APPLICATIONSAFETY
Switching safety
Spurred by an increasingly long list of fire issues from PV products, IMO Precision Controls recently wrote to an international electrical body expressing their concern regarding a safety critical component being used in many installations of PV equipment. Graham Viney, Quality Manager at IMO discusses the safety issues with AC to DC switching and highlights a lack of industry awareness of the challenges or the solutions
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s any electrician is aware the nature of DC switching has to be considered with care because on disconnection an arc can occur that is more arduous than that produced with an AC load because there is no zero point on DC. The nature of this arc means that design considerations have to be made within the switch in order to quench this phenomenon that not only includes significant contact gaps with high speed of operation, but also thermal transmissive materials.
What you have to consider is that any AC isolator is predominantly designed with materials chosen such that the load will be AC. This means that the load supply will be a 50/60Hz sine wave, whether it be 230Vac or 400Vac, etc. When switching AC it should be remembered that the nature of the load supply will always pass through 0Vac and therefore although loads can be arduous in type the supply is self extinguishing. By that I mean that even if the isolator switches at peak load and an arc between contacts is formed the action of the supply reducing to 0V means that the load will tend to zero and the arc be extinguished.
DC load, on the other hand, is always there and unless the load becomes zero the power being pulled through the contacts will always be the same, so if the load is 500Vdc 25A it will be 500V 25A now, in 1s , in 1min, in 1hour – that is constant. If this is the case unlike the AC above if you switch “OFF” on load you will also be switching “ON” on load; DC does not go through a 0V level unless there is system supply failure (or some other fault). Slow switching action or paused action could potentially allow the dangerous arcing
of the DC across the contacts. Dependent upon the failure mode of the switch the DC arcs could cause fire damage or worse still failure of the isolation of the product and possible DC electric shock. At the very least, if installers are going to use AC type switches modified for DC use, we feel that they should label the isolator as indicated by the manufacturer. This may still not be clear to the consumer but will at least reduce the level of safety risk from the current high level.
Problem solving
In principle these modified AC isolators will allow the user to switch DC, however, the market design for this product is such that the disconnecting action is normally linked to the speed of operation by the user. It is also important that the action be a continuous movement rather than with any kind of pause in the switch operation which might create a sustained arc.
Whilst IMO will not modify and de-rate its AC isolator switches for use in DC applications, we respect the rights of both our competitors and installation companies to use these switches if they have evaluated them as safe. However, many of the manufacturers of these AC switches endeavour to protect their position by inserting the phrase “with rapid handle operation” or “continuous rapid handle operation” in their technical literature and this essential piece of safety information is not being passed on to the consumer representing a substantial safety risk.
This is something IMO feels passionate about and makes every effort to educate potential users about safety in AC or DC installations.
www.solar-pv-management.com Issue IV 2011
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