, dirt, etc.

Complete installation kits are available, but installers are well advised to study the requirements of intended installations. Sketch plans should be done, with indications for conductor length, connector location and, where appropriate, junction boxes and isolator switches.