High Energy Three Phase

[Dane Bear]

Today I was told that when you run say 480v 3 phase current to a machine tool to not use two grounds, such as using the ground wire in the conduit, and installing a ground stake at the machine. Reason being, if there is more than one ground there can be a fault by "hunting" and could have no effect at all. Is this true?

[Rerouter]

slightly off, what you get is a ground differential, over 100m depending on what currents and voltages your using you can have a few hundred volts difference between the 2 grounding points, this causes a large current to flow through your ground conductor to try and equalise it,

equally it is much safer to ground local to the machine, as it reduces resistance, and is less likely to kill someone touching the case,

[Kremmen]

This, you will have to explain the reasoning behind.

In all discussions please bear in mind that neutral (N)  and Potential Equalization or ground/earth (PE), where labeled, are distinct circuits and PE is never a current carrying one. This means that under normal circumstances PE conductor must not be part of any functional circuit. (Let's not go into EMI filters and such here).

You (OP) don't say what kind of power your machine tool draws but let's assume it is at most tens of kW. If it is significantly larger (in the hunderds of kW / megawatts range) then we are into industrial installations and some adjustments to the reasoning below are needed.

I realize we probably come from places with differing electrical code and different reasons for doing things in a specific way. Nevertheless, i would have thought that multiple grounding in grounded systems would be a good thing only. At least in premises wiring here where we use TN-C-S distribution system end-to-end, the hallmark of which is that the neutral wire incoming from substation to premises is split into N and PE at the main panel, and physically grounded there as well. The strong recommendation is that the PE conductor should then be further grounded every 200 meters between buildings/sites/subpanels. This principle is variously called Protective Multiple Earthing or Multiple Earthed Neutral (even if it is actually PE, not N that is earthed).

So in my eyes the original recommendation not to use a PE conductor at all is a clear code violation. The reasoning behind this "hunting" thing does not compute at all.
Also, hundreds of volts potential difference in PE conductors are possible, but i seriously doubt you will ever see anything like that in domestic installations. The energy levels required for such are high enough that the fuses have cut the power long before. I have seen such voltages in distribution substation ground faults, but then you also get fireworks like you woudn't believe. Just doesn't happen in your average home wiring.

So, at least you must have the PE conductor. Feel free to locally ground the tool as well, it will improve the grounding and it won't hurt.

[G7PSK]

The only reason that I can see for not having a local earth on a machine is where there is some form of earth current leakage breaker in use, but  they are rare these days. Most circuits in the UK are multiple earth. When I install generating plant I always put an earth at the plant end on occasions that might be 3 or more then another earth where the power enters the building then the electricians who take over from there put their own earth in, The reason for an earth both ends of the feed which is very often less than 20 meters is power plant vibrate and an earth wire could break as can the other wires but that is more noticeable to the users and is not as serious as if the earth failed.

[ejeffrey]

You MUST use the grounded conductor that is bundled in the same conduit as your power wiring.  You can have additional ground connections, but you can't disconnect the 'official' one.  In large installations you may need multiple grounding points (Kremmen knows more about this than me), but in smaller installations that is not needed.  The other things you can't do is ever connect neutral to ground -- either via the PE wire or a different ground connection except for the point specifically called out in the code.  Doing so would allow the PE conductor to be an alternate return path and force it to carry substantial current.

Ground fault protection circuits don't care: they measuring live/neutral current imbalance and open if there is leakage to ground.  They don't care which path to ground the current takes.  Otherwise they wouldn't work in the designed situation when you throw your hair dryer into the bathtub

[Gall]

It is very important to understand the diggerence between the PE earthing wire and direct earthing of the equipment.

Short answer: never use both unless you know what you're doing.

Long answer.

There are several ways to connect the secondary winding of the Big Transformer that provides power to your outlet.
http://en.wikipedia.org/wiki/Earthing_system
Generally, one of the wires is earthed and then split in two - the "white wire" (N) and the "green-yellow wire" (PE). Both are connected to the same terminal somewhere (electrically but NOT mechanically so they cannot fail both at the same time). The reason to do so is that the N wire may fail in case of overcurrent. The PE wire carries no current and is (should be) mechanically very robust so its failure is very unlikely. It never has any devices (i.e. switches or breakers) that may disconnect PE without disconnecting other lines. (I.e., Russian rules prohibit any kind of any commutation or non-permanent connections on PE with one and only exception for wall outlets).

There are cases when PE wire should be earthed in several places. Also there are cases when PE should NOT be earthed. Consider the following:
* Your device has metal enclosure which is connected to PE.
* PE is broken somewhere.
* Someone's else device is connected to PE and broken. PE is connected to L.
If your device is earthed directly, it may happen to be one and only connection between PE and earth. This is VERY dangerous. Your PE wire and your earth connection are not rated for the currents that may appear in this case. They cannot earth the whole building!

Another case is when PE has a lot of good earth connections. I.e., an outdoors overhead line may have earth at every tower. In this case it is perfectly safe to ground everything. It may be even required. The exact circuit depends on local regulations and may vary.

[SeanB]

PME is only used on the supplier side, almost never on the consumer side, aside from TV and satellite antenna systems, which have to be connected to a dedicated lightning ground system. In that case anyway the equipment is double isolated and is not provided with a mains earth. Your supply will be earthed at the consumer meter, and again at the pole connection and the substation. The cable used there will be substantial, as the prospective fault currents are high.

[Kremmen]

If your device is earthed directly, it may happen to be one and only connection between PE and earth. This is VERY dangerous. Your PE wire and your earth connection are not rated for the currents that may appear in this case. They cannot earth the whole building!

Otherwise i mostly agree but this bit is not logical. In fact the TT distribution system does _not_ require a PE wire so there the local earth is by design the only ground connection.
Assume you _do_ have a PE conductor from the device back to the distribution panel PE busbar. How is this conductor then going to handle the current if the locally grounded one couldn't? Like i said earlier, if the code requires a PE wire, the you must bring one from the panel.
Additionally grounding the device will not make things any worse, instead it will be better _provided_ you do the local grounding according to code. There are strict minimums in the conductor sizing that are there to assure that things will not catch fire.
Saying that the earth connection is not rated is a poor excuse. It _must_ be rated to handle the maximum short circuit current, or it is a code violation.

Always bearing in mind that the code may allow different implementations. But on these days of residual current devices it is real hard to see why local grounding would be a bad thing, provided the PE wire is also there when required.

[Gall]

If your device is earthed directly, it may happen to be one and only connection between PE and earth. This is VERY dangerous. Your PE wire and your earth connection are not rated for the currents that may appear in this case. They cannot earth the whole building!

Otherwise i mostly agree but this bit is not logical. In fact the TT distribution system does _not_ require a PE wire so there the local earth is by design the only ground connection.

TT networks do not have PE at all, they only have "your" earth connection. So this has nothing to do with TT.

Assume you _do_ have a PE conductor from the device back to the distribution panel PE busbar. How is this conductor then going to handle the current if the locally grounded one couldn't?

Oh I got your point. The problem is that the "uplink" PE may be much thicker than yours. And cable to the faulty device too. Let's say we are in an apartment on the 10th floor. The wire coming from the basement to the distribution box of the 10th floor is AWG4. It is earthed in the basement. There is a AWG7 wire between the apartment's distribution box and the floor's box. And there is AWG13 wire between the circuit breaker and the wall outlet. (All wire sizes are taken from Russian rules for 220VAC in suck buildings, approximate AWG equivalents). Consider the elevator that has its motor at the top. Its connection is AWG7 too.

Let's say that the PE wire is cut between 5th and 6th floor. And we managed to connect one of our wall outlets to the earth. Consider the elevator's motor is shorted. Now we have AWG7 L connected to earth via AWG13 PE. The short-circuit current is then limited by our long AWG13 wire and our poor earth connection. It may be not enough to trigger the circuit breaker of the powerful AWG7 elevator. Guess what will burn.

In the very unlikely case of such failure one has to choose between electrical hazard and fire hazard. Fire will happen for sure, electrocut will not. People do not touch equipment too often. Even if they do, the floor and the shoes are usually dielectric. Thus it is unwise (and often prohibited) to earth a PE wire after distribution boxes. It is Ok to earth PE if this connection is able to carry the whole load of the whole building. Unfortunately this never happens unless PE is intentionally made to have multiple earth connections.

In Russia the most painful thing is retrofitting water boilers in old buildings. The pipes are made from steel and are earthed (poorly). The PE wire is retrofitted and thus cannot be fully protected from mechanical breakage. The boiler connects to both PE and steel pipes and may make a "parasitic" earth connection. The one and only way to avoid this is to use plastic dielectric inserts in the pipes.

[Kremmen]

[...]
The problem is that the "uplink" PE may be much thicker than yours. And cable to the faulty device too. Let's say we are in an apartment on the 10th floor. The wire coming from the basement to the distribution box of the 10th floor is AWG4. It is earthed in the basement. There is a AWG7 wire between the apartment's distribution box and the floor's box. And there is AWG13 wire between the circuit breaker and the wall outlet. (All wire sizes are taken from Russian rules for 220VAC in suck buildings, approximate AWG equivalents). Consider the elevator that has its motor at the top. Its connection is AWG7 too.

Let's say that the PE wire is cut between 5th and 6th floor. And we managed to connect one of our wall outlets to the earth. Consider the elevator's motor is shorted. Now we have AWG7 L connected to earth via AWG13 PE. The short-circuit current is then limited by our long AWG13 wire and our poor earth connection. It may be not enough to trigger the circuit breaker of the powerful AWG7 elevator. Guess what will burn.
[...]

No, no, no. That kind of setup is illegal right there.
The PE conductor is absolutely required to carry the full short circuit current relevant to the supply branch it is protecting. Combining an elevator motor circuit with an apartment circuit the way you speculate is out of the question. Those circuits must share neither live or PE wiring from the building entry panel onwards (unless every part of the circuit can handle whatever the fuses let through). The general and easy principle is that all circuit wire sizes (including PE wires) must match the fuses / overcurrent breakers protecting that specific circuit. And you must not share PE with differently fused circuits unless all parts of the PE wiring can handle the maximum current the fuses can pass.

One part of installation approval is to measure the minimum short circuit current capability of the wiring, including PE. For say a 16 amp fused circuit you typically aim at something like 200 amps minimum, preferably more, to guarantee a swift fuse rupture in case of a hard short. This means exactly what you are talking about - the PE circuit must be able to handle whatever current the fuses let through before rupturing.

A point here is that you cannot prevent local grounding from happening by chance. Take the classic example - your toaster with grounded metal chassis lying on its side on the grounded kitchen sink. Now the toaster housing is grounded and nothing can prevent your scenario above from happening - nothing but correct wiring, that is.

Sorry, i don't really do AWG, this being Europe. mm2 is more familiar but i get the idea.
 

[ejeffrey]

No, no, no. That kind of setup is illegal right there.

No, he has a weird double fault condition where there is a secondary distribution panel and it loses its earth connection.  Each circuit is fused for the wires involved.  The big heavy equipment has a Live-PE fault, but can't blow the breaker because the PE is disconnected.  However, if some other small circuit on the same distribution panel has an accidental earth connection, the fault current can flow through there to earth, and back up the main building grounding rod.  I think this condition is possible in the US.  I have definitely seen breaker panels that have circuits of different sizes, although I only know for sure I have seen it in a primary distribution panel, not a secondary panel.

Nevertheless, it is a stupid problem.  It shouldn't be possible to lose the earth wire without losing the live wires as well.  That is why PE must always be run along with power wiring, in the same conduit or cable so that if something cuts the earth wire it also cuts the power.  It is also why you never switch or disconnect the ground except for an outlet which should be designed to always connect ground first and disconnect it last.  Saying that shoddy wiring is dangerous is a tautology, and the answer is not to avoid accidental earth connections, but to do your wiring properly.

People do not touch equipment too often.

People touch household appliances essentially every time they use them.  Since the scenario you describe won't flip any breakers or otherwise shut off the power, it will persist until someone is electrocuted.

In Russia the most painful thing is retrofitting water boilers in old buildings. The pipes are made from steel and are earthed (poorly). The PE wire is retrofitted and thus cannot be fully protected from mechanical breakage. The boiler connects to both PE and steel pipes and may make a "parasitic" earth connection. The one and only way to avoid this is to use plastic dielectric inserts in the pipes.

The reason steel pipes are earthed has nothing to do with the live-chassis fault problem.  All large metal structures within a building should be earthed for potential equalization to avoid large induced voltages due to nearby lightning strikes, power lines, or whatever.  They do not need to and should not be relied upon to carry the full fault current of an electrical circuit, but adding a plastic pipe section to avoid running a proper PE wire is kind of insane.  And it is a hopeless cause anyway.  Maybe with something simple like a water boiler you can avoid inadvertent ground connections... but what happens when someone decides they need an alarm for if the boiler temperature drops, and they hook up a serial cable to a PC.  Now your ground connection is through the little foil shield of your serial cable.  For many appliances it is even harder.  The correct solution is to run a proper power cable that has the PE bundled with the live and neutral.  If you can't get a good earth connection, you need to switch to alternate protection measures like RCDs with the associated annoyances.

[SeanB]

Electrical standards are pretty much country specific. What is allowed in the US is not allowed in other countries, and the converse also applies. The basic principle of safety is the same, but the implementation and rules are different around the world

[ejeffrey]

Electrical standards are pretty much country specific. What is allowed in the US is not allowed in other countries, and the converse also applies. The basic principle of safety is the same, but the implementation and rules are different around the world

True, E&M works the same everywhere.  Running a physically separate grounding wire that is not well protected against mechanical damage is a bad idea no matter what the code allows.  The primary purpose of a grounded conductor is to provide a path for fault currents sufficient to trip the fuse/breaker.  If it can't do that reliably when needed, there really isn't much point in having it.

[Gall]

Electrical standards are pretty much country specific. What is allowed in the US is not allowed in other countries, and the converse also applies. The basic principle of safety is the same, but the implementation and rules are different around the world

True, E&M works the same everywhere.  Running a physically separate grounding wire that is not well protected against mechanical damage is a bad idea no matter what the code allows.  The primary purpose of a grounded conductor is to provide a path for fault currents sufficient to trip the fuse/breaker.  If it can't do that reliably when needed, there really isn't much point in having it.

Exactlier: If it can't do that reliably when needed, it may be very dangerous.

If your local regulations require the PE wire be equally thick everywhere, it does not matter where exactly it is earthed. The weakest place here is the earthing itself. If if meets the resistance and current requirements, if the earthing terminal is thick and robust enough and will definitely not fail if all the current flows through it - go on and earth your equipment! It is safe and it makes it more safe. Your local rules probably require this earthing too.

There is a different case if the PE wire is thick near the earthing and thinner near the consumer. This kind of setup is used in ex-USSR and some other countries where residental towers are common. In such a tower there are many hundreds of wall outlets in about 100 apartments. In this case each apartment cannot have its own earth connection because one cannot have 100 thick cables going to the basement. Local rules in these countries permit to use the common "bus" going from the basement vertically and earthed in the basement. This "bus" uses five thick wires, i.e. 25mm^2 or more, three phases + N + PE, thus forming TN-S system. Each apartment has its own distribution box connected to this "bus" with i.e. 10mm^2 cable. In this case it is dangerous and prohibited to connect anything but the thick "bus" to earth.

There may be even pathological situations if the building does not meet the requirements. This may happen in some poor countries i.e. if the inspection was bribed (and is not uncommon in ex-USSR, mostly in villages). It may be even worse as there may be no possibility to fix the wiring without rebuilding everything. The one and only option in this case is to separate "our" equipment from "theirs" equipment by making a TT system. In many cases expensive protection equipment is needed like overvoltage and phase control relais as it is not uncommon to sudenly get 380V out of the single-phase 220V outlet in such a building...

A typical distribution box of an old USSR building if the inspection was bribed.
Note the 0.75mm^2 flexible wire running nobody knows where and free-hanging DIN-rail circuit breakers connected by 1mm^2. And - this is not the worst I've seen!

[SeanB]

I still can beat you, but travelling through the Cato Manor "Informal Settlement" at night is rather discouraged at present.

During the day you can see the distribution wiring, most commonly a single strand of cable, 0.2mm to 2mm, without insulation, and draped from tree to tree and across sheet metal roofing on wooden stick poles, with a feed to the shack to run a TV, 2 plate stove and a few lights ( and of course a cellphone charger) with a well watered earth return buried outside somewhere close by. Fed from a wire tied to a street light pole, or by cutting the cable feeding some house. Current is enough to make the wire glow brightly. the culture of entitlement is such that they do not want to pay for power even if it is offered, but will rather steal it.

[Gall]

Ouch. I definitely do not want such kind of connection

The worst problem in old buildings in many countries is that the connection is only supposed to be made according to the rules. Even if there is a good thick wire, somebody may cut it and join back together either to steal electricity or to steal copper. Thus the main safety rule in such buildings is: "If you cannot see something, assume it is in worst possible condition and wired incorrectly. If you cannot see something not because the door is locked, assume it is or will be stolen."

[Gall]

And a specific Russian rule: "If you see a circuit breaker made by IEK, multiple its rating by two. If you and your neighbor share the same distribution box, do not trust your neighbor's breakers. He may change them to 100A being tired by blackouts caused by his electric kettle."

[M. András]

i should have made a photo of our fuse box few years ago. 10 16A breakers  we have 3 phase coming in without a 3 phase outlet. the 3x240volts wire  all black around 4mm²  1 8mm² and 1 somewhere in the middle, i dont remember how it was distributed to 10 breakers but none of the distribution strips were used on that wallmount box, earth and neutral was simply wrapped around the larger wires fairly burned insulation etc  all of the wires were black without any marking. it was a shocking sight i have to admit. the rest of the building is in the same state built in the 80's  all of the wires aluminium

[Dane Bear]

Wow! just got back, this is alot of reading!! Many thanks.

[HackedFridgeMagnet]

If you see a circuit breaker made by IEK, multiple its rating by two

that's great.

Aus/NZ uses what we call an MEN system which is really just our flavour of the TN-C-S system.

In AS3000, I think it is mandatory to only have one connection between Neutral and Earth per building.
An MEN system is described as a good connection to from the Earth Bar to physical ground and a connection from that Earth bar to the Neutral, plus all the earth conductors that go to each sub circuit.

 A separate building can have it's own MEN system with its own Neutral to Earth connnection and earth stake, but you have to make sure there is no earth run between the two buildings, such as electrically continuous water pipes and telecoms shields. These improper connections are what gives people those tingles that scare the hell out of them.

I am pretty sure that as long as you maintain the integrity of the MEN system as described above you can then add other ground stakes.

What you cant do is add other Neutral - Earth connectors within the same building.