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#25 Reply
Posted by
IanB
on 09 Apr, 2013 13:39
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Earthing equipment has nothing to do with personal safety it is about protecting supply and distribution equipment. earthing dates back to the late 19th century when generating equipment was driven by steam engines that were connected to earth by all the pipes involved. I attach a PDF of a page from The electrical year book of 1951 which explains that very well.
This statement is not at all correct. You have made here an error of fact, of logic and of comprehension.
The book extract you attached says that earthing protects supply equipment in the event of fault conditions. The same reference says
nothing at all about whether earthing also protects users of electrical equipment at the point of delivery. The first case is not exclusive of the second case and both may be true at the same time.
In both cases the principle is to ensure that a fault condition where a live conductor connects to earth potential causes a fault current that will trip a breaker or fuse before harmful effects can occur. Such harmful effects might include equipment damage in some cases and human injury in others.
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#26 Reply
Posted by
Monkeh
on 09 Apr, 2013 14:45
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I always thought that its primary role was to prevent a massive potential difference between, say, the chassis of the item in question and a water pipe, for example, if you were to touch both simulatneously.
That's equipotential bonding, not earthing.
However, one of the requirements for protective (earth) conductors is that they be large enough not to present a hazardous voltage in case of a short circuit before the fault clears (ie. the protective device trips).
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I may be a bit of a stickler, but current prefers the path of least impedance.
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#28 Reply
Posted by
G7PSK
on 09 Apr, 2013 15:29
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Earthing equipment has nothing to do with personal safety it is about protecting supply and distribution equipment. earthing dates back to the late 19th century when generating equipment was driven by steam engines that were connected to earth by all the pipes involved. I attach a PDF of a page from The electrical year book of 1951 which explains that very well.
This statement is not at all correct. You have made here an error of fact, of logic and of comprehension.
The book extract you attached says that earthing protects supply equipment in the event of fault conditions. The same reference says nothing at all about whether earthing also protects users of electrical equipment at the point of delivery. The first case is not exclusive of the second case and both may be true at the same time.
In both cases the principle is to ensure that a fault condition where a live conductor connects to earth potential causes a fault current that will trip a breaker or fuse before harmful effects can occur. Such harmful effects might include equipment damage in some cases and human injury in others.
I was trying to point out not to rely on earthing to protect you or others, If none of the system was earthed and there was no way that the live wires could connect to earth you would not need one, but as the text I attaching here says no insulation is perfect especially when this one was written (1921) I have some earlier stuff around but I have to dig that out.
And yes earthing equipment will help to protect people but just don't rely on it to do so use your grey matter as you don't make a good fuse if you do the least is it hurts and the worst is your light's go out and that's not good your fuse is non serviceable.
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#29 Reply
Posted by
ptricks
on 09 Apr, 2013 21:32
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Grounding has more of a role to play in North America than some other countries that use only one voltage to the home. North American power is supplied to the home at 1-16KV through a center tapped step down transformer , often called a pig on a pole. Between the two outer secondaries is 240VAC and from either secondary to the center tap is 120VAC. The center tap is grounded at the transformer to establish the neutral point , it is like when you use a transformer with a center tap as ground and then the other secondaries to produce a + and - DC supply for powering opamps ,etc. Watch out for people that don't understand how this system works, even people who work in the field often refer to the system as having different phases, it is only single phase and has nothing in common with split phase power.
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#30 Reply
Posted by
IanB
on 10 Apr, 2013 06:21
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Watch out for people that don't understand how this system works, even people who work in the field often refer to the system as having different phases, it is only single phase and has nothing in common with split phase power.
Technically, it does have two different phases though.
In a normal three phase distribution system the three phases share a common neutral (assuming a wye configuration) and each phase is 120 degrees apart from the others. In this supply the phase to phase voltage is higher than the phase to neutral voltage (by a factor of 1.73). All of the three phases are "hot" compared to ground.
In a typical US domestic supply there are two phases that share a common neutral and each phase is 180 degrees apart from the other. The phase to phase voltage is higher than the phase to neutral voltage (by a factor of 2). Each of the two phases is "hot" compared to ground.
So in practical terms you have a two phase supply instead of a three phase supply. There is a similarity in kind that differs only in the details.
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#31 Reply
Posted by
G7PSK
on 10 Apr, 2013 11:46
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Are the two halves of the secondary wound in opposite directions then in order to get 2 phase. The only true two phase that I have seen requires a 4 wire system which is one reason the 3 phase system won out.
If the two haves are 180 deg. apart. surely you would not have 240 voltage across the two live wires only 120 V. live to neutral as each half would equally oppose the other.
With 3 phase here you have 230 phase to neutral and 415 phase to phase but it is still single phase using two live wires but at 415 instead of 230 volts but the resultant power is due to the phase angle being 120 degrees at 180 deg they are equal and opposite.
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#32 Reply
Posted by
SeanB
on 10 Apr, 2013 19:40
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Yes, you have 240V between the 2 "hot" wires. You also get a 3 phase variant that has 115V between phase and neutral, but 208V between phases. Not seen in a residential install often, you will with a factory have 2 supplies fed from separate transformers. A 208V 3 phase and a 120V 2 phase supply, each with a separate meter. You also get a 277V supply often used for lighting purposes, as you can run a 1kW mercury lamp direct from this with only an inductive ballast. No need for an autotransformer HX or CWA ballast then with the additional capacitors and extra losses.
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#33 Reply
Posted by
ptricks
on 11 Apr, 2013 02:14
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In a typical US domestic supply there are two phases that share a common neutral and each phase is 180 degrees apart from the other. The phase to phase voltage is higher than the phase to neutral voltage (by a factor of 2). Each of the two phases is "hot" compared to ground.
This is a common misconception about residential power in the USA. I think people confuse the measuring of the L1 or L2 to N and saying that is 2 phases but it is wrong to say that because it isn't 2 phases as would normally be considered in power distribution. That would be like me plugging in a power transformer into an outlet and saying because it has a center tap it is now 2 phase power.
The 2 wires L1 and L2 are not separate phases, they are a single phase . Tapping the neutral which is the center tap of L1 to L2 results in half the voltage because it is only one half the winding. There are only 2 wires connected to the transformer that powers the home so only one phase can exist.
L1+L2 = 240VAC , L1+N = 120VAC , L2+N = 120VAC , all are the same phase.
This explains it better:
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#34 Reply
Posted by
c4757p
on 11 Apr, 2013 02:27
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Why does it matter how they are derived? One is 1/120Hz (180° of 60Hz) shifted relative to the other. Sounds like a "phase" to me.
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#35 Reply
Posted by
David_AVD
on 11 Apr, 2013 02:34
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I've heard this "it's not 2 phase" discussion several times over the years. I've always thought that if the 2 hot legs are not in-phase, then that's two phase. How they are derived is irrelevant I reckon.
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#36 Reply
Posted by
c4757p
on 11 Apr, 2013 02:47
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I think it would be correct to argue that split-phase isn't "two-phase", since that term has specific meaning for power distribution, but I don't think the minutiae of power distribution/electrician's terminology really matters when someone's just trying to explain the basics. Phase is just the frequency domain extension of a time offset, and it's perfectly valid for something to have a phase angle of 180°. Split-phase isn't "two-phase", but the two legs of it are most certainly phases.
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#37 Reply
Posted by
ptricks
on 11 Apr, 2013 03:06
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The problem with using the word phase when talking about AC power is that you can't start calling every transformer with a tap as having multiple phases , there is only one wave of power moving through the coil from the start of the winding to the end of the winding, that is one phase, no matter how many taps you put on the winding you will still only have one phase moving through the winding till it reaches the end. The reason grounding is so important in these systems is that without it serious shock hazards exist and is why the first power systems were 2 phase or 4 wire systems.
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#38 Reply
Posted by
JoeyP
on 11 Apr, 2013 03:11
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Anyone know what the actual resistance of "Earth" is? Not talking about locally to a grounding rod. I know it varies considerably in a give location by soil type and moisture etc. I'm talking say Sydney to Los Angeles or North pole to South pole, excluding the resistance of the local connecting point.
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#39 Reply
Posted by
ptricks
on 11 Apr, 2013 03:17
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Anyone know what the actual resistance of "Earth" is? Not talking about locally to a grounding rod. I know it varies considerably in a give location by soil type and moisture etc. I'm talking say Sydney to Los Angeles or North pole to South pole, excluding the resistance of the local connecting point.
I don't know of anywhere that list a general all around number as it would vary a lot from place to place but you might want to read about how submarines communicated in the past. Submarines used ELF communications that required lines 32 + miles long to be buried in the ground due to the high resistance of the earth. They probably have more info on it than most.
http://en.wikipedia.org/wiki/Communication_with_submarineshttp://en.wikipedia.org/wiki/Project_Sanguine
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Anyone know what the actual resistance of "Earth" is? Not talking about locally to a grounding rod. I know it varies considerably in a give location by soil type and moisture etc. I'm talking say Sydney to Los Angeles or North pole to South pole, excluding the resistance of the local connecting point.
you have to measure it.
http://www.weschler.com/_upload/sitepdfs/techref/gettingdowntoearth.pdf
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And when I said measure it I literally meant you should measure it.
I'm pretty sure no one knows, but with that book and some really long cables you might become famous.
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#43 Reply
Posted by
IanB
on 11 Apr, 2013 03:59
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That would be like me plugging in a power transformer into an outlet and saying because it has a center tap it is now 2 phase power.
It is exactly like that, because that is exactly what is going on!
If you take the center tap from a transformer as a common reference and look at the voltages on the outer terminals you do indeed have two phases.
The see the illustration below. In each case the phases are evenly distributed around the circle.
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#44 Reply
Posted by
JoeyP
on 11 Apr, 2013 04:06
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And when I said measure it I literally meant you should measure it.
I'm pretty sure no one knows, but with that book and some really long cables you might become famous.
I don't think the leads for my DMM are quite long enough. I thought maybe some really smart people had already calculated it from volume/resistivity of elements etc. Maybe it's the next final frontier. I should go get a big fat government grant...
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#45 Reply
Posted by
G7PSK
on 11 Apr, 2013 17:16
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If the 2 halves of the transformer windings were 180 deg. apart both the halves would go positive or negative at the same time so if you connected to the 2 outer trappings without the neutral no power would flow, also there is only a single phase going into the transformer shown so the input is single phase.
https://www.eevblog.com/forum/Smileys/default/bangheadonwall.gif
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#46 Reply
Posted by
ivan747
on 11 Apr, 2013 21:27
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Since we are discussing grounding, I have a question. My house wiring is a mess. We are not sure if we have grounding, but after looking around, I figured the earth wires connected to our outlets join the Neutral at the service panel. This service panel is old, so it doesn't have terminals for ground, only for neutral.
It took me a long time to figure out things because there's absolutely no consistent color coding on anything. Now, I know it is correct to join ground and neutral at the service panel, but I am still not 100% sure this is actually connected to earth with a grounding rod. There is a grounding rod outside the house, but color coding is not consistent (the wire is green, but so are both of the lines inside the service panel and possibly the neutral).
Basically, we have no visual way to know if our earth terminals are actually connected to earth. We just know earth in our outlets join neutral at the service box and the current goes back to the 17kV-240V transformer outside somehow.
How can I solve this?
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#47 Reply
Posted by
IanB
on 11 Apr, 2013 21:34
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One test would be to use a DMM to measure voltages. On an AC volts range, check that the measurement between neutral and ground in each socket is no more than a few mV, then check that the voltage between the circuit ground and metal objects like water pipes and gas pipes is also no more than a few mV. If you really want to get creative, drive a metal stake into the ground and measure between that and the circuit ground in electrical sockets. That also should be close to zero.
This is not a guarantee that everything is good, but it will certainly give you an indication if things are bad.
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#48 Reply
Posted by
ddavidebor
on 11 Apr, 2013 21:34
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Call someone with an earth tester, an insurance company and a church because is better that you make peace with god before go in the sky because you don't have working protection in your home.
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#49 Reply
Posted by
ivan747
on 11 Apr, 2013 21:45
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Call someone with an earth tester, an insurance company and a church because is better that you make peace with god before go in the sky because you don't have working protection in your home.
The scary thing is that there's a water heater with the earth lead directly connected to the water pipes inside it. An electrician came to install a ground lead yesterday, and he bolted it to the service box chassis, but the service box chassis is floating. He says it is grounded because it is embedded into the concrete wall but I wouldn't trust in that (that's not an Ufer ground by any means). The box is floating because the neutral terminals are isolated with a plastic separator.
One test would be to use a DMM to measure voltages. On an AC volts range, check that the measurement between neutral and ground in each socket is no more than a few mV, then check that the voltage between the circuit ground and metal objects like water pipes and gas pipes is also no more than a few mV. If you really want to get creative, drive a metal stake into the ground and measure between that and the circuit ground in electrical sockets. That also should be close to zero.
This is not a guarantee that everything is good, but it will certainly give you an indication if things are bad.
I just measured earth and neutral at an outlet far away from the service panel against the grounding rod, it's around 300mV AC on both cases. We use PVC pipes for water and gas pipes are not grounded because in this country you just pay for a propane gas tank and when it's empty you pay to get it swapped. They are not bolted to the floor or anything.
The closest thing to earth potential is the grounding rod and I measured 360mV between this and the earth pin on the socket. Maybe if I used a low impedance multimeter it would drop down further.