Earthing rod question

[John Smillie]

Hi, I was thinking of taking a spur from the ring main in my house
out to the garage where I plan to use a tumble dryer and a freezer. I
would install an RCD protected double socket in the garage:

http://www.screwfix.com/p/nexus-2g-13a-rcd-switched-socket/91095

I thought I would provide a separate earth. We are on clay so I think
I can get away with banging an earthing rod through the garage floor.

I read that I need a low resistance, say 50 ohms. But what is the
circuit? Form where to where? I can connect one terminal of my meter
to the earth rod but where does the other terminal go?

 

[ARWadsworth]

Hi, I was thinking of taking a spur from the ring main in my house
out to the garage where I plan to use a tumble dryer and a freezer. I
would install an RCD protected double socket in the garage:

http://www.screwfix.com/p/nexus-2g-13a-rcd-switched-socket/91095

I thought I would provide a separate earth. We are on clay so I think
I can get away with banging an earthing rod through the garage floor.

I read that I need a low resistance, say 50 ohms. But what is the
circuit? Form where to where? I can connect one terminal of my meter
to the earth rod but where does the other terminal go?

Providing a TT setup for a shed power supply could be overkill.

Are there any reasons why you cannot export the earth from the house? Have a
look at

http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside

 

[John Smillie]

Thanks for both answers, which provided very comprehensive links.

 

[ARWadsworth]

The test procedures are described here:

http://wiki.diyfaq.org.uk/index.php?title=TT_Earthing

(nice and simple if you have an earth loop impedance meter, bit more
complex if not. Sometimes buying a second hand one on ebay and
flogging it after is a sensible approach)

Except that you will not sell it

The worst case is having to hide it behind a tin of old paint in the shed
and just telling SWMBO that you have sold it.

 

[NT]

Hi,  I was thinking of taking a spur from the ring main in my house
out to the garage where I plan to use a tumble dryer and a freezer.  I
would install an RCD protected double socket in the garage:

http://www.screwfix.com/p/nexus-2g-13a-rcd-switched-socket/91095

I thought I would provide a separate earth.  We are on clay so I think
I can get away with banging an earthing rod through the garage floor.

I read that I need a low resistance, say 50 ohms.  But what is the
circuit?  Form where to where?  I can connect one terminal of my meter
to the earth rod but where does the other terminal go?

You connect a nice thick earth wire from the earth rod to the earth
connector block by the CU. The meter isnt generally earthed.

As mentioned, I dont know whether its worth doing this, or simply
using the main house earth.

You dont need a fancy tester to determine the rod earth's resistance,
very basic equipment will do that. A car battery and a multimeter
should suffice.


NT

 

[robgraham]

You connect a nice thick earth wire from the earth rod to the earth
connector block by the CU. The meter isnt generally earthed.

As mentioned, I dont know whether its worth doing this, or simply
using the main house earth.

You dont need a fancy tester to determine the rod earth's resistance,
very basic equipment will do that. A car battery and a multimeter
should suffice.

NT

And it doesn't need to be a 'nice thick wire' as the ground resistance
is sufficiently high that even the resistance of standard 2,5mm
earthing cable is insignificant.

Rob

 

[Tim Watts]

robgraham wrote:

And it doesn't need to be a 'nice thick wire' as the ground resistance
is sufficiently high that even the resistance of standard 2,5mm
earthing cable is insignificant.

Rob

But there are minimum CSAs set depending on the route, so that the wire is
suficien from a mechanical POV. If it's in the open unclipped or buried
direct, it's something like 6mm2 (I don't have the book to hand, could be
4mm2, but definately bigger than 2.5)

 

[Andy Wade]

I read that I need a low resistance, say 50 ohms. But what is the
circuit? Form where to where? I can connect one terminal of my meter
to the earth rod but where does the other terminal go?

To the 'general mass of earth' (which does not exist as a convenient
physical terminal...

Re-repost:

In the absence of a fancy earth tester[1], the following simple DIY
procedure that I've posted a few times before will give results of
entirely adequate accuracy:

[Repost from 20/04/1998, Message-ID: <[email protected]>]

Anyone with a bit of electrical common sense, knowledge of Ohm's law,
and a decent multimeter can measure earth electrode resistance quite
easily. You need to isolate the electrode in question and then find a
way of getting some current to flow into it. A safe way to do this is
to use a double-wound mains transformer with a secondary voltage of
around 24 (exact value not critical). Connect one end of the secondary
via a suitable length of wire to the main earth terminal in the house
and connect the other end to your earth electrode via an ammeter.

Energise the primary of the transformer, and the secondary current which
flows will immediately give you a rough idea of the total resistance in
the circuit, most of which will be attributable to your electrode. For
a more accurate result, drive a second temporary earth electrode (a 2ft
offcut of 15mm water pipe will do) into the ground at a distance of 10m
or more from the one you're measuring. Then use the meter on volts to
measure the voltage drop between the two electrodes. Dividing this
figure by the electrode current measured earlier gives you the earth
resistance. (Reactance in the circuit will be negligible.) Move the
temporary reference electrode to a second position and repeat. Average
the two values obtained, but if they are significantly different, try
further positions for the reference electrode.

[1] Not to be confused with a loop tester for measuring earth fault loop
impedance, which *is* an indispensable piece of kit.

 

[NT]

I read that I need a low resistance, say 50 ohms.  But what is the
circuit?  Form where to where?  I can connect one terminal of my meter
to the earth rod but where does the other terminal go?

To the 'general mass of earth' (which does not exist as a convenient
physical terminal...

Re-repost:

In the absence of a fancy earth tester[1], the following simple DIY
procedure that I've posted a few times before will give results of
entirely adequate accuracy:

[Repost from 20/04/1998, Message-ID: <[email protected]>]

Anyone with a bit of electrical common sense, knowledge of Ohm's law,
and a decent multimeter can measure earth electrode resistance quite
easily.  You need to isolate the electrode in question and then find a
way of getting some current to flow into it.  A safe way to do this is
to use a double-wound mains transformer with a secondary voltage of
around 24 (exact value not critical).  Connect one end of the secondary
via a suitable length of wire to the main earth terminal in the house
and connect the other end to your earth electrode via an ammeter.

Energise the primary of the transformer, and the secondary current which
flows will immediately give you a rough idea of the total resistance in
the circuit, most of which will be attributable to your electrode.  For
a more accurate result, drive a second temporary earth electrode (a 2ft
offcut of 15mm water pipe will do) into the ground at a distance of 10m
or more from the one you're measuring.  Then use the meter on volts to
measure the voltage drop between the two electrodes.  Dividing this
figure by the electrode current measured earlier gives you the earth
resistance. (Reactance in the circuit will be negligible.)  Move the
temporary reference electrode to a second position and repeat.  Average
the two values obtained, but if they are significantly different, try
further positions for the reference electrode.

[1] Not to be confused with a loop tester for measuring earth fault loop
impedance, which *is* an indispensable piece of kit.

how to make life hard. Just connect car battery between mains earth
and the new rod via an ammeter. So much easier.


NT

 

[NT]

And it doesn't need to be a 'nice thick wire' as the ground resistance
is sufficiently high that even the resistance of standard 2,5mm
earthing cable is insignificant.

Rob

according to the rules it does. There are reasons why.


TN

 

[robgraham]

On 01/10/2011 14:42, John Smillie wrote:

To the 'general mass of earth' (which does not exist as a convenient
physical terminal...

Re-repost:

In the absence of a fancy earth tester[1], the following simple DIY
procedure that I've posted a few times before will give results of
entirely adequate accuracy:

[Repost from 20/04/1998, Message-ID: <[email protected]>]

Anyone with a bit of electrical common sense, knowledge of Ohm's law,
and a decent multimeter can measure earth electrode resistance quite
easily.  You need to isolate the electrode in question and then find a
way of getting some current to flow into it.  A safe way to do this is
to use a double-wound mains transformer with a secondary voltage of
around 24 (exact value not critical).  Connect one end of the secondary
via a suitable length of wire to the main earth terminal in the house
and connect the other end to your earth electrode via an ammeter.

Energise the primary of the transformer, and the secondary current which
flows will immediately give you a rough idea of the total resistance in
the circuit, most of which will be attributable to your electrode.  For
a more accurate result, drive a second temporary earth electrode (a 2ft
offcut of 15mm water pipe will do) into the ground at a distance of 10m
or more from the one you're measuring.  Then use the meter on volts to
measure the voltage drop between the two electrodes.  Dividing this
figure by the electrode current measured earlier gives you the earth
resistance. (Reactance in the circuit will be negligible.)  Move the
temporary reference electrode to a second position and repeat.  Average
the two values obtained, but if they are significantly different, try
further positions for the reference electrode.

[1] Not to be confused with a loop tester for measuring earth fault loop
impedance, which *is* an indispensable piece of kit.

how to make life hard. Just connect car battery between mains earth
and the new rod via an ammeter. So much easier.

NT

I'm obviously suffering from thickness this morning as I fail to see
how either the ac or dc methods work.

Draw a box representing your house - draw another box inside
representing the load items. Two wires go into the box and are
connected by a coil or resistor; thus representing L,N and the load
and are all isolated from the building. A third wire now goes from
the outside of the load box to either the battery or transformer
2ndry, with the other lead of the power source going to ground.

No current can flow because the case of the load is floating so how
does this work ?

I can only assume I'm missing something somewhere as the instructions
are written with the utmost confidence, and seeming clarity !! :>)

Rob

 

[tony sayer]

Anyone with a bit of electrical common sense, knowledge of Ohm's law,

and a decent multimeter can measure earth electrode resistance quite
easily.  You need to isolate the electrode in question and then find a
way of getting some current to flow into it.  A safe way to do this is
to use a double-wound mains transformer with a secondary voltage of
around 24 (exact value not critical).  Connect one end of the secondary
via a suitable length of wire to the main earth terminal in the house
and connect the other end to your earth electrode via an ammeter.

Energise the primary of the transformer, and the secondary current which
flows will immediately give you a rough idea of the total resistance in
the circuit, most of which will be attributable to your electrode.  For
a more accurate result, drive a second temporary earth electrode (a 2ft
offcut of 15mm water pipe will do) into the ground at a distance of 10m
or more from the one you're measuring.  Then use the meter on volts to
measure the voltage drop between the two electrodes.  Dividing this
figure by the electrode current measured earlier gives you the earth
resistance. (Reactance in the circuit will be negligible.)  Move the
temporary reference electrode to a second position and repeat.  Average
the two values obtained, but if they are significantly different, try
further positions for the reference electrode.

[1] Not to be confused with a loop tester for measuring earth fault loop
impedance, which *is* an indispensable piece of kit.

how to make life hard. Just connect car battery between mains earth
and the new rod via an ammeter. So much easier.


NT

I bow to my learned friend Mr Wade's knowledge and respect for Brother
Meow, but would this be sufficient volts to drive meaningful amps
through the ground especially if its bone dry like at the moment?..

 

[Andy Wade]

[...]
No current can flow because the case of the load is floating so how
does this work ?

Uh? The current flow is between two things independently connected to
earth:

(i) the means of earthing for the house installation, usually the TN
earth provided by the electricity supplier (DNO). By statute this must
have a resistance to the general mass of earth of less than 20 ohms. In
practice it will be very much lower than that in most urban areas; and

(ii) the isolated earth electrode that you are testing.

If the house earthing is TT (with its own electrode and no metallic
earth from the DNO) then the resistances may be comparable so you would
have to use the reference electrode method to get valid results.

DC methods can be affected by incidental electrolytic PDs (different
metals, salts in the soil, etc.) so AC testing is preferred. If you
must use DC, reverse the battery and average the results.

 

[robgraham]

On 01/10/2011 14:42, John Smillie wrote:
I read that I need a low resistance, say 50 ohms.  But what is the
circuit?  Form where to where?  I can connect one terminal of my meter
to the earth rod but where does the other terminal go?
To the 'general mass of earth' (which does not exist as a convenient
physical terminal...
Re-repost:
In the absence of a fancy earth tester[1], the following simple DIY
procedure that I've posted a few times before will give results of
entirely adequate accuracy:
[Repost from 20/04/1998, Message-ID:<[email protected]>]
Anyone with a bit of electrical common sense, knowledge of Ohm's law,
and a decent multimeter can measure earth electrode resistance quite
easily.  You need to isolate the electrode in question and then finda
way of getting some current to flow into it.  A safe way to do this is
to use a double-wound mains transformer with a secondary voltage of
around 24 (exact value not critical).  Connect one end of the secondary
via a suitable length of wire to the main earth terminal in the house
and connect the other end to your earth electrode via an ammeter.
Energise the primary of the transformer, and the secondary current which
flows will immediately give you a rough idea of the total resistance in
the circuit, most of which will be attributable to your electrode.  For
a more accurate result, drive a second temporary earth electrode (a 2ft
offcut of 15mm water pipe will do) into the ground at a distance of 10m
or more from the one you're measuring.  Then use the meter on volts to
measure the voltage drop between the two electrodes.  Dividing this
figure by the electrode current measured earlier gives you the earth
resistance. (Reactance in the circuit will be negligible.)  Move the
temporary reference electrode to a second position and repeat.  Average
the two values obtained, but if they are significantly different, try
further positions for the reference electrode.
[1] Not to be confused with a loop tester for measuring earth fault loop
impedance, which *is* an indispensable piece of kit.

how to make life hard. Just connect car battery between mains earth
and the new rod via an ammeter. So much easier.

That's ok for this particular application where there is a mains earth
to make use of. However with a TT only install you won't have a "known
good" earth connection to take advantage of.

--
Cheers,

John.

/=================================================================\
|          Internode Ltd -  http://www.internode.co.uk          |
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|        John Rumm - john(at)internode(dot)co(dot)uk              |
\=================================================================/

Having re-read the Wiki to see what I had missed, and decided that it
really isn't very clearly explained for a TT system (there is a little
bit of an indication that the author was either bored at this point or
knew so much that his wording isn't quite clear) - anyway I went off
to see what Mr Google could find and came up with this far clearer
explanation, which I would commend to anyone seeking to test their TT
system.

http://www.electrical-testing-safety.co.uk/the-electrical-tests-part-2.html

Rob

 

[NT]

[...]
No current can flow because the case of the load is floating so how
does this work ?

Uh?  The current flow is between two things independently connected to
earth:

(i) the means of earthing for the house installation, usually the TN
earth provided by the electricity supplier (DNO).  By statute this must
have a resistance to the general mass of earth of less than 20 ohms.  In
practice it will be very much lower than that in most urban areas; and

(ii) the isolated earth electrode that you are testing.

If the house earthing is TT (with its own electrode and no metallic
earth from the DNO) then the resistances may be comparable so you would
have to use the reference electrode method to get valid results.

DC methods can be affected by incidental electrolytic PDs (different
metals, salts in the soil, etc.) so AC testing is preferred.  If you
must use DC, reverse the battery and average the results.

The resistance you get (Vbattery/i_flow) is the R of the 2 earth
electrodes added together. In a lot of cases the main house earth will
be of very low R, giving only a slight addition to the total R. If the
main house eaeth is also a local rod, it will have significant
resistance. This can be measured fairly easily, with enough accruacy
for this purpose, but hopefully it wont be necessary.


NT

 

[NT]

Anyone with a bit of electrical common sense, knowledge of Ohm's law,
and a decent multimeter can measure earth electrode resistance quite
easily.  You need to isolate the electrode in question and then finda
way of getting some current to flow into it.  A safe way to do this is
to use a double-wound mains transformer with a secondary voltage of
around 24 (exact value not critical).  Connect one end of the secondary
via a suitable length of wire to the main earth terminal in the house
and connect the other end to your earth electrode via an ammeter.
Energise the primary of the transformer, and the secondary current which
flows will immediately give you a rough idea of the total resistance in
the circuit, most of which will be attributable to your electrode.  For
a more accurate result, drive a second temporary earth electrode (a 2ft
offcut of 15mm water pipe will do) into the ground at a distance of 10m
or more from the one you're measuring.  Then use the meter on volts to
measure the voltage drop between the two electrodes.  Dividing this
figure by the electrode current measured earlier gives you the earth
resistance. (Reactance in the circuit will be negligible.)  Move the
temporary reference electrode to a second position and repeat.  Average
the two values obtained, but if they are significantly different, try
further positions for the reference electrode.
[1] Not to be confused with a loop tester for measuring earth fault loop
impedance, which *is* an indispensable piece of kit.

how to make life hard. Just connect car battery between mains earth
and the new rod via an ammeter. So much easier.

NT

I bow to my learned friend Mr Wade's knowledge and respect for Brother
Meow, but would this be sufficient volts to drive meaningful amps
through the ground especially if its bone dry like at the moment?..

12v is enough to overcome electrolytic potentials. Any effcetive earth
rod is in soil that's always wet, even when the surface is dry.


NT

 

[NT]

On 04/10/2011 09:09, robgraham wrote:
[...]
No current can flow because the case of the load is floating so how
does this work ?
Uh?  The current flow is between two things independently connected to
earth:
(i) the means of earthing for the house installation, usually the TN
earth provided by the electricity supplier (DNO).  By statute this must
have a resistance to the general mass of earth of less than 20 ohms.  In
practice it will be very much lower than that in most urban areas; and
(ii) the isolated earth electrode that you are testing.
If the house earthing is TT (with its own electrode and no metallic
earth from the DNO) then the resistances may be comparable so you would
have to use the reference electrode method to get valid results.
DC methods can be affected by incidental electrolytic PDs (different
metals, salts in the soil, etc.) so AC testing is preferred.  If you
must use DC, reverse the battery and average the results.

The resistance you get (Vbattery/i_flow) is the R of the 2 earth
electrodes added together. In a lot of cases the main house earth will

Not forgetting the internal resistance of the battery...

dont forget that 0.05 ohms


NT

 

[robgraham]

On 04/10/2011 08:43, NT wrote:
On 01/10/2011 14:42, John Smillie wrote:
I read that I need a low resistance, say 50 ohms.  But what is the
circuit?  Form where to where?  I can connect one terminal of my meter
to the earth rod but where does the other terminal go?
To the 'general mass of earth' (which does not exist as a convenient
physical terminal...
Re-repost:
In the absence of a fancy earth tester[1], the following simple DIY
procedure that I've posted a few times before will give results of
entirely adequate accuracy:
[Repost from 20/04/1998, Message-ID:<[email protected]>]
Anyone with a bit of electrical common sense, knowledge of Ohm's law,
and a decent multimeter can measure earth electrode resistance quite
easily.  You need to isolate the electrode in question and then find a
way of getting some current to flow into it.  A safe way to do this is
to use a double-wound mains transformer with a secondary voltage of
around 24 (exact value not critical).  Connect one end of the secondary
via a suitable length of wire to the main earth terminal in the house
and connect the other end to your earth electrode via an ammeter.
Energise the primary of the transformer, and the secondary current which
flows will immediately give you a rough idea of the total resistancein
the circuit, most of which will be attributable to your electrode.  For
a more accurate result, drive a second temporary earth electrode (a 2ft
offcut of 15mm water pipe will do) into the ground at a distance of 10m
or more from the one you're measuring.  Then use the meter on volts to
measure the voltage drop between the two electrodes.  Dividing this
figure by the electrode current measured earlier gives you the earth
resistance. (Reactance in the circuit will be negligible.)  Move the
temporary reference electrode to a second position and repeat.  Average
the two values obtained, but if they are significantly different, try
further positions for the reference electrode.
[1] Not to be confused with a loop tester for measuring earth fault loop
impedance, which *is* an indispensable piece of kit.
how to make life hard. Just connect car battery between mains earth
and the new rod via an ammeter. So much easier.
That's ok for this particular application where there is a mains earth
to make use of. However with a TT only install you won't have a "known
good" earth connection to take advantage of.

Having re-read the Wiki to see what I had missed, and decided that it
really isn't very clearly explained for a TT system (there is a little

Which bit of the wiki in particular? Quite happy to go and reword things
to make it clearer.

bit of an indication that the author was either bored at this point or
knew so much that his wording isn't quite clear) - anyway I went off

Well the section on electrode testing in the TT article was a direct
lift of Andy's words as posted above - so perhaps the latter!

I suspect a diagram might help ;-)

to see what Mr Google could find and came up with this far clearer
explanation, which I would commend to anyone seeking to test their TT
system.

http://www.electrical-testing-safety.co.uk/the-electrical-tests-part-...

--
Cheers,

John.

/=================================================================\
|          Internode Ltd -  http://www.internode.co.uk          |
|-----------------------------------------------------------------|
|        John Rumm - john(at)internode(dot)co(dot)uk              |
\=================================================================/

Sorry John - I will admit to being old and therefore in some people's
minds a bit dottled and slow, but also to having been an electronics
engineer for all my career and dabbled in electrics all my life, and
I'm afraid I found the description in the the 'other ' website totally
clear whereas the Wiki one I struggled with. I think that struggle is
evident from my earlier post on this matter too.

Rob