450 volts on internet coax line! - Page 1
Forum section: electronics / beginners

author: testtube44
author_rank: Regular Contributor
timestamp: May 14, 2019, 03:44:48 am
content: Why does my oscilloscope measure 400+ AC volts on my internet coax line? Yet I short it to ground and it doesn't arc over or anything? I was scared shitless to touch it, what is causing this?

author: testtube44
author_rank: Regular Contributor
timestamp: May 14, 2019, 03:49:45 am
content: I'm afraid that if I touch it I will die. What is this? EDIT: its mains at 60 hz, not RF

author: soldar
author_rank: Super Contributor
timestamp: May 14, 2019, 09:46:18 am
content: What happens if you connect a resistor of say 100K or 200K? Still same voltage?

author: StillTrying
author_rank: Super Contributor
timestamp: May 14, 2019, 10:39:36 am
content: Going by your other posts I'd have to ask what you're connecting the scope's GND clip to!

author: stj
author_rank: Super Contributor
timestamp: May 14, 2019, 11:54:36 am
content: it could be capacitive coupling to a powerline in the street because of shit earthing at the cable mux gear,put a multimeter across the coax.btw, is this solid, or oscillating at a fixed frequency?

author: radiolistener
author_rank: Super Contributor
timestamp: May 14, 2019, 12:41:12 pm
content: 400Vpp at 50 Ohm coax line is 1600 W power. This is twice more than microwave owen.There is no need to touch it, just to be near is enough, because 100 MHz at 1600 W is crazy power.If you don't feel smell of burning meat, there is no 400V

author: testtube44
author_rank: Regular Contributor
timestamp: May 14, 2019, 08:05:21 pm
content: Quote from: StillTrying on May 14, 2019, 10:39:36 amThe coax ground? If there is that big of a difference between my scope and the line, why didn't it arc over? Is it like static electricity where there's voltage but it dissipates quickly?

author: james_s
author_rank: Super Contributor
timestamp: May 14, 2019, 08:07:52 pm
content: Sounds like user error to me, I don't see any way you'd get 450V even if it's just capacitive coupling. Either way if you don't get a spark when you short it then there is no safety risk.Static electricity from walking across a carpeted room on a cold day can easily be more than 50kV, it doesn't kill you because the current available is extremely low.

author: langwadt
author_rank: Super Contributor
timestamp: May 14, 2019, 09:11:10 pm
content: capacitors in the input filter of an ungrounded scope means it is floating on mains voltage with regards to the grounded internet coax

author: radiolistener
author_rank: Super Contributor
timestamp: May 14, 2019, 09:43:03 pm
content: Quote from: blueskull on May 14, 2019, 09:08:12 pmAs I know, Ethernet over coax works at least on 10 MHz and requires terminators.Quote from: blueskull on May 14, 2019, 09:08:12 pmFor 10 MHz unloaded 1 kV on 50 Ohm line = loaded 500 V on 50 Ohm = 2500 W RF power.This is more than enough to roast chiken

author: stj
author_rank: Super Contributor
timestamp: May 14, 2019, 10:40:13 pm
content: not ethernet, internet - most likely DOCSIS.

author: radiolistener
author_rank: Super Contributor
timestamp: May 15, 2019, 07:33:24 am
content: Quote from: stj on May 14, 2019, 10:40:13 pmthe same for DOCSIS. It doesn't needs 2 kW power in order to transmit the data.These 450V looks just like induced voltage from mains.