ELECTRICAL I SAFETY


How “Fit for Purpose” are switches in the photovoltaic Industry?


There is a flip side to using photovoltaic energy supplies in that there is a need to use DC electrical supplies. In a world where AC based electrical supplies have monopolised there are a number of potential dangers with the more volatile DC currents. Kevin McCarthy, Managing Director at Katko UK believes there is not enough awareness or reaction to the potential dangers.


A


lthough growth in the photovoltaic market has created an efficient means of generating electricity


in an environmentally friendly way, it has also created a potentially lethal danger - the introduction of powerful DC supplies into domestic and commercial environments that previously functioned on AC.


The dangerous DC difference


At 50Hz AC (Alternating Current) the current and voltage pass through a zero point at 100 times per second. A person touching a live component will of course receive a nasty shock, but possibly will be able to withdraw from the component when the supply goes through the zero point. DC (Direct Current) however, never goes through zero unless it is switched off. A person coming into contact with live parts will be unable to withdraw from it, often with fatal consequences.


Next, consider that the present AC supply in domestic properties is 240 V, and sometimes 415 V in commercial environments. The DC supply from photovoltaics is normally a potentially lethal 500 V to 600 V. This is why it is absolutely vital for photovoltaic suppliers and installers to ensure that ‘Fit for Purpose’ safety


26 www.solar-international.net I Issue IV 2012


switches are used, and not the cheap options, which could spell disaster. Price difference between safe, high quality switches and the cheap alternatives would, on say a £10,000 installation, be around 0.008%. Quite apart from the vital safety aspects, the potential labour costs of return site visits to rectify faulty inferior switches could make them a costly false economy. How can suppliers/installers of photovoltaic systems know that a switch is “Fit For Purpose?”


Genuine DC Switching


There is a fallacy being generated by some switch manufacturers that their products are true DC switches and not AC switches which have been de-rated or rewired for DC operation. This claim is totally untrue.


There are two main switch designs. The first uses a double break contact on a contact bridge, similar to the operation of a contactor. The second method employs a knife-operated mechanism. Both systems have their pros and cons. A double break system permits increased spring pressure on the contacts, giving better electrical contact and therefore less heat. It is also able to switch more quickly to extinguish the arc on breaking.


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