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
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,
Yes, the touch-current will just sit there waiting for a victim.
However, in the case of a TN-S system with no extraneous-conductive-parts, in the event of a PE break, a consumer's earth electrode connected to MET will help divert at least 83 % of the available touch-current (assuming body resistance of 1 kΩ, and earth electrode resistance at the 200 Ω maximum recommended for stability).. So, let's say we had a potentially lethal touch-current of 30 mA, this is reduced to a much less problematic 5 mA (at least in dry conditions).
When you say "recommend" you really mean "should"
In standards speak, 'in a 'specification' (such as BS 7671) should' is synonymous with 'recommended', and such recommendations are not necessary for conformity to the standard. On the other hand, 'shall' is 'normative' and is necessary for conformity to the standard.
There is a tacit assumption that we consider only un-monitored TNS as it used to be when introduced in the 1930s. We can of course have a modern and much smarter TNS and actually log that leakage and the NE offsets, and either report it or ADS on it . You cannot do this as successfully on PME as there is no other conductor to compare voltages with and you cannot have any kind of EFR at the substation, as fault current and legit neutral current look the same.
Mike.
We can of course have a modern and much smarter TNS and actually log that leakage and the NE offsets, and either report it or ADS on it .
Food for thought ... although N-PE voltage is also affected by line currents (or, in a three-phase portion of the network, balance of line currents).
And, since we are talking about such small currents that might be indicated in an N-PE break, I'd not therefore immediately be a proponent of 'ADS' on it, as the impact of supply interruption could outweigh the benefits of monitoring. Quite possibly, this is possibly (from a correlation vs scientific "actuality of a fault") monitoring L-N (or L-PE) voltages for a single-phase supply to detect a PEN break.
But certainly 'we' (i.e. the distributor) could act more quickly upon a repeated or unexpected N-PE voltage condition.
Indeed. I do not know how common these sort of monitors are in new substations, but it would only be a modest extension of the design to include some extra channels of ADC to cover the sorts of thing that would be needed. I think 21st century power distribution will need to be quite a bit smarter than it was in the 20th.
M.
That fire is not caused by a broken PEN, there are image look up tools that will identify it for you. Fine art America has it listed as an image of a practice drill in Calafornia, from 2013, and shutterstock and adobe both list it as part of their collections of 'fires and firemen' but with later date stamps.
II is just a 'clip-art' image.
Indeed, hence my tongue-in-cheek observation. One might expect such sensationalist photos in the tabloids for common consumption, but I would expect better from a more serious source such as EandT!
However, in the case of a TN-S system with no extraneous-conductive-parts, in the event of a PE break, a consumer's earth electrode connected to MET will help divert at least 83 % of the available touch-current (assuming body resistance of 1 kΩ, and earth electrode resistance at the 200 Ω maximum recommended for stability).. So, let's say we had a potentially lethal touch-current of 30 mA, this is reduced to a much less problematic 5 mA (at least in dry conditions).
Agreed - however proper TN-S systems are rapidly going the way of the Dodo - practically every domestic installation these days if it has a DNO supplied Earth will be under PME conditions - so the current available under a PEN break will likely be several amps - and 17% of serval amps is hardly any less lethal than 100% of those same several amps.
- Andy.
however proper TN-S systems are rapidly going the way of the Dodo - practically every domestic installation these days if it has a DNO supplied Earth will be under PME conditions - so the current available under a PEN break will likely be several amps - and 17% of serval amps is hardly any less lethal than 100% of those same several amps.
Andy, you can't compare apples and oranges.
My reply regards controlling touch-currents in certain cases of TN-S systems with broken PE in the distribution network, is in no way relevant, applicable, or comparable, to the case of trying to control touch-voltage PME with broken PEN conductors!
I think the assumed value of 200 Ω in the example calculation gave that away!
As above, to control PME touch voltage, you really need low values of resistance - typically sub 1Ω, which is seen as not realistic in a number of areas in the country (unless you have a large amount of steelwork from a large steel-frame building to use as your electrode).
However, it's also the case that 5 no. premises with sub 5 Ω resistance (that might be achievable with foundation or perimeter-ring style electrodes) could well help ... as discussed above.
5 no. premises with sub 5 Ω resistance (that might be achievable with foundation or perimeter-ring style electrodes)
We have to be careful when the local connection to terra-firma on the fault side gets too good, as now it starts to matter about the "electrode" at the substation side as well.
The traditional assumption is that the transformer star point is well earthed, and the fault is not. If the reverse is true, then it is all the folk on the non- fault side that get shocked as the whole transformer is in effect lifted off true earth by a voltage set by the total current coming back via terra-firma, and the combined electrode resistances on that side.
In effect the planet is at the mid point of two resistors straddling the fault.
I can see this being OK in a street of terraced houses and lots of plumbing and so on, I am less sure about a country lane with overhead singles zigzagging over the road from pole to pole, where one modest pole-pig transformer feeds 10 properties spread over half a mile and there is a 20 ohm or so DNO spike at the base of every other pole. (we get a lot of that here but mostly the houses are TT. )
Mike.
In effect the planet is at the mid point of two resistors straddling the fault.
I can see this being OK in a street of terraced houses and lots of plumbing and so on, I am less sure about a country lane with overhead singles zigzagging over the road from pole to pole, where one modest pole-pig transformer feeds 10 properties spread over half a mile and there is a 20 ohm or so DNO spike at the base of every other pole. (we get a lot of that here but mostly the houses are TT. )
Agreed ... but that raises more questions - like what voltage do you choose for protecting yourself, which we've not delved into yet either?
We're about to take you to the IET registration website. Don't worry though, you'll be sent straight back to the community after completing the registration.
Continue to the IET registration site
