ELECTRICAL I SAFETY
The downside is that there is less distance between the points on breaking. A knife-operated system allows more distance between the contacts on breaking, but the downside is that spring pressure on the contacts is much lower, thus creating more heat.
Switch manufacturers have used and still use both designs for AC switches. It is totally untrue, as some switch manufacturers claim, that AC switches use the double break contact system and that DC switches use the knife system.
Door /Cover Interlocking To prevent anybody from unscrewing a cover and gaining access to live contacts, standards require there to be a method of interlocking the door or cover of a switch when it is in the ‘ON’ position. While most manufacturers provide this facility, some allow covers to be unscrewed and removed in the ‘ON’ position, and some even allow removal of the switch with the cover - this can kill.
Standards also require the facility for fitting a padlock to the handle in the ‘OFF’ position, to prevent unauthorised personnel
from gaining access. Again, while most manufacturers supply this facility, some allow the cover, complete with padlock, to be unscrewed and removed. This totally defeats the object of fitting a padlock in the first place, again with potentially fatal consequences.
Manufacturers arrive at the ratings for their switches by very different routes. Some calculate ratings without testing. Others test their products in-house, from where results can be massaged. The best quality manufacturers have their products tested by completely independent authorised test facilities for ratings and compliance with standards.
Conclusion
If you purchase safety switches, you can gain peace of mind and help to drive up standards in the industry simply by asking your supplier for confirmation of how their products are tested, and requesting copies of test certificates as proof. All reputable manufacturers will be very happy to oblige.
© 2012 Angel Business Communications. Permission required.
Issue IV 2012 I
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