APPLICATIONSAFETY
In a DC rated AC Isolator, there is usually a direct link between the operator turning the handle and the contacts switching, therefore if the operator turns the handle slowly then the contacts will break slowly leading to arcing times of up to 100ms or more. As these kinds of AC isolators have direct action the operator can stop the making/breaking of the contacts thereby oscillating them about a point that could make/break the arcing causing significant contact wear and excessive product heating. With the IMO range it is impossible to stop the make/break once it has started movement and therefore the operation must go to completion before any secondary movement can occur.
Check the fine print
In many instances where AC isolators are modified for DC use, the DC ratings are often covered with a caveat such as “quick switching only” in small print and raises the question, “What is quick switching?”
Losses & Failures
IMO products operate with a “knife switch” mechanism meaning that when the unit is operated the operation gives a double break but the arcing effect occurs on the corners of the switch only and so the main contact is made on an area where no arcing has occurred. The contact mechanism has a rotary nature which also means that when the isolator is operated a self-cleaning action occurs on the arcing points thereby producing good contact integrity over the life of the product. An advantage of this is that in the event of the supply to earth failure the high short circuit current pulls the contacts together thereby giving an high short circuit withstand up to 1700A (product dependant).
If you consider the AC Isolator based type of DC product, this as a norm uses a double break but on a contact bridge, similar to that in a contactor; therefore, although this also offers a double break like the IMO the mechanism, the arcing occurs at the switching/contact point and any subsequent operation leads to continuity being made (or trying to be made) at the same point. Should contact welding occur where these contacts touch then of course the isolation of the unit drops and therefore its effectiveness for switching the higher powers.
If we then consider, as above, a short circuit situation then the capability of an AC based isolator is of the order of up to 400A only (product dependant). As stated initially, the AC supply goes through a 0V point so there has been very limited commercial consideration to designing arc suppression mechanisms into AC isolators.
The knife type mechanism gives a set of contacts per pole however, as indicated previously the typical AC based isolator uses a contact bridge mechanism therefore will incur losses due to contact resistance. This means that self heating will occur within the PV device. If you consider a DC installation where, to obtain the isolation, a four pole AC Isolator is wired with each pole set in series, this will actually give the customer 8 contact sets, leading to 8 losses per pole, and 8 heating effects per pole; which at high current levels can produce product heating as well as system losses.
Summary
The PV industry is moving towards grid parity and eventually a sustainable industry that is not reliant on government subsidies, but the future success of the industry is dependant on public and customer perception. A lack of concern for specific safety issues has led to fire hazard issues for PV installation overseas and every mistake is bad publicity reducing public confidence.
Although the industry is moving at an incredibly fast pace, it requires only a basic understanding of electrical engineering issues and a small amount of corporate foresight to prevent most safety issues the solar industry faces. Educating the value chain to the dangers is the first step. Now the industry needs to respond with a clear message of safety to potential PV users..
© 2011 Angel Business Communications. Permission required.
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www.solar-pv-uk.com Issue III 2011
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