13 ms·

In a properly executed TN-C-S distribution, the TN-C and TN-S sections are separated with a 4 way single junction point where the PEN conductor coming from the

by yholio 6y ago

In a properly executed TN-C-S distribution, the TN-C and TN-S sections are separated with a 4 way single junction point where the PEN conductor coming from the station is connected to the local grounding rod and after this the separated PE and Neutral conductors are drawn into the property. Absolutely no circuit breaker is allowed on the PEN conductor upwards of this separation point. This arrangement remains safe regardless of any single fault in any of the 4 wires. If the PEN conductor breaks but the local earthing still has decently low resistance, the internal PE protection that connects to the case of electronic appliances is still connected to the ground. If strong currents are drawn the voltage of the PE in relation to the real ground will increase, but the mains voltage will also fluctuate with the difference; so you might get a 20-30V drop in the mains voltage and the same voltage appears on the PE, not usually dangerous and you can see something is wrong with the power. Differential and overload protections still work since they have a safe return via the grounding rod. If, on the other hand, the local grounding malfunctions, the PE is still secured to the PEN, which should be grounded at the nearest pole or local transformer. This is a harder defect to detect and periodic checks are recommended on the local ground, but at least you have single fault protection, and two things need to happen before a dangerous condition can occur (both local grounding and the PEN conductor must have a fault). An adjacent benefit of having good local grounding in this configuration is cleaner and more stable voltage, since all groundings of all houses in an area are connected in parallel to the same PEN line, providing a path to the ground that might have a lower impedance than the PEN line feeding it.

aaronmdjones 6y ago
That would be safer, yeah, and I agree that's how it should be done. However, we don't do that. Usually, our TN-C-S supplies are just augmented with PME, where the PEN conductor is earthed at multiple points along its run. This is almost equivalent to your description of a separate earth rod in every property, but it's on the main run instead of each branch, and it's not done for every point (e.g. house), nor even every other point or perhaps not even every 5 points. This usually saves you from the consequences of a PEN conductor fault if it's broken along its run somewhere (because there's still an earth rod after the break), but it can't save you if the neutral is broken after it splits off to feed your property, and it can't save you if your property is the last on the line. Where we have separate property earthing rods (either as the sole earthing arrangement, as in a TT earthing arrangement, or instead, in addition to a TN-S or TN-C-S supply, for example in an outbuilding containing extraneous conductive parts like copper water pipes), these are not permitted to be connected to a TN-C-S supply earth because practically no-one else in the street will have such a setup and then a broken PEN conductor sees your earth rod carry, perhaps, half of that street's supply current, which it could obviously never safely do. I imagine we don't mandate supplementary earth rods in TN-C-S installations here because: 1) PME is common, reducing the impact of a PEN conductor fault, and 2) As I said, TN-C-S installations here use concentric cable, which is very hard to break in such a manner as to cause a PEN conductor fault without also killing the line conductor, rendering the fault moot.
- yholio 6y ago
  > Where we have separate property earthing rods ... these are not permitted to be connected to a TN-C-S supply earth because practically no-one else in the street will have such a setup and then a broken PEN conductor sees your earth rod carry, perhaps, half of that street's supply current, which it could obviously never safely do. If no one on the street has an earthing rod and the PEN breaks, then all the PE internal wiring of all houses will be connected, via transformers, light-bulbs etc. to the live wire. So every grounded appliance in the area is live. This is such a dangerous scenario that a grounding rod carrying half the current of the street is the least of your worries. All differential protections are also disabled. Additionally, houses will typically be spread around on a three phase supply with the PEN carrying the disequilibrium current. If PEN breaks uphill from such a distribution cell then you end up with two houses connected in series to two phases, giving a voltage √3 = 1.732... higher than the mains (208 vs 120V in US or 400V vs 230V in Europe). The voltage will spread depending on the resistance of the two houses. So if a house has large consumers it will see a very low voltage, while it's series pair might see a huge voltage spyke. This is an incredibly dangerous scenario from a fire hazard perspective. A similar thing can happen in a 120/240V single phase system. So, if you want good, solid, safe voltage in TN-C-S systems, a good grounding (preferably less than 1 Ohm) should be connected at the separation point between the TN-Combined supply and your internal TN-Separated wiring. This guarantees your differential protections will work regardless of what happens uphill. Of course, if this is an uncommon practice in the area and not allowed by local code then that's that. But I wouldn't feel very safe without, at a minimum, a ground fault detection relay (which has it's own mini-earthing rod and detects when PEN floats at a too high voltage), and also a over-voltage protection. PEN faults are very common, it's not a question of if, it's a question of when. Coaxial supplies are used around here too, with recommended local earthing and mandatory ground fault detection relay if local earthing does not exist. Still, it happens that lighting destroys both the central conductor and the electronic ground relay, yet line voltage still finds a way to shock people (you don't need a very good connection for that). Other times, the aluminum-steel core of the coax breaks down. Other times, idiot electricians work on the network and create PEN faults, exploding all the devices of a certain home by exposing them to three phase voltage.

by yholio 6y ago

aaronmdjones 6y ago

That would be safer, yeah, and I agree that's how it should be done. However, we don't do that. Usually, our TN-C-S supplies are just augmented with PME, where the PEN conductor is earthed at multiple points along its run. This is almost equivalent to your description of a separate earth rod in every property, but it's on the main run instead of each branch, and it's not done for every point (e.g. house), nor even every other point or perhaps not even every 5 points. This usually saves you from the consequences of a PEN conductor fault if it's broken along its run somewhere (because there's still an earth rod after the break), but it can't save you if the neutral is broken after it splits off to feed your property, and it can't save you if your property is the last on the line. Where we have separate property earthing rods (either as the sole earthing arrangement, as in a TT earthing arrangement, or instead, in addition to a TN-S or TN-C-S supply, for example in an outbuilding containing extraneous conductive parts like copper water pipes), these are not permitted to be connected to a TN-C-S supply earth because practically no-one else in the street will have such a setup and then a broken PEN conductor sees your earth rod carry, perhaps, half of that street's supply current, which it could obviously never safely do. I imagine we don't mandate supplementary earth rods in TN-C-S installations here because: 1) PME is common, reducing the impact of a PEN conductor fault, and 2) As I said, TN-C-S installations here use concentric cable, which is very hard to break in such a manner as to cause a PEN conductor fault without also killing the line conductor, rendering the fault moot.

yholio 6y ago
> Where we have separate property earthing rods ... these are not permitted to be connected to a TN-C-S supply earth because practically no-one else in the street will have such a setup and then a broken PEN conductor sees your earth rod carry, perhaps, half of that street's supply current, which it could obviously never safely do. If no one on the street has an earthing rod and the PEN breaks, then all the PE internal wiring of all houses will be connected, via transformers, light-bulbs etc. to the live wire. So every grounded appliance in the area is live. This is such a dangerous scenario that a grounding rod carrying half the current of the street is the least of your worries. All differential protections are also disabled. Additionally, houses will typically be spread around on a three phase supply with the PEN carrying the disequilibrium current. If PEN breaks uphill from such a distribution cell then you end up with two houses connected in series to two phases, giving a voltage √3 = 1.732... higher than the mains (208 vs 120V in US or 400V vs 230V in Europe). The voltage will spread depending on the resistance of the two houses. So if a house has large consumers it will see a very low voltage, while it's series pair might see a huge voltage spyke. This is an incredibly dangerous scenario from a fire hazard perspective. A similar thing can happen in a 120/240V single phase system. So, if you want good, solid, safe voltage in TN-C-S systems, a good grounding (preferably less than 1 Ohm) should be connected at the separation point between the TN-Combined supply and your internal TN-Separated wiring. This guarantees your differential protections will work regardless of what happens uphill. Of course, if this is an uncommon practice in the area and not allowed by local code then that's that. But I wouldn't feel very safe without, at a minimum, a ground fault detection relay (which has it's own mini-earthing rod and detects when PEN floats at a too high voltage), and also a over-voltage protection. PEN faults are very common, it's not a question of if, it's a question of when. Coaxial supplies are used around here too, with recommended local earthing and mandatory ground fault detection relay if local earthing does not exist. Still, it happens that lighting destroys both the central conductor and the electronic ground relay, yet line voltage still finds a way to shock people (you don't need a very good connection for that). Other times, the aluminum-steel core of the coax breaks down. Other times, idiot electricians work on the network and create PEN faults, exploding all the devices of a certain home by exposing them to three phase voltage.