Steve Pester, BRE, discusses the issues surrounding DC isolators
BRE was recently sent an example of a DC isolator (switch) which had filled up a room with smoke in a few seconds. It burnt through the side of the enclosure before it could be disconnected. Luckily, in this case, the solar engineer was on hand to deal with this dangerous situation but there’s no need to spell out the potential consequences for life, property and the reputation of the solar industry from such incidents.
The cause? Almost certainly an incorrectly specified isolator. All installers will know that DC is much more dangerous than AC and requires special components. You may think, therefore, that it’s a clear-cut case of the installer not doing their homework, or deliberately using a cheaper AC switch instead of a DC type but that is not necessarily true. We have been made aware of a worrying practice whereby one or two manufacturers are ‘redefining’ isolators originally designed for AC use as suitable for DC, with or without minor modifications. Occasionally, distributors have incorrectly advised installers on this issue as well. The advice ranges from using a ‘rapid action’ when turning the switch to wiring two poles in series in order to increase the switch gap in the ‘off’ position. If you hear this kind of thing, be suspicious! Purpose-designed DC isolators will rapidly suppress switching arcs and are of a different design from AC types.
MCS requires installers to use DC-rated components for all DC functions, but the MCS product scheme does not currently certify isolators. So as an installer, how can be sure you are using a safe device if you cannot rely on data sheets or advice from suppliers? The best option is to ask the supplier or manufacturer to provide a copy of the test certificate from an independent, accredited, test laboratory, which shows the device to be DC-rated. This is also the best way to check that it is rated for the voltage and current, allowing for the safety factors specified in the MCS PV installer’s guide (currently the DTI guide ed. 2, shortly to be replaced with an updated MCS guide).