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Old 3rd Jul 2011, 6:59 am   #1
Neil Purling
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Default Is the mains 'earth' good enough?

In the context of the home-brewed radio set.
A design in Dec 1962 Pract Wireless. The power was via a 2 core cable, but the design had a transformer for HT & LT so the chassis was 'cold'. There was a socket for your own 'earth' connection wired to the chassis.
If I use 3-core mains cable and connect the earth wire to chassis is it going to be good enough?

I know considerations of electrical safety were different then. I also suppose that some people might not have had 3-pin plugs in their homes too.
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Old 3rd Jul 2011, 7:46 am   #2
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Default Re: Is the mains 'earth' good enough?

I've always found it perfectly adequate to use the mains earth, but it may depend on your installation. In the last house I lived in, there was a strap down to earth in any case, via a thick twisted copper conductor from the back of the fuse box down the outside wall of the house (over head lines, old installation - originally late 20s, early 30s).

The problem with your own earth is it is likely to be variable, depending on soil resistance / dampness (etc)
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Old 3rd Jul 2011, 8:04 am   #3
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Default Re: Is the mains 'earth' good enough?

Mains earth is for safety, not RF earthing. Nowadays it's very noisy. Even modern equipment can be designed to might safety regulations and have a metal cabinet and yet have no Earth via mains plug.

Never disconnect an earth wire on three wire flex.

Anything that needs an external aerial has either a signal return (balanced or coaxial cable to dipole etc) or needs an earth wire to an outdoor ground spike at least as the "mains" Earth is too noisy.

If it's a two core flex isolated flex, just connect the Earth wire direct. You may want a ferrite choke on mains cable. But that doesn't work well at MW/LW. See below.

If it's a 3 core flex, put a few ferrite chokes on mains cable or 5 to 10 turns of entire cable on an old LOPT core and isolate aerial and earth sockets with 1nF 1000V ceramic capacitors, at the radio.

Water pipes are not reliable anymore as an earth. One galvanised steel 4' (1.2m) earth spike from Electrical wholesale and 4mm wire to it is a minimum. A number of spikes around the garden (more the better) tied together is even better.
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Old 3rd Jul 2011, 8:07 am   #4
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Default Re: Is the mains 'earth' good enough?

Now that most houses have all the metal bonded together it is quite good, a good external earth can reduce noise a bit as can using a balanced aerial (no earth). Some old plugs has a hole near the earth terminal just for this.
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Old 3rd Jul 2011, 8:11 am   #5
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Default Re: Is the mains 'earth' good enough?

I missed Neon's reply, obviously it depends on how your house is earthed http://en.wikipedia.org/wiki/Earthing_system and what is near you causing interference. Give it a go, if it is noisy invest in a few 8 foot earth rods.
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Old 3rd Jul 2011, 8:52 am   #6
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Default Re: Is the mains 'earth' good enough?

You may find it helpful (not to mention advisable) to read the attached RSGB leaflet on the subject of earthing. Don't be put off by the title - it's about safety rather than EMC.

Hugh
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File Type: pdf emc-leaflet-07.pdf (61.9 KB, 290 views)
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Old 3rd Jul 2011, 9:32 am   #7
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Default Re: Is the mains 'earth' good enough?

Interesting article Hugh.
If you do set your own local TT system (like I have), you must make regular checks of the Earth impedance (Ze) to ensure it meets requirements.
Unless you're operating a transmitting station, and capable and competant to modify your earthing system, I'd leave well alone.
In answer to the original question, the mains earth must be connected to chassis for safety, and will most likely suffice as an RF earth (for a receiver).
Rob.
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Old 3rd Jul 2011, 10:08 am   #8
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Default Re: Is the mains 'earth' good enough?

Historically UK (inc N.I.) was fig1. Ireland for a long time has been fig2.

If you are Fig1. and the radio uses mains earth, then the "aerial" connections, especially the "Earth" are best isolated by 1000nF 1kV.

If you are Fig2. or Fig3, then an isolation capacitor may be not important in one sense. But if there is a wiring fault in the house, a capacitor stops your Radio Earth conducting fault currents.

If the Mains Earth is poor, then all the filters in all the SMPSUs etc may put current via your mains earth on radio and flow to "earth" via your RF earth. The current will be a mix of 50Hz and switcher noise. Ferrite clamp near box end on all interference producing equipment and ferrite clamp on plug end at Radio set hugely reduces HF and higher interference. For LW & MW multiple turns of the entire cable (interference producer and/or radio) on an old LOPT core is needed to get enough common mode inductance. The 1nF capacitor will mostly stop 50Hz earth currents.

If the equipment has the concentric box symbol and a two core flex, then don't fit a three core flex and don't connect chassis to earth.

If it's before the days of that symbol and test, then megger live and neutral to the chassis. If if passes and came with a 2 core flex, don't fit a 3 core. If it fails investigate why, if there is no fault, then connect a 3 core flex. If the leakage is too high at mains voltage (more than ??), then there is a fault and adding an earth will trip a correctly installed RCD.

I don't have the figure for leakage. If you touch an old metal case Sky box that is plugged in and not connected to anything you may feel leakage. It's at a permitted "safe" level. Someone may know what that is for UK.

Because neighbouring houses (or possibly apartments) are on different phases (380v to 440V between "lives") then any un-isolated inter-house wiring or shared aerial, satellite dish etc must meet EXTRA safety regulations of Earth bonding, how and where the earth bonding is done and minimum size of earth wires for bonding.

Don't share wiring, aerials or dishes without qualified advice. A computer RJ45 / Cat5e or ethernet cable can be shared (as these use built in isolation on network adaptors), but there is a right and wrong way of doing even that! Especially if there is a screen on Cat5e or BNC on coax ethernet can come in contact with a chassis.
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Old 3rd Jul 2011, 10:10 am   #9
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Default Re: Is the mains 'earth' good enough?

The mains earth is for electrical saftey as already mentioned..using it as a reception earth it can sometimes be a bit noisy depending on location and other stuff buzzing around in the house as the mains earth is bonded to neutral back at the box, this means that you are depending on the power distribution company for a decent signal earth and it does vary somewhat!!
At my location it is just a wire comming down the utility pole and buried in the ground.
A word of caution: if putting in seperate mains feeds to outbuildings, shack etc a seperate earth spike at the end can cause earth loop problems.
I put an armoured cable to our new chalet in the garden and the distribution company would NOT advise me all they said was speak to an electrician (ie a disclaimer) I used the exported PME only, no seperate earth.
As a saftey note I never rely on a home made earth for electrical saftey, only the boards earth.
Hope this is of some use
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Old 3rd Jul 2011, 11:52 am   #10
David Simpson
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Default Re: Is the mains 'earth' good enough?

Hello Neil, If you look in "Search" you'll see a heafty thread about 3 years back regarding modern domestic earthing - PME in particular.
The RSGB website has some very good info on earthing aswell.

Regards, David
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Old 3rd Jul 2011, 12:07 pm   #11
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Default Re: Is the mains 'earth' good enough?

Some mains earthing arrangements can introduce some noise with some radios tuning some bands. I haven't found my mains earth to be a problem with normal broadcast reception, and mains borne interference mostly arrives down the live wire. It's quite a lot of work to set up different radio earthing arrangements so it's well worth trying the mains earth first and seeing what happens.

It was normal practice for the chassis of valve sets to be left floating before the 60s, meaning there was either no earth connection or connection to a separate radio earth. It is considered bad practice to leave metalwork floating like this now for safety reasons.
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Old 3rd Jul 2011, 4:19 pm   #12
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Default Re: Is the mains 'earth' good enough?

Earthing the chassis will put some extra strain on the mains transformer insulation, as of course the mains live is looking for a way to get to earth. The insulation should be good enough to take the extra electrical stress and would be better for safety.
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Old 3rd Jul 2011, 7:35 pm   #13
Neil Purling
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Default Re: Is the mains 'earth' good enough?

On the chassis the incoming cable is wired to a connector block.
The L & N to the switch of course.
Should I thus wire the capacitor between the the terminal of the earth wire & chassis?
Can you confirm the value & voltage as well?
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Old 3rd Jul 2011, 8:14 pm   #14
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Default Re: Is the mains 'earth' good enough?

Which earth wire? Mains or External RF earth?

If the L or N to chassis leakage is OK, then you can wire the mains earth wire direct to chassis, using only a 3 core flex. If the Leakage is NOT OK, then you have a problem anyway as the RCD may trip and there must be a fault.

Only an RF earth should optionally use a 1000nF 1kV capacitor to stop Mains Earth wirring hum loop to your RF outdoor Earth spike(s).
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Old 3rd Jul 2011, 9:17 pm   #15
Neil Purling
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Default Re: Is the mains 'earth' good enough?

Perhaps I should proceed and make the project exactly as originally designed, but with 3-core cable. I can experiment with either earth method, leaving the set with such a arrangement as gives best performance.

The only independent earth I can access is the domestic cold water pipe as it enters the property. A property of this age has metallic pipes and I'm pretty sure they're copper.
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Old 3rd Jul 2011, 10:47 pm   #16
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Default Re: Is the mains 'earth' good enough?

Quote:
Originally Posted by Neil Purling View Post
The only independent earth I can access is the domestic cold water pipe as it enters the property. A property of this age has metallic pipes and I'm pretty sure they're copper.
This should be bonded directly to the mains earth anyway, so may not make any difference.
Rob.
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Old 3rd Jul 2011, 11:03 pm   #17
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Default Re: Is the mains 'earth' good enough?

Usually at the hot tank / immersion / willis heater. But the pipes in the ground can be plastic. Or become plastic!
Also at any central heating boiler.
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Old 3rd Jul 2011, 11:11 pm   #18
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Default Re: Is the mains 'earth' good enough?

Maybe regs are different in Eire, but any incoming service (water / gas /oil etc) should be bonded directly to the main earthing terminal, as close as reasonably possible to where it enters the building.
Rob.
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Old 4th Jul 2011, 12:05 am   #19
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Default Re: Is the mains 'earth' good enough?

Sometimes in older property copper pipe is a replacement for the old bsp galv. pipe, usually the rest of the old pipe is buried underground and is usually in a very poor state if it is the original and probably would not make a decent signal earth.
It is true to say that bonding should be at the nearest place to the board as already mentioned with other bonding at the appropriate places, assuming that there is a continuity of the copper/...s/steel pipe, quite often there is not in these days of push fit plastic and diy alterations.I have "heard" that sinks do not have to be earthed if fed by plastic pipe ? DO NOT QUOTE ME ON THIS!!
Always best to check with current regs. as they are changing so fast
Hope this is of use to someone.
Cheers.

Last edited by ms660; 4th Jul 2011 at 12:06 am. Reason: spelling
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Old 4th Jul 2011, 8:00 am   #20
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Default Re: Is the mains 'earth' good enough?

All this talk about earth bonding rather hides the main point of doing it. It isn't to earth things via the water and/or gas pipe. It's to ensure that all exposed metalwork in the house is at the same potential to minimise risk of shock if there's an insulation fault. Hence a house with plastic gas and water mains still needs bonding if the internal pipework is metal.

There's some professional disagreement about exactly what does and doesn't need earth bonding but that's not the point of this thread. If a mains earth works for you, then great. If it injects too much noise etc then you need a rod. Try it and see.
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