"Sub-main only requires 2 s disconnection time. Also, in TT systems, even in final circuits you're not limited to 30 mA RCD for some circuits, provided earth electrode resistance is sufficiently low to ensure 2x residual current rating. You can achieve 0.04 s disconnection with any non-delayed RCD if you can achieve 5x residual current rating."
Right, but you still have the issue of remote earth, whereby the user is subjected to full 230 volts for 2 seconds while outside the zone of bonding extraneous bonding. Where as with a TN supply, sub-mains typically result in some level of voltage sag across the terminals of the transformer.
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"If we use a 4-core cable, with the armour or outer wrap as earth, we will have the same problem in TN-S systems. It will be a broken PE rather than PEN - although this time you won't know about it until you need it. At least with broken PEN, most of the time it's known about before it's a real issue.
There are things you can do to monitor PE - but there are also things you can do to monitor PEN (perhaps less reliable, though, if you have a single-phase installation in a three-phase network)."
For a TN-S supply, in order for the earthing system to become live you need 3 failures: loss of the incoming PE, failure of an RCD, and a fault in the system. Where as in TN-C-S, you only require one failure, the rupture or disconnection of the PEN.
Because of this, the only advantage I can see that TN-S has over TN-C-S is that there are no diverted neutral currents in "normal operation". Diverted neutral currents can cause "tingles" in certain conditions, and of course not recommended in locations with explosive atmospheres. Where it may cause tingles, or livestock are involved, a buried earth grid can help control voltage effects of diverted neutral currents.
The required bonding and earthing to control ever increasing neutral currents ends up being more costly in the long run. You also have the issue of magnetic fields, where a gamble is being waged on the hopes of eventually discovering 50Hz fields have no effect on organisms.
"Sub-main only requires 2 s disconnection time. Also, in TT systems, even in final circuits you're not limited to 30 mA RCD for some circuits, provided earth electrode resistance is sufficiently low to ensure 2x residual current rating. You can achieve 0.04 s disconnection with any non-delayed RCD if you can achieve 5x residual current rating."
Right, but you still have the issue of remote earth, whereby the user is subjected to full 230 volts for 2 seconds while outside the zone of bonding extraneous bonding. Where as with a TN supply, sub-mains typically result in some level of voltage sag across the terminals of the transformer.
.
"If we use a 4-core cable, with the armour or outer wrap as earth, we will have the same problem in TN-S systems. It will be a broken PE rather than PEN - although this time you won't know about it until you need it. At least with broken PEN, most of the time it's known about before it's a real issue.
There are things you can do to monitor PE - but there are also things you can do to monitor PEN (perhaps less reliable, though, if you have a single-phase installation in a three-phase network)."
For a TN-S supply, in order for the earthing system to become live you need 3 failures: loss of the incoming PE, failure of an RCD, and a fault in the system. Where as in TN-C-S, you only require one failure, the rupture or disconnection of the PEN.
Because of this, the only advantage I can see that TN-S has over TN-C-S is that there are no diverted neutral currents in "normal operation". Diverted neutral currents can cause "tingles" in certain conditions, and of course not recommended in locations with explosive atmospheres. Where it may cause tingles, or livestock are involved, a buried earth grid can help control voltage effects of diverted neutral currents.
The required bonding and earthing to control ever increasing neutral currents ends up being more costly in the long run. You also have the issue of magnetic fields, where a gamble is being waged on the hopes of eventually discovering 50Hz fields have no effect on organisms.
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