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True DC


TRUE DC refers to a type of Switch Disconnect (Isolator) used in Solar Photovoltaic installations, in accordance with EN 60364-7-712. Pioneered by UK based IMO Precision Controls Ltd, and later adopted by other manufacturers in this marketplace such as Santon and ABB, the isolator design incorporates a user independent switching action so as the handle is moved, it interacts with a spring mechanism which, upon reaching a set point, causes the contacts to "SNAP" over, thereby ensuring a very fast break/make action. This mechanism means that the disconnection of the load circuits and the suppression of the electrical arc, produced by a constant DC load, is normally extinguished in a maximum of 5ms using the specific pole suppression chambers incorporated into the design.

Many alternative solutions, particularly those based upon an AC Switch Disconnect design which use bridge contacts, have been modified and rated for DC operation. These types of product have a switching speed that is directly linked to the operator speed, therefore slow operation of the handle results in slow contact separation of the contacts which can produce arcing times of 100ms or more. Additionally in these switches, the contact surface is also the surface upon which electrical arcs tend to form; therefore any surface damage or sooting caused by arcing is likely to have a detrimental effect on the isolators contact resistance and its longevity.

TRUE DC Solar Isolators use a rotary knife contact mechanism so when the unit is operated, the handle movement gives a double make/break per contact set. As DC load switching creates electrical arcing, the design is such that this only occurs on the corners of the switching parts meaning that the main contact is made on an area where no arcing has occurred. The rotary contact mechanism methodology used in TRUE DC solar isolators means that when the isolator is operated, a self-cleaning action occurs on the arcing points and contact surfaces thereby producing good high-vibration resistant contact integrity, with reduced contact resistance. This contact system ensures that power loss per pole is kept as low as possible, and consistent over the life of the product, unlike conventional style isolators where entrapment of contaminants, and then subsequent compression on lateral operation, can lead to variable and increasing contact resistance, and hence per pole losses.

The overall design of a TRUE DC solar isolator is satisfactory for use in installations classified as either DC-21A, DC-21B or DC-22A, and so suitable for a high number of "off load" operations (without current) and also a high number of operating cycles "on load" (with current).


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