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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

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Old 26th Apr 2022, 6:35 pm   #1
stevehertz
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Default Mains filtering

As we all know, interference from SMPS and many other sources greatly affects, and can totally ruin attempts to receive AM transmissions in a domestic situation. I'm constantly dabbling with solutions and I recently fitted clip on ferrite rings to every device's mains lead (computer, printer, USB hub, monitor etc) in an area where I also have a couple of vintage radios nearby - they had ferrite rings fitted too. The result? no better. So I really got down to tracing the source using a modern portable transistor radio. To cut a long story short, the interference peaked whenever the radio was placed near to any mains cable on the ring main - the interference is 'everywhere'. BTW, I have a long wire external aerial that provides (in theory!) for a good signal to noise ratio, so I'm not just hanging a bit of wire out of the back of my radios.

It got me thinking about a 'global' mains filter unit and how effective it may be. I did some googling and found this as used by a radio ham and seems just the job: https://www.m0nwk.co.uk/mains-electr...ise-reduction/

Has anyone tried this kind of global filtering?
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Old 26th Apr 2022, 9:41 pm   #2
wireman
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Default Re: Mains filtering

Maybe I'm missing something but I thought you were only allowed one double socket on a spur unless it was protected by a 13A fuse.
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Old 26th Apr 2022, 11:49 pm   #3
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Default Re: Mains filtering

That is correct, a spur being specifically a branch from a ring final circuit that would be protected at 32A in the distribution board. But any number of points may be connected to a radial circuit wired in 2.5 sqmm protected at 20A. I don't see anything in the link that states the supply is a spur off a ring final.

The reason for the limitation on spurs is that regardless of the cable rating used for the spur itself, too many points connected to it would tend to concentrate too much of the ring final's total 32A in one place, increasing the probability of one leg of the RFC being overloaded. This in turn is because the RFC is a special case that violates the principle Ib <= In <= Iz (Design load does not exceed OCPD rating, OCPD rating does not exceed derated CCC of cable). It is an empirical solution that relies on a distributed load being somewhat shared between the two legs. There are situations in which multiple points on a spur would be perfectly safe but they do not fall within the specifically defined configuration of an RFC.
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Old 27th Apr 2022, 6:59 am   #4
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Default Re: Mains filtering

I made up a filter for my shack by winding L N and E individually over 3 ferrite rods. It is not connected to a spur, just goes to a 4-way multi connector which has my ham gear connected to it.
Not sure if it reduces noise much but every little helps.
You can buy ready made filters.

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Old 27th Apr 2022, 2:18 pm   #5
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Default Re: Mains filtering

I've now added ferrite rings to my incoming mains supply (house), Still no better.
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Old 27th Apr 2022, 2:50 pm   #6
Roger Ramjet
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Default Re: Mains filtering

On the presumption that a battery powered radio is used, has anyone tried switching off the whole house mains supply off at the consumer unit to see if the interference then disappears ? This would prove or disprove re radiation via mains supply into the ring circuit or any other house wiring for that matter.

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Old 27th Apr 2022, 3:14 pm   #7
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Default Re: Mains filtering

Good point Roger.
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Old 27th Apr 2022, 5:12 pm   #8
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Default Re: Mains filtering

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Originally Posted by Lucien Nunes View Post
That is correct, a spur being specifically a branch from a ring final circuit that would be protected at 32A in the distribution board. But any number of points may be connected to a radial circuit wired in 2.5 sqmm protected at 20A. I don't see anything in the link that states the supply is a spur off a ring final.
Good point, I was not thinking of the other situation.
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Old 27th Apr 2022, 6:12 pm   #9
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Default Re: Mains filtering

Pragmatically, to understand what supply-filltering will work for you. you need to understand the supply-schema used by your DNO.

Viddy here, my Droogs, as Alex would say in "Clockwork Orange" - https://www.lsp-international.com/power-supply-system/

If you're working with an old "Protective multiple earth" [PME] system then then at the incomer the customer 'earth' will be strapped to the neutral- whixh means that opening the customer 'main switch' still leaves the customer-side 'earth' connected to the DNO Neutral - so any RF noise on the Neutral still gets coupled into the building's cabling.

[I don't know precisely what the capacitance between the earth-conductor and the L/N conductors of typical premises-wiring cable is, but I would hazard a guess that at HF/MF frequencies it will be sufficient that in a typical domestic-install where there aew hundreds ofv Metres of such cable the inter-wire capacitance is sufficient that you can assume that L/N/E are at the same RF potential]

So "switching-off' the mains-switch as a diagnostic-technique, without knowing the DNO topology could ne a fool's errand.

Same goes for fitting chokes on the L/N incomer cables... noise-on-the-neutral could be getting a free-pass round your efforts via the earth-conductor into your entire domestic wiring infrastructure, where it will happily re-radiate to the detriment of your MF/HF reception.
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Old 27th Apr 2022, 6:58 pm   #10
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Default Re: Mains filtering

In that case, any suggestions then?
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Old 27th Apr 2022, 7:13 pm   #11
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Default Re: Mains filtering

Quote:
Originally Posted by stevehertz View Post
In that case, any suggestions then?
First, consult your DNO and identify what supply-schema you have to your property.

Without knowing this, adding 'filters' could be futile - and potentially dangerous!
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Old 28th Apr 2022, 7:57 am   #12
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Default Re: Mains filtering

I have exactly the same problem. It seems to be at its worst at the fusebox, indicating it may be coming in on the cable from outside. It's so strong I get modulation hum on strong signals when using a portable around the house. Luckily, my Wellbrook loop mounted at gutter height at the back of the house gets rid of most of it on my Amateur Radio/SWL stuff.
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Old 28th Apr 2022, 8:01 am   #13
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Default Re: Mains filtering

Quote:
Originally Posted by G6Tanuki View Post
Quote:
Originally Posted by stevehertz View Post
In that case, any suggestions then?
First, consult your DNO and identify what supply-schema you have to your property.

Without knowing this, adding 'filters' could be futile - and potentially dangerous!
So, exactly what is the question that I ask of Western Power, as sure as eggs are eggs they'll throw a curved ball back at me?!
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Old 28th Apr 2022, 8:08 am   #14
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Default Re: Mains filtering

Have a look here

https://en.wikipedia.org/wiki/Earthing_system

Ask them what kind of earth your property has (if you cannot work it out for yourself) and when the come back with say TN-S or TN-C-S when you can see exactly what that means.

Cheers

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Old 28th Apr 2022, 8:33 am   #15
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Default Re: Mains filtering

My earth is of my own making. I drove a long copper earth bar into the ground and brought a wire from it up to the electrical distribution box. What's that called?
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Old 28th Apr 2022, 8:49 am   #16
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Default Re: Mains filtering

It could be called "a pretty impressive ground loop" if it's competing with your electricity provider's earthing system. Under some circumstances I believe* it may also be hazardous, particularly in the event of certain types of faults elsewhere in your neighbourhood. As always, whether it's a problem and/or a hazard will depend on the details of your provider's earthing scheme.

Cheers,

GJ

*I wouldn't class myself as competent to advise authoritatively on this - if you're in any doubt then you should consult someone who is.
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Old 28th Apr 2022, 8:55 am   #17
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Default Re: Mains filtering

Quote:
Originally Posted by stevehertz View Post
My earth is of my own making. I drove a long copper earth bar into the ground and brought a wire from it up to the electrical distribution box. What's that called?
That's similar to my own installation. The "company" never provided an earth terminal even when the supply was converted from overhead to underground.
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Old 28th Apr 2022, 8:58 am   #18
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Default Re: Mains filtering

If no earth is provided by the DNO then it will be TT

You will need to be using RCD at the very least as the ELI will be quite high.

Cheers

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Old 28th Apr 2022, 9:03 am   #19
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Default Re: Mains filtering

What is TT? ELI? My distribution unit does use RCDs.
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Old 28th Apr 2022, 9:08 am   #20
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Default Re: Mains filtering

Follow the Link in post #14

ELI is Earth Loop Impedance.

TT is Terre Terre ie a local earth usually provided by a spike or a mat.

If you want to improve the ELI you can use Marconite concrete which is what we use.

Cheers

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