Dumb questions about PEN fault and solution

[Helix737]

Sorry if these questions are dumb, but I thought I'd ask you guys as I'm brand new to this field. I was watching this video from John Ward and had a few questions about it.

1. In a TN-C-S system, if there was a PEN fault before the service head, the current would go through the protective devices as normal and wouldn't detect an imbalance. But when it returns through the neutral and reaches the break in the PEN, it has to come back on itself. So would the current return through both the earth and the neutral conductor at all or would it return entirely through the earth (presumably because the earth conductors are designed to be a faster route to true earth, and because there is more potential difference in the earth path than the neutral path back to the appliance/live conductor)?

2. Also what is the point in using TN-C-S system other than it being cheaper because you have one less conductor to have separately running through the street? Is this worth the safety concern it comes with?

3. What is the best solution to this problem? TT systems with plastic incoming pipes?

Many thanks

 

[pc1966]

Sorry if these questions are dumb, but I thought I'd ask you guys as I'm brand new to this field. I was watching this video from John Ward and had a few questions about it.

1. In a TN-C-S system, if there was a PEN fault before the service head, the current would go through the protective devices as normal and wouldn't detect an imbalance. But when it returns through the neutral and reaches the break in the PEN, it has to come back on itself. So would the current return through both the earth and the neutral conductor at all or would it return entirely through the earth (presumably because the earth conductors are designed to be a faster route to true earth, and because there is more potential difference in the earth path than the neutral path back to the appliance/live conductor)?

In the example drawing you have it would attempt to return via the person to true Earth off the CPC connected metalwork.

Depending on the property there may be bonded conductive service pipes that also provide some means to return to Earth. But if that path is not basically a low impedance due to a metal service pipe to unbroken segment of the network then you will see a high voltage N/CPC to true Earth.

2. Also what is the point in using TN-C-S system other than it being cheaper because you have one less conductor to have separately running through the street? Is this worth the safety concern it comes with?

That is its main goal, lower cost.

The DNO are supposed to manage the risk by the design of joints and provision of multiple earth rods over the segment, but it seems older cables with aluminium outer PEN are corroding through (at least that is what I have heard).

Considering the current s-show in the Ukraine and impact on our energy prices, as over half the UK electric comes from gas, there are bigger issues in terms of how investment has been spent. Or not.

3. What is the best solution to this problem? TT systems with plastic incoming pipes?

There is no "ideal" solution as all have their issues.

For people inside the property, with "extraneous conductive parts" (i.e. metalwork) bonded as per the regs, there is no real shock danger. But over-voltage from an imbalanced 3-phase network losing its N can cause damage and conceivable a fire.

Going TT avoids the open-PEN risk but then you need a reliable earth rod, and hammering in one of those with care and possible excavation to check could strike a buried cable or pipe cause a whole lot more damage. Also you are then dependant on the RCD electronics for fault protection, and that is going to be less reliable than the simple thermal-magnetic MCB side (or fuse) used on a TN system that can disconnect due to high CPC current.

Having both a delay RCD incomer and RCBO or final circuit RCD is one way to increase the certainty that one of the two series RCD would still trip, but more cost. But off hand I don't know the relative statistics of "RCD failures when needed" versus "open PEN failures" to decide what is the actually lower risk.

 

[Lister1987]

In the example drawing you have it would attempt to return via the person to true Earth off the CPC connected metalwork.

Depending on the property there may be bonded conductive service pipes that also provide some means to return to Earth. But if that path is not basically a low impedance due to a metal service pipe to unbroken segment of the network then you will see a high voltage N/CPC to true Earth.

That is its main goal, lower cost.

The DNO are supposed to manage the risk by the design of joints and provision of multiple earth rods over the segment, but it seems older cables with aluminium outer PEN are corroding through (at least that is what I have heard).

Considering the current s-show in the Ukraine and impact on our energy prices, as over half the UK electric comes from gas, there are bigger issues in terms of how investment has been spent. Or not.

There is no "ideal" solution as all have their issues.

For people inside the property, with "extraneous conductive parts" (i.e. metalwork) bonded as per the regs, there is no real shock danger. But over-voltage from an imbalanced 3-phase network losing its N can cause damage and conceivable a fire.

Going TT avoids the open-PEN risk but then you need a reliable earth rod, and hammering in one of those with care and possible excavation to check could strike a buried cable or pipe cause a whole lot more damage. Also you are then dependant on the RCD electronics for fault protection, and that is going to be less reliable than the simple thermal-magnetic MCB side (or fuse) used on a TN system that can disconnect due to high CPC current.

Having both a delay RCD incomer and RCBO or final circuit RCD is one way to increase the certainty that one of the two series RCD would still trip, but more cost. But off hand I don't know the relative statistics of "RCD failures when needed" versus "open PEN failures" to decide what is the actually lower risk.

Let's not forget the elephant in the room; the technology having no decernible product standard anywhere, like SY but without the fallback on VDE standards.