ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Actually, that coil has very little to do with lightning.
What it is meant for is a bit of spare length in case either the building moves on it's foundations or the earth stake needs to be re-terminated for whatever reason.

The MEN system of supply down in this part of the world depends very heavily on this one wire.

I have seen that before on low voltage wires in HVAC. It is sort of a shock absorber to relieve some of the strain on the wire from flexing etc. Sort of like the curly cord on your phone handset.

It WILL add a bit of inductance, which will decrease the effectiveness of the grounding system at handling fast risetime transients.

If one was to make the rod 'flush' with grade, would the curly Q be long enough to allow that? Perhaps, the inspector wanted to see the rod, and clamp prior to it being driven flush.

Or, are the codes in Australia not the same as stateside that the rod must be 'flush'?

How much inductance and how much more damage from a lightning strike will this inductance cause? My GUT feeling is not much. That wire looks to be equivalent to about 10 AWG.

No they aren't, John.
You are allowed to install them flush, but it is not a requirement.

Another difference between Aussie practice and US practice is that here the use of a green (or green / yellow) wire for this application is actually forbidden.

The style of the connection has long been disallowed here - by local rules, if nothing else - since the 70's. At that time, there was a campaign against that method, asserting that such connections were too easily damaged by lawn mowers, weed whackers, and the like.

I think 'inductance' is the wrong word; I think the word we ar thinking of is 'impedence.' As in, a coil of wire has a greater impedence than a straight wire. Since impedence is an AC phenominum, I am not sure the coil would have any effect on current flow caused by a (DC) lightning strike.

Lightning though not AC, shares more in common with AC than DC with its sharp rise times and varying currents. If you look at strikes with a spectrum analyzer you would find that each strike is built of multiple frequencies across the specturm. To do a complete analysis of the effect this pracitce would have on the lightning strike depends on each individual strike.

With that said let's assume that we happen to have a direct building strike to the electrical equipment with its major component frequencies in the 100 kHz range (somewhat typical but again vaires widely) and the that the coil provides 0.001 mHenrys the impedence to the major component of this assumed strike would be (1j + negligible resistance) ohms. If the current portion of the frequencies where a small 5000 amps (strikes can range from a couple thousand amps to 200,000 with super strikes easily exceeding this range) then the voltage drop across this coil is 5 kV. That is significant and will be the cause of loss equipment inside the building in a direct strike.

Someone who is a ham radio operator would quickly be able to estimate the inductance of the coil, but say the inductance is 100 times greater (a mere 0.1 mHenries), now we are looking at 500 kV. I am not going to look up the formula though I should have it memorized somewhere.

I recommend against this practice. NFPA 780 inhibits bends to not exceed 90 degrees from straight and that the radius of the bend be 8" (about 20 cm) minimum.

John,
The colour of earth (ground) wires, was changed down this way some years ago, mainly because of the earth wire in flexible cords.
Colour-blind people wiring plug-tops on to the ends of flexes are most likely to transpose the red (phase) and green earth wires.
(Red-Green colour blindness is the most common type)

In effect, all earth wires are coloured with the yellow stripes, and I must say it makes it easier to identify an earth wire in a switch-board panel where you don't have a lot of ambient light.

BTW, that wire in the photo is #10 AWG, it is known as 6mm² down this way.
The clamp looks rather old though, they are a LOT more substantial these days.

I will reinforce the folks who say the coil is a bad idea.
It slows the leading edge of the spike and while that is happening it can go the other way.

In surge protection we do exactly the opposite. We put inductors (ferrite beads) in the signal lines and very short, straight grounding conductors. The theory is, by the time the shot could get down the signal lines, the ground has shunted it out. I know one engineer who swore tying a few overhand knots in the power cord would mitigate a transient. A lot of people told him he was nuts but if you looked later they had a few knots in their power cords. A ferrite bead is a lot better.

Mike, I was highlighting one of the quirks of the wording in our code.

Green, or green/yellow, is reserved for the 'equipment grounding condustor' alone. That's the 'earth' wire from the receptacle to the panel. This is also sometimes referred to as the 'bonding' conductor.

The wire from the panel to the ground rod to the panel has a completely different name: the 'grounding electrode conductor,' or GEC. No color is specified; indeed, it need not be insulated at all.

The same issue arises with the 'water bond,' or wire from the panel to where the water line enters the house.

So, we look to other parts of the code which reserve other colors for specific uses. The only reserved colors are green, green/yellow, gray, white, and in one special circumstance, orange.

In common practice, the wire is solid and bare, while the similar 'water bond' wire is stranded and has black insulation.

6mm2 main earth? What service conductor size would that typically be used with? (here I think the main earth/PEN bond has to equal the cross-section of the phases, usually ends up being 16mm2 or up).

I suppose that's another difference between our codes!

Here, while larger conductors are often specified, the largest wire we're ever REQUIRED to run to a ground rod is #6, which is (seat-of-pants guess here) perhaps 12mm2.

A lot has been written on the topic, but it all boils down to this question: Just what doest the ground rod do? Whatever else it does, it certainly does not help to clear faults.

Ragnar,
That would be used with a 16mm² phase conductor, note that this only applies to single phase 230V installations.
Anything poly-phase would have a minimum earthing lead size of 10mm².



John,
#6 is equivalent to 10mm².
Believe me, I've seen some pretty weird things offered up as an Earth Electrode in my time, it seems to be the further you get out in the rural areas here, the more bizarre they become.

One old guy once tried to convince me that wrapping the Earthing Lead around an old axle off of a tractor (with the wheels and tyres still on it) and burying it, "should comply with the most stringent of electricity regulations".

This is from table 8. It will give you a conversion of AWG to MM2

http://gfretwell.com/electrical/mm%20to%20awg.txt

It appears 10 mm is more like #8

According to that table, #6 would be pretty close to 16mm2.

Why are there such considerable differences between stranded and solid conductors? Around here, we use the total cross section, e.g. stranded 1.5mm2 means a wire with a number of strands, the sum of strand cross sections equaling 1.5mm2. So 1.5mm2 ist 1.5mm2, regardless of whether it is stranded or solid. The effective current carrying capacity is the interesting thing, not the cross section of the wire itself (there IS a considerable difference in that regard, which is why I hate using stranded wire for fixed wiring where space is ntoriously tight).

There does seem to be a problem with table 8.
The sq/mm and sq/in is bigger with stranded wire but the resistance is higher. I bet they just use the area of the outer diameter, not the cross section of the strands.

http://gfretwell.com/electrical/Table%208.jpg

Greg,
I've always found with Imperial-Metric conversions there is always some sort of a concession.
Wires over here are stated as {so many wires @ a given measured diameter), therefore we have something like 4.0mm² that has 7/0.85mm in the conductor twist.

That list of Table 8 makes me wonder if we don't have accountants in there somewhere.
I've never seen a 1.3mm² wire here, mind you, with inflation the way it is these days, you just never know.

Table 8 is just what you get if you have a calculator and a lot of time on your hands.
It is clear the area in in2 or MM2 in that table is just what you get when you compute the area of the circumscribed circle at the widest diameter, not the cumulative area of the strands.
The metric numbers are simply the calculated size, not any trade size you would find in a metric country.

It does make me wonder if there is some rounding going in in the actual wire since that is an international commodity. I know we can't really get 1/2" plywood anymore except for a few specialty suppliers. It is 12mm that they call 15/32 and most people think of as 1/2".
I guess I should get some wire and "mike" it.

If what I heard is true you'd be surprised if you did! Apparently the nominal cross section is defined based on copper with a certian specific resistance. If copper with a lower resistance is used, the actual wire size can be smaller than the nominal cross section!

It is clear that the NEC table doesn't show actual trade sizes, I'd consider it a tool for easier comparison. However, it'd be much more useful if it used the cumulative area of strands in case of stranded wire since that's what you want in an ampacity table.

I guess the main use of said table would be something along the lines of "Ok, I've got NEC tables, I am working to US specifications but have to use metric wire. What size conductors do I need to use?".

For those interested: this PDF shows what such an ampacity table usually looks like in Europe (page 4)
http://library.abb.com/GLOBAL/SCOT/SCOT209.nsf/VerityDisplay/C6ED3B4782C5F804C12572A5003A1CA0/$File/2CDC401002D0102.pdf

Page 5 shows maximum ampacities under specific conditions and associated standard sizes of OCPDs.

You folks talk funny

I think I was able to sort it out.

Nennquerschnit = wire size
Kabeln = Cables
Anzahl der gleichzeitig belasteten Adern = number of conductors
Strombelastbarkeit = amp rating
Nennstrom = Ampacity

Am I close?

You nailed it pretty close!
It's current-carrying conductors though, so if planning a 2w 1ph circuit it's 2, in case of 3ph 4w it's only 3.
Nennstrom is the nominal breaker/fuse rating and Strombelastbarkeit is the wire ampacity.

Cold-drawn metal of any description will probably be right on the bottom limit of the spec., on instructions from the bean counters. Pull a few miles of wire through a die and 0.002" on diameter adds up to a heap of $$$, especially where copper is concerned.