Earth fault

[evanrob]

I recently 'drove' a 6 ft copper earth rod fully, into the ground beneath my upper floor ,bedroom window to provide , what I thought would be a 'good earth point' ! Surprised to find that on connecting a substantial earth wire(about 7metres in length) to a radio metal chassis , there was a spark, on touching the metalwork.!!
An A.C voltmeter showed a reading of 56.5 volts,.???? Whilst I appreciate the recommendation to use a central earthing point, I am (in my naivety) rather puzzled where the 56.5 volts is actually coming from ???
Helpful comments greatly appreciated .. Many thanks

 

[Paul_C]

It will almost certainly be due to capacitance between the supply and the chassis of the radio. You'll find this to be common where the radio/TV/VCR etc. has a metal casing but is supplied via only a two-core flex (i.e. not earthed directly via the plug).

 

[bernardgreen]

In some old ( antique ) valve radios the HT supply was derived straight from the mains with neutral connected to the chassis and live via a diode and smoothing choke to provide 300 volts DC. Reversing the Live and Neutral at the plug meant the chassis was live. As a result these radios came close to being " double insulated "

The give away was plastic screws in the control knobs or sealing wax filling that hole to prevent fingers touching the metal screw head.

 

[dingbat]

I recently 'drove' a 6 ft copper earth rod fully, into the ground beneath my upper floor ,bedroom window to provide , what I thought would be a 'good earth point' !

Why?
What were you hoping to achieve?
What is wrong with your existing earthing arrangement?

 

[ericmark]

I recently 'drove' a 6 ft copper earth rod fully, into the ground beneath my upper floor ,bedroom window to provide , what I thought would be a 'good earth point' !

Why?
What were you hoping to achieve?
What is wrong with your existing earthing arrangement?

I would assume the earth was for the radio to receive or transmit without either picking up signals mains born or creating signals mains born. It is a standard method of EMC control.

Although radio's do sometimes have a live chassis, this would normally mean a class II supply so there would be no connection to the house earth. So since there does seem to be a connection to the house earth one questions the whole arrangement.

It is all too common for a house to rely on a collection of extraneous conductive parts to gain an earth and as over the years these are reduced with water and gas supplies being replaced with plastic although the whole house is at the same earth polarity because of the supplementary protective bonding conductors this may not be the same as the ground outside.

Even when we use earth loop impedance testers this does not show with a TN system that the house is at the same polarity as the earth outside it only shows that the connection to the supply authorities earth is sound.

I have in the past seen problems where TN-C-S systems are connected to a good radio earth and the neutral was broken due to road works. It melted the 4mm earth wire. Although as a result he had turned off his mains supply so was only house in street not to lose equipment as a result.

If the reading had been below 10 volt I would not be really worried but at 56.5 volts then it does need investigation. That voltage is common with generator supplies where the centre tap is used on one winding for both 110v and 230v options but fail to see where it would arise in a standard house supply.

What sort of meter did you use? I noted with an old TV voltages of that sort generated by the switch mode power supply and with my AVO Mk8 I noted the different voltage as the range was changed. Does the radio use a switch mode supply?

Also how is the house earthed? Does it use an earth rod (TT) and have a RCD (100ma) or does the earth come from the supply authority (TN). And what is the radio you are using. Are you transmitting? If so what bands. How is the radio powered direct from mains or with a power supply? If you are a radio ham have you got some one who will visit your house to give you advice.

73 Eric GW7MGW, Ex VR2ZEP and VP8BKM

 

[automationman]

I used to repair TVs back in the mid/late 70s.... Quite often came across early 60s sets that had a live chassis and consequently a live screen on the ariel connector and cable when it was plugged in!

It was common practice to use the metal chassis as a "common" on old TVs/Radios

Some of the old TV power supplies use to use multiple high wattage voltage dividers which were wire wound resistors wound over a single insulated rod with one end electrically connected to the main chassis... You could warm your hands up in front of them on a cold day.... lovely!

Just wondering why you are trying to ground the chassis anyway and why you are using an earth rod?

 

[bernardgreen]

I used to repair TVs back in the mid/late 70s.... Quite often came across early 60s sets that had a live chassis and consequently a live screen on the ariel connector and cable when it was plugged in!

Did you ever come across the dropper resistance for the series filaments being a resistive wire in the mains lead connected to live in the plug. Shortening the lead a little meant the valves glowed brighter, good for increasing emmission in old and tired valves but cut too much of the lead and the filaments burnt out.

 

[evanrob]

Thanks gentleman for the replies to my query... Paul c, appreciate the remarks re capacitance , earth/chassis , which I assume also has a bearing with BernardGreen's reference to the old AC/DC , live chassis of 'yesterday'. Perhaps I should have been a little clearer in my explanation re the 'radio'.. This is a 'communication type of receiver, which is , as you can imagine , is rather sensitive to noise, 'hash ' etc,,, hence my concern re -- (to answer to a point , i.e. Dingbat's 'helpful'? query,WHY WHY !! )
Please do not expect me to believe that the general earth points throughout a dwelling are ideal, especially in older property, although as stated , I quite understand the reasons for a 'common' earthing point . (earth loops etc) While I would gladly acknowledg that I do not fully understand the full workings , re phase relationships ,AC theory, etc. All that I asked , was , 'Where does the arbitrary voltage of 56.5 come from?? Bearing in mind the capacity, between earth and 'so called ' neutral :
Right or wrong (I,m sure I will be corrected) .Is there some degree of 'leakage ' somewhere ??? Thanks for your time .

 

[bernardgreen]

This is a 'communication type of receiver, which is , as you can imagine , is rather sensitive to noise, 'hash ' etc,,,

but it should also be more selective between signal and noise.

Please do not expect me to believe that the general earth points throughout a dwelling are ideal, especially in older property,

They are far from ideal for any communication system "earth". They are only suitable for safety purposes ( and not always ideal for that ).

'Where does the arbitrary voltage of 56.5 come from?? Bearing in mind the capacity, between earth and 'so called ' neutral :

Measure it with a 230 volt filament lamp between the earth rod and the chassis. It will almost certainly have been reduced to zero by the lamp. Your high impedance meter is reading the current flowing through stray capacity between wires and through any filter capacitors connected to the chassis. If it isn't reduced to zero ( less than 0.5 volts ) or the lamp glows dimly then you have a fault that needs looking into by an electrican.

Often comms equipment is fed by an isolation transformer so the electronics have no direct connection to the mains to remove any problems with mains "earths" that are not at true ground potential. The isolation transformer is often the 230 to lower voltage transformer in the equipment. But often there is a filter on the incoming mains which has capacitors between live and mains earth and this capacitors create current in the "earth" wiring.


Right or wrong (I,m sure I will be corrected) .Is there some degree of 'leakage ' somewhere ??? Thanks for your time .[/quote]

 

[automationman]

[
Did you ever come across the dropper resistance for the series filaments being a resistive wire in the mains lead connected to live in the plug. Shortening the lead a little meant the valves glowed brighter, good for increasing emmission in old and tired valves but cut too much of the lead and the filaments burnt out.

Now you dragging back memories of a simpler time!! I do indeed remember that and attempting to restore valves (especially the PL509 line output pentode) by getting out the "rheostat" and super-heating the filaments in a feeble attempt "clean off" the cathode


It does sound like the 56.5 volts is capacitively coupled from perhaps line filters... The low impedance lamp should drop that to near zero.

 

[evanrob]

Again gents, many thanks for your replies, including the nostalgic memories!
First of all Ericmark, you hit the nail on the head: Yes 'ham radio ' is the name of the game here, but let me hasten to add ,chaps , A qualified electrical engineer I am definitely NOT,. Above all ,my original query was one of puzzlement and , if you like; 'a need to know ' in my dotage,??
The power suppply in the radio is 'normal' A.C. NOT, switched mode,(thank goodness). The voltage was measured with my old Avo 8, , and yes the overall house wiring does have RCD 30ma connection with I presume adequate earthing.. Bernardgreen; this is really a concern regarding mains borne general interference, not signal to noise ratio. By the way, Bernard,and you to automationman, in passing Yes I well remember, the old 'line cord' and the 'barretter' etc, you mention, used to replace then with 'series capacitors' for the valve heaters..!! ( That should date me ?)
Back to you Eric, I repeat, all this is really a 'niggle' to me;I was just wondering, as always, even now, how can I improve things , both electrically and reception-wise. This brought me to 'thumping 'the earth rod into the ground in the first place and then discovering the ,' minute 'spark on connecting to receiver chassis, which equated to the 56.5 volts?? from a 'isolated'?? earth rod ?? many thank again , trust I have'nt bored you[/b]

 

[automationman]

Evanrob, the earth rod you planted is not really isolated. It forms a high resistance path back to the sub-station transformer within a few miles of your house where the centre tap of a star-wound xformer is connected to earth (it also joins many other parallel paths too)

The main point here is that your radio power supply may be leaking capacitively coupled charges from the mains (where the neutral is near earth potential so common with the earth rod give or take a few hundred ohms) The spark you saw is simply that coupled charge trying to find a way home since it has a potential difference with respect to the earth rod which is connected back to the supply xformer via the mass of earth.

In a sense, it does not really matter assuming the lamp experiment suggested by bernardgreen causes the measured voltage to approach zero. Your AVO 8 still offers 20KOhms per volt resistance where the lamp will offer something like 100 ohms total. The main point here is the question: Did connecting the chassis to the earth rod improve (reduce) what I beleive you are saying is mains borne inteference?

 

[automationman]

Evenrob, just a note on your RCD. The RCD is not in any way connected to earth and "knows nothing" about the earth circuit.
A modern RCD simply looks to see if there is a difference in current flow between LIVE and NEUTRAL conductors. It assumes that if there is a difference then the difference must be flowing to the earth circuit (which of course it is either directly or indirectly) A domestic RCD trips when the differental current meets or exceed 30mA.

Don't worry about being dated... We are all younger than the pyramids though my wife sometimes suggest that I can remember them being built

 

[Paul_C]

Quite often came across early 60s sets that had a live chassis and consequently a live screen on the ariel connector and cable when it was plugged in!

Live chassis TV sets should have some sort of isolation between the chassis and the aerial sockets, either by way of transformer coupling or by capacitors of low enough value to present a relatively high reactance to 50Hz but not to the VHF and/or UHF radio signals.

Now you dragging back memories of a simpler time!! I do indeed remember that and attempting to restore valves (especially the PL509 line output pentode) by getting out the "rheostat" and super-heating the filaments in a feeble attempt "clean off" the cathode

And don't forget the little CRT heater booster transformers which were sold specifically with the intent of overrunning the tube's heater a little in order to try and squeeze a little more life out of it before it would need replacing.

Did you ever come across the dropper resistance for the series filaments being a resistive wire in the mains lead connected to live in the plug. Shortening the lead a little meant the valves glowed brighter, good for increasing emmission in old and tired valves but cut too much of the lead and the filaments burnt out.

Some of the "American Midget" sets imported in the 1940's modified for use on 200 to 250V supplies were equipped with very long cords of such type to drop the extra 100+ volts for the heaters.

Paul c, appreciate the remarks re capacitance , earth/chassis , which I assume also has a bearing with BernardGreen's reference to the old AC/DC , live chassis of 'yesterday'.

All related in the basic principles involved. Just as with the TV sets mentioned above, live-chassis domestic radios would employ coupling capacitors if provided with external aerial connections in order to isolate the latter from the possibility of being connected directly to a live chassis. These capacitors tend to be of higher value than those for a VHF/UHF TV aerial input, and if starting to go leaky after all these years can result in a little more current at 50Hz being available through them than was originally intended.

The isolation from direct connection to chassis wasn't just for where a 2-pin unpolarized plug was used or to guard against incorrect wiring, but also because in those areas which had D.C. mains supplies live-chassis radios & TV in homes fed from the negative side of the 3-wire distribution system had to be wired so that the chassis would end up live, otherwise they wouldn't work.

A lot of the communication receivers of the sixties used transformer supplies (and were thus A.C. only), but there would still often be capacitors connected between the incoming supply and chassis for R.F. filtering, which also result in a little leakage and the chassis potential rising somewhat if it isn't earthed. A lot of the Japanese receivers were like that.

The voltage was measured with my old Avo 8

Which is 1000 ohms per volt on the 100V range and above, so assuming you were using the 100V range to measure your 56.5V, the meter would still present a resistance of 100 kilohms. It would take a capacitance of less than 0.01uF to result in a low enough reactance at 50Hz to give the 56.5V reading. Put a load across the meter, like the lamp suggested earlier, and the voltage will drop. Switch the meter to the 250V range and you'll see the voltage rise, because the meter resistance would then increase to 250K.

 

[ericmark]

Oh evanrob you have really brought them out on this one. Well you underestimate your ability having taken the RAE it is not an easy exam and far harder than the 2382 which today seems to label a bloke as an electrician.

As to the voltage the Avo Mk8 being an old analogue meter I think from memory 20,000 ohms per volt presents the circuit with a resistance. Without pulling mine from it's resting place can't remember the scales but seem to remember something like 30, 100, 300 volt. Think the Mk7 was 50, 100, 500 volt but long time ago.

So measure at 100 volt range and 300 volt range and note the difference. What you are looking at is a 300 x 20,000 or 6Mohm resistor and a 100 x 20,000 or a 1Mohm resistor. We would expect to see a smaller voltage on the 100 volt range to 300 volt range and the over voltage is that small may be worth reading on 30 volt range. As already stated a bulb between the two earths should give some idea of current flow.

Now to question as to why. Well there is always a voltage gradient and we expect to see up to 10 volts between earths. But the 50 odd volt you are reading seems a bit over the top.

There are two basic reasons and next is to find out which it is.
1) There is something wrong with house earth.
2) The radio has some inductive or capacitive link and needs an isolation transformer.
So first point is without radio is there a voltage gradient between the house earth and earth rod? If this is 50 odd volt then the house earth needs checking.
If the house earth is within 10 volt. Normally a double diode will stop any gradient so normally no more than 1.2 volt. But if within 10 volt no real worries and we are not looking for a fault in the house but a radio fault.

If the house is OK then we are looking at the radio. You say not switched mode which considering the sort of load produced with SSB transmitter is just as well. So as talked about in many posts you are looking at a old design or a fault.

You say it is a PMR radio? Not the Icom or Yaesu or one of the other common Ham radios. I would guess it has been re-crystal to ham band or is it a phase lock looped with modern PC link to set frequency? But the make and model would help. Pye Westminster and Cambridge were common in my day but these used an external power supply.

I love my clamp on meter. No worry about wrong scale or overload. However with the old AVO although there is a overload trip at just 10A I would not want to try and measure current. Can you get another ham to help you. I know I have many times tested fellow hams shacks with my earth loop impedance meter and have also tested equipment with the PAT tester and joining a local club would likely help.

I watch with interest the replies.