I've just seen this article in E&T highlighting the increased incidence of reported broken PEN's from 57 in 2003 to 474 in 2021. It seems they are becoming less of a rare event.
David
I've just seen this article in E&T highlighting the increased incidence of reported broken PEN's from 57 in 2003 to 474 in 2021. It seems they are becoming less of a rare event.
David
Only if by 'electrode' you mean 'drive a rod' (which I'd agree is very risky).
There are other options.
What other option would you recommend Graham?
I'm building a house, it is a TN-C-S supply, but, it is not coming into the house, the meter and cut out are located near the front gate, the house is 20 metres away (not the one you know JP!), so I have the choice of making it TN-C-S or, not exporting the earth from the meter, and making the house TT. I am tending toward TT, as I can get a number of earth rods in around the property.
What would you do?
I'm building a house, it is a TN-C-S supply, but, it is not coming into the house, the meter and cut out are located near the front gate, the house is 20 metres away (not the one you know JP!), so I have the choice of making it TN-C-S or, not exporting the earth from the meter, and making the house TT. I am tending toward TT, as I can get a number of earth rods in around the property.
What would you do?
If I were building a house, I'd put in a foundation electrode. How does the risk of an RCD failing (in 6 months between presses of the test button ;-) ) compare with the risk of a broken PEN; and what is the risk with aerial supplies compared with under ground?
Too late for the foundation electrode, as the footings and drains are finished, my only option is a ring of earth rods around the building, or, keep the TN-C-S. The supply from the meter will be protected by a time delayed 300mA RCD, as well as a type 1 SPD. Of course, internally will be 30mA and type 2 SPDs.
Not all DNOs used Consac and quite a lot of Consac has already been overlaid. It might help if the DNOs produced some form of risk tool. Perhaps a post code look up with risk ratings derived from an analysis of broken PENs/DNC by cable/network type. Another way forward would be to make measurement and reporting excess values of current through gas meters part of the meter inspection regime. This would be easy and cheap enough to do. No doubt some enterprising soul will make an alarm that can be fitted around pipes and cables, I can even see this finding its way into the regs!
. But on the other hand if you happen to be the only house on the street that's been upgraded to have an additional electrode, I could be a lot higher than your own load.
No different to the situation if you're the only house downstream of the break with extraneous-conductive-parts.
In my view we should return to TN-S, with 4 core street mains and and with wire armouring used as the earth.
Option one. Old style mains with the wire armouring exposed and in direct contact with the general mass of earth. This approach gives a very low earth resistance since the armouring provides both a metallic earth path back to the transformer AND numerous fortuitous additional earth connections along the cable.
Option two, use the more modern type of 4 core cable with a plastic outer covering. This protects the wire armouring against rust. If selecting this option then extra connections to earth should be provided at each joint.
We could now add to that bring back TN-s campaign, some basic condition monitoring at the substation, and a little box with a SIM card that rings in its earth leakage current readings, electrode resistances and instantaneous HV and LV load currents, for the price of digging a decent foundation, thus allowing the DNO transformer and cables to be preemptively maintained and overload to be realized before the paint blisters off the transformer.
Unfortunately the same acute constriction of the wallet that gives us the broken PEN problem in the first place probably means this is also off the table. But I agree, it would be good.
The safest alternative is probably to stop worrying about earthing altogether and adopt a similar approach to that now taken for garden appliances outdoors, and make everything 2 pin and double insulated.
Mike.
If we have no extraneous-conductive-parts in an installation, TN-S could be more dangerous than TN-C-S, if the PE back to the substation breaks, due to "leakage currents". Sure, commercial/industrial installations with lots of opportunistic Earthing, TN-S is definitely the correct choice ... but perhaps not always for distribution to domestic installations.
In addition, earth fault loop impedances are higher (in general) than TN-C-S.
Overall, I think each of the three usual earthing arrangements has its own advantages and disadvantages.
Conclusion is, though, perhaps BS 7671 should continue to recommend a consumer earth electrode connected to MET for TN systems.
When you say "recommend" you really mean "should" 
Do DNOs have a policy when connecting power to new properties to add an earth "pigtail" at the point of connection or where the cable exits the ground? Should they? Could be quite easy with a stainless steel braided sheath applied on site attached to the pigtail.
I think they add one where they take the joint from an existing cable, but this could be some way away from the termination.
If we have no extraneous-conductive-parts in an installation, TN-S could be more dangerous than TN-C-S,
That's an interesting statement. Is that just down to a broken PEN being "more obvious" to consumers than a broken PE, so will likely persist for a shorter duration, as it'll be called in to the DNO earlier? In terms of the risk up to that point, I suspect I'd rather discover the "tingle" of a broken PE (leakage currents) rather than the "belt" of a broken PEN (load currents) - not to mention the additional fire risks from a broken PEN.
- Andy.
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