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Hello everyone ?
It's totally confusing for me. It is my understanding that the neutral and the ground busbars should be connected at the main distribution board (schematic attached), but not at the auxiliary or sub panel. But why?
I know that if I connect them in sub panel the current from neutral will flow through ground wires and metalic in the house, and it's dangerous. But, what is difference? The other sides of this wires are already connected together, their origin is neutral and ground busbar in main panel that are already connected!!?? ?
 

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The rules on the N-E link vary a lot by country. In the UK, for example, the link is only done by the electricity provider (DNO = distribution network operator) and not within an installation. However, if using a generator then the generator has the link (as if it were the electricity supplier) but transfer switches here in the UK have to switch neutral as well for this reason (so only one external N-E link is present at any point in time).

In other countries it is done at the main DB (as you show) and goes by various different names.

The key points behind having only one N-E link are basically down to safety:
  • Shock - an open fault on a cable carrying current does not lead to "earthed" metalwork becoming live.
  • Fire - a high neutral current does not circulate through earth metalwork and bonding conductors that is not rated for such a sustained level of I^2R heating.
  • Testing - so you can easily separate N and E at one place close to the power source to perform an insulation test on the N conductor to make sure the above are true.
 
The rules on the N-E link vary a lot by country. In the UK, for example, the link is only done by the electricity provider (DNO = distribution network operator) and not within an installation. However, if using a generator then the generator has the link (as if it were the electricity supplier) but transfer switches here in the UK have to switch neutral as well for this reason (so only one external N-E link is present at any point in time).

In other countries it is done at the main DB (as you show) and goes by various different names.

The key points behind having only one N-E link are basically down to safety:
  • Shock - an open fault on a cable carrying current does not lead to "earthed" metalwork becoming live.
  • Fire - a high neutral current does not circulate through earth metalwork and bonding conductors that is not rated for such a sustained level of I^2R heating.
  • Testing - so you can easily separate N and E at one place close to the power source to perform an insulation test on the N conductor to make sure the above are true.
This answer is great. I could be wrong on this, but my take is everything connected to the neutral will add to the neutrals resistance to ground. Adding the ground to the MDP only, will ensures that the resistance to ground is retained.
 
I think you are a bit confused when talking about resistances.
Put simply :
The neutral wiring (which should always be treated as "live") carries the load current. If it gets broken, then something doesn't work but nothing exposed is pulled high by the load current.
The earth wiring is there to keep exposed conductive parts at a safe voltage relative to ground.

There are electrical systems where there is no connection between "neutral" and earth known as IT. They are pretty well unheard of in domestic environments as a single fault will "do nothing" - everything still works, but internally it could be that "live" is now earthed and "neutral" is "hot"), and nothing will trip until there's a second fault. it's also difficult to get a reliable floating supply other than from a standalone generator or isolating transformer. In my day job I deal with IT power systems (it's fully floating with no intended connection to earth), and we have a) monitoring equipment to detect faults, and b) full time monitoring/control of the installation by qualified people.

But back to regular domestic supplies. As PC1966 says, over here our DNO usually provides the earth - though that does have some issues. Over in the US it seems it's normal for the householder to provide an earth rod and to earth the neutral - which would seem to make it a customer provided multiple earth system (effectively you and your neighbours are sharing your earth rods).
The basic principle is that if you have a fault in your installation or an appliance, then current will flow from line (hot) to ground via the earth (ground) wires, and that current will be sufficient to blow a fuse or trip a breaker. At the same time, during the time between the fault occurring and the fuse/breaker disconnecting the supply, the touch voltage (i.e. what someone would feel if stood on ground and touching some "earthed" metalwork) needs to stay low enough that it isn't dangerous (under our rules, that's 50V for AC circuits).
It would actually work for the electricity supplier to earth the neutral at the substation (transformer), for you to provide just your local earth, and not to link neutral and earth anywhere else. But that makes the above fault handling (automatic disconnection of the supply to faults by a fuse/circuit breaker) heavily reliant on the quality of those two earths - and the ground in between.
My assumption is that those who write your wiring codes decided that having every property provide an earth rod AND earth the neutral to it would significantly reduce the fault impedances - and avoid problems where a poor earth rod means that a fault persists, the fuse doesn't blow, and in the mean time there are dangerous voltages present on "earthed" bits of metalwork. Over here that's done by the DNO (the people who pipe the lecky to our houses) putting lots of earth electrodes in the local network - called protective multiple earthing - and so providing a low impedance earth to most premises.
 
I think you are a bit confused when talking about resistances.
Put simply :
The neutral wiring (which should always be treated as "live") carries the load current. If it gets broken, then something doesn't work but nothing exposed is pulled high by the load current.
The earth wiring is there to keep exposed conductive parts at a safe voltage relative to ground.

There are electrical systems where there is no connection between "neutral" and earth known as IT. They are pretty well unheard of in domestic environments as a single fault will "do nothing" - everything still works, but internally it could be that "live" is now earthed and "neutral" is "hot"), and nothing will trip until there's a second fault. it's also difficult to get a reliable floating supply other than from a standalone generator or isolating transformer. In my day job I deal with IT power systems (it's fully floating with no intended connection to earth), and we have a) monitoring equipment to detect faults, and b) full time monitoring/control of the installation by qualified people.

But back to regular domestic supplies. As PC1966 says, over here our DNO usually provides the earth - though that does have some issues. Over in the US it seems it's normal for the householder to provide an earth rod and to earth the neutral - which would seem to make it a customer provided multiple earth system (effectively you and your neighbours are sharing your earth rods).
The basic principle is that if you have a fault in your installation or an appliance, then current will flow from line (hot) to ground via the earth (ground) wires, and that current will be sufficient to blow a fuse or trip a breaker. At the same time, during the time between the fault occurring and the fuse/breaker disconnecting the supply, the touch voltage (i.e. what someone would feel if stood on ground and touching some "earthed" metalwork) needs to stay low enough that it isn't dangerous (under our rules, that's 50V for AC circuits).
It would actually work for the electricity supplier to earth the neutral at the substation (transformer), for you to provide just your local earth, and not to link neutral and earth anywhere else. But that makes the above fault handling (automatic disconnection of the supply to faults by a fuse/circuit breaker) heavily reliant on the quality of those two earths - and the ground in between.
My assumption is that those who write your wiring codes decided that having every property provide an earth rod AND earth the neutral to it would significantly reduce the fault impedances - and avoid problems where a poor earth rod means that a fault persists, the fuse doesn't blow, and in the mean time there are dangerous voltages present on "earthed" bits of metalwork. Over here that's done by the DNO (the people who pipe the lecky to our houses) putting lots of earth electrodes in the local network - called protective multiple earthing - and so providing a low impedance earth to most premises.
Oh sorry for the confusion, I was not explaining how ground works. I was attempting with my best knowledge to explain why in the US you would only ground the neutral at the MDP and not every sub panel.
"I could be wrong on this, but my take is everything connected to the neutral will add to the neutrals resistance to ground. Adding the ground to the MDP only, will ensures that the resistance to ground is retained."
 
"I could be wrong on this, but my take is everything connected to the neutral will add to the neutrals resistance to ground. Adding the ground to the MDP only, will ensures that the resistance to ground is retained."
I'm not sure my parsing of that makes sense!

The short answer is safety. If you deliberately link N-E at locations other than the origin of the system you run the risk of open faults making metalwork live and so presenting an electric shock risk (failure of combined N+E path) and/or small bonding conductors overheating and starting fires (open N diverting normal current via ground wires that are sized only for adiabatic limit of fault clearing).

Even the 'origin' location link has its issues as @Simon47 says. In the UK it is only the DNO (supplier) that is allowed to do it as they generally have higher standards of planning and joint-making than is the norm for domestic electrics. (No offens intended for the great domestic sparks on this forum, but you are not the ones I am thinking of here!)
 
I'm not sure my parsing of that makes sense!

The short answer is safety. If you deliberately link N-E at locations other than the origin of the system you run the risk of open faults making metalwork live and so presenting an electric shock risk (failure of combined N+E path) and/or small bonding conductors overheating and starting fires (open N diverting normal current via ground wires that are sized only for adiabatic limit of fault clearing).

Even the 'origin' location link has its issues as @Simon47 says. In the UK it is only the DNO (supplier) that is allowed to do it as they generally have higher standards of planning and joint-making than is the norm for domestic electrics. (No offens intended for the great domestic sparks on this forum, but you are not the ones I am thinking of here!)
Right, thanks for your explanations.
 

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