Sizing PV isolators have never been the easiest thing for sparkies. Sizing miniature circuit breakers (MCB) for your normal light and power electrical job is easy. For light and power jobs, all you need to think about is current. Sizing PV isolators is much harder because solar installers need to juggle the voltage and current based on how many panels are installed.
Rating the DC isolator is not something you want to put off until you rock up to a job on a Monday morning. Many solar installers went through the process of figuring out what size isolators to use when they got their accreditation. In order to comply, they have committed DC isolator configurations versus number of panels to memory. The problem is, things are about to change.
As many installers will be aware AS/NZS 5033: 2014 was amended in the middle of last year. There were two parts: amendments one and two. Amendment one was mandatory at the time of publication while amendment two will be mandatory on June 28 this year. The second amendment details the new way to size an isolator.
There are three things you need to do to get ready for the change:
- Refamiliarize yourself with the calculations for PV array maximum voltage and the short circuit current;
- Understand how to size isolators based on the new requirements, and;
- Get your hands on the specifications from the isolator manufacturer so you can put the theory into practice.
Calculate voltage and current
Start by calculating the maximum possible volts that the solar array can produce. The colder the temperature, the more volts the PV panel makes. Find the expected minimum daily temperature from the Bureau of Meteorology (www.bom.gov.au) to get climatic data on the town where you are installing. Then you correct the voltage that is listed on the solar panel specifications. There is a six-minute presentation in the technical information section at the Clean Energy Council’s solar accreditation website www.solaraccreditation.com.au on how to do this.
The next step is to calculate how much current the PV panels can produce. In real life, the sun could give your panels more energy than the standard test condition, so you need to adjust current value on the solar panel specifications. To get the current right, multiply the short circuit current from the specs sheet by 1.25.
The calculations above should sound familiar, because nothing has changed with PV array maximum voltage and sizing the maximum current. However, the next step is new.
Size the isolator for normal operation and fault conditions
Before amendment two was introduced you needed to figure out the current flow through the switch on the worst possible fault and size it accordingly. The worst possible fault on a transformer-less inverter can have the full load current and the PV array maximum voltage flowing through the negative circuit, returning through the MEN.
This fault is still possible today but the likelihood of it happening is very low. So low, that AS/NZS 5033: 2014 amendment two now has a new way of sizing the isolator. The new way requires you to size the isolator twice for transformer-less inverters. Firstly for normal operation (referred to by the standards as I(e)) and secondly for the worst possible fault (referred to by the standards as I(make) & I(break)) like in the scenario mentioned earlier. It’s a win for the industry because we can get away with smaller isolators, especially on larger commercial systems.
The manufacturers should provide an I(make) & I(break) column and an I(e) column on their specification. For clearer instructions on how to size isolators to AS/NZS 5033: 2014 amendment two watch the CEC toolbox talk on sizing DC isolators. You will save yourself a bit of time by watching this rather than thumbing through your standards and trying to figure it out for yourself, although we would recommend doing that also.
Getting your hands on DC isolator specifications
Just understanding the theory isn’t enough because you need an up-to-date manufacturer’s specification to comply. Unlike old isolator specifications, DC isolator specifications need to include a column for normal operation (I(e)) and a column for fault conditions (I(make) & I(break)). Without this, you can’t apply your calculations.
In most cases, manufacturers will find it hard to display the required information on the side of an isolator because the amount of information required will not fit in such a small space. For this reason, do your calculations before you go to site so that you have the correct information at your disposal.
As of June 28 this year, isolators installed in Australia will need to comply with AS 60947.3, which is the new Australian isolator product standard. Be careful when you are checking, though, because previously DC isolators were required to meet the international product standard with a similar name of IEC 60947.3, so it can be confusing.
The specifications also need to sight utilization category of DC-PV2, whereas previously the utilization category needed to be a minimum of DC21B. On top of this, DC-PV2 and AS 60947.3 need to be printed on the front cover of the isolator along with other information to be compliant. Section 8.1.3 of the new CEC Install and Supervise Guidelines for Accredited Installers goes into more detail. If you haven’t downloaded the most recent CEC guideline released on April 15, I would recommend doing to so because it will help you meet isolator sizing obligations.
Many installers have asked the CEC to provide lists of compliant DC isolators in the same way we do for solar panel, inverters and batteries. At this point in time, the CEC has decided not to do this because it would be duplicating the Electrical Regulatory Authorities Council’s (ERAC) list found at equipment.erac.gov.au/Public/. The CEC recommends only using isolators on this list because some state regulators make it a mandatory requirement.
What to do next?
Firstly, start using your old stock that complies with the international product standard IEC 60947.3 because these will no longer comply come June 28. Download a copy of the latest CEC guidelines from www.solaraccreditation.com.au to help you get across AS/NZS 5033 amendment 2. Also watch the CEC toolbox talks on AS/NZS 5033 Amendment 2 and Sizing DC isolators to help you do your calculations.
At the same time, you should start researching isolators that comply to AS 60947.3 and a good place to start is with the ERAC list. When you’ve chosen the isolator, gather the relevant specifications and do your calculations before you go to site.
Remember to give yourself a bit of time to learn the new procedure, because it can take a bit to get your head around the new requirements. Finally, make sure anyone working for you is across the changes as well.