RF noise on the mains - Page 1
Forum section: electronics / beginners

author: Dave Turner
author_rank: Frequent Contributor
timestamp: January 14, 2022, 03:11:39 am
content: A small thought.Dave and others have correctly debunked the claims of power saving devices which simply consist of a capacitor placed across the mains input.Nevetheless I note that most power supply circuits, particularly switched mode varients, use capacitors, chokes etc in series/parallel with the mains in order to reduce noise transmission from an item of equipment into the said mains.Would there be any advantage gained by introducing a device to electrical distribution boards to 'short out' RF interference?

author: radiolistener
author_rank: Super Contributor
timestamp: January 14, 2022, 06:25:53 am
content: No. RF interference should be blocked as close as possible to the source.

author: fordem
author_rank: Regular Contributor
timestamp: January 14, 2022, 01:39:04 pm
content: Whilst I don't disagree with the answer above, I don't think it answers the question that was asked.Yes, RFI should be removed at source, but what happens if it isn't - how should it be dealt with? With the introduction of mains powered LED lighting, there have been a number of current limiting LED drivers wired directly to the utility power with no more than an on/off switch between them and the distribution panel, and some of these drivers have proven to be quite "noisy" - this sort of situation is what I think the question was aimed at.Personally, I'm not aware of any commercially available device that can be wired to the distribution panel to deal with this sort of interference, are they necessary?The only commercial filtering product I've seen was the Tripp Lite "Isobar", basically an inline surge suppressor that incorporated LC filters between the banks of outlets, and at the input.

author: SeanB
author_rank: Super Contributor
timestamp: January 14, 2022, 01:52:12 pm
content: Yes you do get RFI filters that are DIN rail mounting, typically intended for installation on industrial control roacks to reduce the RFI from the internal parts, and often also combined with a MOV block to reduce transients as well. Nothing stopping you from buying them and putting in breaker panels, to reduce the hash from light circuits getting into the mains supply, though the existing wiring will still act as an antenna to radiate it out. Best right by the device itself, so a good reason for somebody like Schaffner to make a small unit with screw terminals input and output, meant to be placed right by the emitter to remedy the cheap manufacturer shortcomings. Likely not going to sell at all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 14, 2022, 09:48:56 pm
content: Sometimes. But depends on the mode. Differential mode can be attenuated by across-the-line capacitors, given that the wiring is good enough. (There's a fair amount of lead length between breakers and buses inside a typical panel for example, but it would be good enough to permit reasonable filtering in the <10MHz range, say.)Common mode, is the wires with respect to free space, or ground. So it needs to be filtered with respect to ground, which assumes both that adding such impedance is acceptable, and that a good enough ground is available.Impedance might not be acceptable, because mains is subject to surges of 2.5kV or more. High rated parts are required, which greatly penalizes the large values needed to affect good filtering, without using a lot of series impedance (i.e. filter inductance). It might not be acceptable anyway due to ground leakage currents (though doing this in the panel may be a lot more acceptable).And filter inductances are penalized for being bulky, expensive, and, I mean it's just another expense really, but the fact that it's a ball of wire that needs to be rated for fault current, really means it's going to be sized for the breaker it's wired to and that's just not feasible in a modular panel system.Anyway, all of this is fairly beside the point, as:1. It's the common mode that radiates, and all that wiring is straight up antennas going everywhere;2. Such a system affords no filtering for external radiated noise, received by the wiring. (So, if you live under a commercial broadcast antenna, or your neighbor is blasting too much CB -- or happens to be a licensed ham but your equipment is just more susceptible than it ought to be, it can't do much about that.)Tim

author: radiolistener
author_rank: Super Contributor
timestamp: January 15, 2022, 06:44:17 am
content: Quote from: fordem on January 14, 2022, 01:39:04 pmif RFI is not removed at source it will be radiated into environment with mains wires between device and filter placed on mains distribution board. Longer wires leads to higher RFI leak. And you cannot eliminate that RFI with filter on your mains distribuition board. Just because it is already radiated by wires, so your filter on distribution board will be useless.

author: m3vuv
author_rank: Super Contributor
timestamp: January 15, 2022, 05:05:33 pm
content: I think mains born rf noise is the one think that will kill amature radio,i used to take part in the uk barrometric net on 3.775 mhz most mornings,now with all the noise here its imposble s9+ of noise from neibours,its killing the hobby for me.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 15, 2022, 05:07:29 pm
content: Don't mess about with the consumer unit or you'll invalidate your fire insurance.Talk to your neighbours and ask them if you can put CM ferrites on thier power leads at your expense.

author: kolbep
author_rank: Frequent Contributor
timestamp: January 15, 2022, 06:30:57 pm
content: Good ol Mr Carlson had a problem in his lab with his coffee machine radiating RF Interference back through the power cord.It caused all the wiring in his lab to act as an antenna, radiating the interference. If he had blocking caps at the Distribution Board, it may have stopped the interference going out to the Grid, but he would most likely still have interference in his lab.All he did in the coffee machine was replaced a cap between the Low Voltage and Mains Section, and then put 2 safety caps (live - earth and neutral - earth), and that was his interference gone.So, Definitely block at the source as much as possible.Watch the start for the problem, and then from 47:50:00 for his solution

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2022, 07:09:03 pm
content: A major source if interference from mains is from power line networking. So much of this is not noise, but intentional produced.