Low voltage lines protected for mains contact - Page 1
Forum section: electronics / projects

author: nunojpg
author_rank: Contributor
timestamp: August 12, 2023, 12:59:05 pm
content: I have a device that talks RS-485 and gets +5V from a UTP cable.This device GND is earthed (PE)The device must be safe if mains voltage (230VAC) comes into contact with any of the UTP lines.RCD protection is not possible.The more or less obvious circuit is this:This means I have to clamp any voltage to max 48V, and be able to either absord the energy or blow the fuse to make it fail safe.For this particular case I don't need the fuses to recover. Device can die.Now, regardless if I use a TVS or a MOV, how can I be sure the fuse will blow?Particularly for the 5V line, that must support 1A at regular operation. If I have a external mains fault that causes eg. 1.5A clamped by the MOV at 30V, how do I make sure the MOV will survive enough to blow the fuse?Or is it guaranteed the MOV will fail short and blow the fuse?Maybe there are calculators for this?

author: SeanB
author_rank: Super Contributor
timestamp: August 12, 2023, 03:02:46 pm
content: The TVS can be assumed to fail short, and easy to add in a pair of them in parallel on the 5V rail, to ensure the fuse will fail, and use a 1A5 fuse. On the ground the same 1A5 fuse, and on the data lines a lower value fuse, 100mA, is simple enough to use, and also comes in SMD versions. Use 2 in series all round, so that even if you have arcing around one, you still have a disconnect. SMD fuses can be soldered, or you can use SMD connectors, and the small size will help with not providing any frequency dependent loss on the data lines. Fuses are rated on carry current, and the time to blow will vary, but the TVS devices and MOV's will survive overload long enough to blow the fuses on direct connection, and on current limited connection will probably still blow after a few3 seconds.Just remember the supply side needs the same protection as well, and the data lines will benefit from adding some low value series resistors as well, to limit current flow into the bus transcievers from any induced noise on the UTP lines. Incidentally use a single pair for the RS485, and parallel the other 3 pairs for the 5V rail, and use an8P8 plug and socket, so you can use cheap premade cables. Use the blue pair for the RS485, so that it is not going to be too bad if somebody plugs it into a proper POE socket, which will protect itself somewhat., and the 3 parallel pairs will reduce voltage drop on the 5V rail.

author: nunojpg
author_rank: Contributor
timestamp: August 12, 2023, 03:24:08 pm
content: Thanks Sean!Quote from: SeanB on August 12, 2023, 03:02:46 pmHow do you come to saying to make 2 TVS on the 5V? If it fail shorts then there is no real risk of it melting before the fuse blows. Are you making any kind of calculations for this?Quote from: SeanB on August 12, 2023, 03:02:46 pmIs this a common failure or design solution? I am really just protecting for mains voltage, there is no lightning protection required.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: August 12, 2023, 03:38:20 pm
content: You could also use PTC polyfuses instead of regular one-time fuses so that your device doesn't die and require repair or fuse replacement to continue working. Although you would need to ensure that your TVS diodes don't then become the weakest link and fail before the fuse fully trips.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2023, 03:49:44 pm
content: I would be worried that a small TVS may not fail short for long enough. Unfortunately fusing rating is not something you'll find on them. Might be worth a test (or, preferably, a call to the mfg to see if they recommend such a rating).

author: coppercone2
author_rank: Super Contributor
timestamp: August 12, 2023, 07:12:27 pm
content: this one is fortunately easy and cheap to testI would just make a board with like 20 of em with push down connectors and apply mains to each one with a switch, should be done in about 5 minutes.