Fuse requirements for 24VAC supply product - Page 1
Forum section: electronics / projects

author: grg183
author_rank: Contributor
timestamp: February 04, 2015, 10:21:09 am
content: Hi,I am designing a board which is powered from a 24VAC (SELV) external supply and a an 18VA load will be powered from the board. Is it required to have a fuse on the supply input ?My question is not if it is better to have a fuse, but specifically if it is required by safety standards... any experience on this ?The problem is that because of the enclosure design, if the fuse blows there will be no way to replace it. Also thinking of using re-settable fuses but let's ignore that for the moment.Thanks,George

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 04, 2015, 10:43:13 am
content: This is an international discussion forum. What is required by "safety standards" in your location depends on where you are.You have not completed your user profile to reveal what country you are in, so nobody can even make an attempt at offering a definitive answer.Furthermore, you have not really described the defined class of your load...http://en.wikipedia.org/wiki/Appliance_classesShould we assume that your load is "Class 3" because it is SELV powered?Note that even Class 3 is insufficient for medical equipment.OTOH, people use external low voltage power supplies ("wall-wart" or "line-lump") specifically to avoid most of the regulatory requirements of mains-powered gear. The external power supplies are required to be compliant with regulatory authorities because they are mains-connected. But that removes the regulatory requirements from your gear which is "downstream" from the external supply.It is NOT common to see fuses downstream from commercial external power supply units. The units themselves are typically protected (explicitly or implicitly) against overload within themselves. Hiding conventional fuses inside equipment labled "No user-servicable parts inside." is just deliberately dumb, and "user-vicious" IMHO.

author: grg183
author_rank: Contributor
timestamp: February 04, 2015, 12:03:50 pm
content: Hi Richard,Thanks for your detailed reply ...yes I realize that I didn't specify much as at this point we are not completely sure of the standard we need to apply. However almost certainly the product is to be approved under UL60730-1. The device will be fully powered from a SELV source. It will also be totally encapsulated in thermoplastic over-molding. There will be an additional 230V/115V connection to a relay dry contact within the device but appropriate clearances will be kept. Because the board will be encapsulated replacing the fuse would be impossible.The thing that is confusing me is because (afaik) a SELV source is not necessarily energy limited and also because we have an 18VA load, 18VA is above the (afaik) 15W SELV energy-limited limitation. So if the supply is not energy limited is a fuse required ?Thanks,George

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 04, 2015, 05:59:20 pm
content: I would think that would be something you would shop around to SELV vendors to see who could offer you a compliant solution.