Consumer units for solar ...

[UNG]

True story from a DNO friend of mine. They lost neutral in the centre of Truro shopping centre. First reports were of gas appliances going haywire, so gas called out intially. By the time DNO got there, the gas pipe, when they dug down to find the electrical fault was glowing red hot!

Had the DNO out to a service head issue a few years ago and got to talking about earthing and he mentioned a job he got called out to by United Utilities the customer complained to UU because her cold water was coming out of the tap quite warm, turned out that one of the supply cables close to the water main had a fault on it and it caused the metal water pipe to heat up and warm the water

 

[Skipper]

Of course, as you leave the bonded area that is when the danger occurs, but, that can happen anyway in a T-N-CS situation ... the well known "cow shed" problem.

It might make sense to bang some earth rods in as recommended by 411.4.2 anyway ... with earth electrodes at the customer premises to support the T-N-CS system rather than converting it to T-T ... whether it will be possible to get an earth impedance value within the limits of table 54 I don't know ... as I said, it is sand around here ... I suspect in summer when it dry as a nun's chuff may prove problematic!

 

[UNG]

It might make sense to bang some earth rods in as recommended by 411.4.2 anyway

From past experience of troubleshooting supply issues, one of the common things I come across is the just bang an earth rod in and all will be fine problem
There always seems to be a total lack of understanding and education these days when it comes to installing earth rods / earthing systems and testing them. Even some of the "specialist" lightning conductor companies seem to be lacking having recently looked at the work of one on a local church who couldn't even test the LPS properly and when it was pointed out to them by the church warden they sent out their compliance / training manager who agreed with my findings and mentioned to the church warden that he needed to go and amend the company training manual, a few years ago this company added an extra earth rod to the LPS without any consideration to it's siting in relation to the existing rods and earth mats

... with earth electrodes at the customer premises to support the T-N-CS system rather than converting it to T-T ... whether it will be possible to get an earth impedance value within the limits of table 54 I don't know ...

The problem with "supporting" the TN-C-S system is it can hide a failure of the PEN and as I and a mate of mine who is also an electrician have found the property can become the earthing point and neutral for a number of properties connected to that DNO supply line which then raises the issue of neutral currents flowing through the property with the earth rod from other co-located installations / properties

as I said, it is sand around here ... I suspect in summer when it dry as a nun's chuff may prove problematic!

If the ground is sand it is unusual to have a TN-C-S supply for the reason you mention, one of the DNO's local to me won't allow TN-C-S supplies in certain parts of their patch because of the ground being too sandy

The installation of earth rods / mats / discs / systems is IMO a subject all on it's own and with rods the assumption that a 4ft rod is all that is needed for an installation to be earthed is in a lot of cases IMO a demonstration of ignorance and the acceptance of some of the impedances that some find acceptable without taking into account the ground conditions at the time can in a lot of cases increase due to the ground drying especially during the summer months
When installing earth rods I generally take more rods than I think I need and always use the extendable type if I can get 8 or 12ft of rod down then I'm likely to get a low impedance if it is likely to need more than one rod I usually stick to 8ft so it is easier to properly co-locate the multiple rods
The problem with BS7671 there is no retest period suggested for earth rods or installations with earth rods and the maximum impedance values make no mention of possible seasonal changes of the measured values whereas BS62305 for LPS suggests an 11 month retest period to take account of the differing ground conditions

Guidance note 8 makes interesting reading

 

[Skipper]

As always, there are compromises. By following the recommendation of 411.4.2 you are not going to make the situation any worse ... all the earth system would be doing is providing some additional protection in the event of a (hopefully rare) PEN failure ... I can see the transformer from here happily balanced on a pole in the corner of the adjacent field, there are limited opportunities for a PEN failure, and an additional earth point is not going to make the situation worse ... and even if in some way it does, by following the recommendations of ammendment 2 you are at least on the right side of the law.

It is worth remembering that 114.1 tells us that the connection between N and Earth (with a capital E) is deemed permanent, so basically it is DNOs problem, all an additional Earth does is provide a bit of non-obligatory protection should the DNO fail to do that.

In practice, all you can hope to achieve with an earth rod or mat is in the event of a PEN failure, to mitigate risks, so if you are in the habit say of standing in a puddle, leaning in through the kitchen window with your hand on the kitchen tap, you might want to put the earth rod near the puddle. If you achieve the values given in table 54, the RCD may even still trip as it should. Thankfully our garden tap is on 20 metres of plastic pipe, before the supply gets to the house. Beyond that, there is limited potential (ho ho) for serious harm in what should be a hopefully very rare event of PEN failure.

 

[UNG]

As always, there are compromises. By following the recommendation of 411.4.2 you are not going to make the situation any worse ... all the earth system would be doing is providing some additional protection in the event of a (hopefully rare) PEN failure ... I can see the transformer from here happily balanced on a pole in the corner of the adjacent field, there are limited opportunities for a PEN failure, and an additional earth point is not going to make the situation worse ... and even if in some way it does, by following the recommendations of ammendment 2 you are at least on the right side of the law.

If your property is the only one off the transformer then the risk should be minimal, adding an additional earth rod connected to the MET will also connect to the neutral on the service head so you will have neutral currents flowing down the additional earth if a PEN break occurs if multiple properties are on the same supply and only one property has an additional earth connection depending on where a PEN break occurs the neutral currents of a number of properties could flow down the additional earth connection

It is worth remembering that 114.1 tells us that the connection between N and Earth (with a capital E) is deemed permanent, so basically it is DNOs problem, all an additional Earth does is provide a bit of non-obligatory protection should the DNO fail to do that.

There is nothing to remember and you neglect to mention the pertinent point in 114.1 which is the reference to the ESQCR of which section 8 deals with the connection of the neutral conductor to Earth at the voltage source, that connection is made to ensure that the neutral conductor reference remains static wrt Earth and does not float when the load on each phase of the transformer is not balanced, this is done whether the supply is TNS, TNCS or TT, on overhead TNCS supplies if you look at the poles along the supply from the transformer ISTR every third pole has an earth rod at the base and cable running up to the neutral on underground TNCS supplies each joint has a short length of copper cable connected to the neutral hanging from it for the PME

In practice, all you can hope to achieve with an earth rod or mat is in the event of a PEN failure, to mitigate risks, so if you are in the habit say of standing in a puddle, leaning in through the kitchen window with your hand on the kitchen tap, you might want to put the earth rod near the puddle.

Not sure of the point you are trying to make

If you achieve the values given in table 54, the RCD may even still trip as it should.

I assume you are actually referring to table 41.5 and the footnote does suggest that the value should be as low as possible and is not considered to be stable if the value exceeds 200Ω. The reality is sub 50Ω is not normally too difficult to achieve

Thankfully our garden tap is on 20 metres of plastic pipe, before the supply gets to the house. Beyond that, there is limited potential (ho ho) for serious harm in what should be a hopefully very rare event of PEN failure.

Not sure what you are suggesting here