The concept of a MET could be implemented as anything - often the Earth bar in a CU in a simple installation for example. I take it you're referring specifically to the twin-screw exposed terminal block (e.g. https://www.tlc-direct.co.uk/Main_Index/Earthing_Index/Earth_Blocks/index.html ) that are sometimes used.

I don't see a particular shock issue (even under broken PEN conditions) - all the bonded pipework and exposed-conductive-parts (including the CU case now they're steel) will be at the same voltage anyway, so insulating 1% of the problem doesn't seem like much of an improvement.

IP protection - e.g. from damp - may have some benefits - but Henley style blocks aren't particularly damp proof - so you'd probably want something a bit better if going down that route. In practice, the (plated) brass blocks and copper conductors seem to stand up reasonably well even when corroded to some extent.

Size of Earthing (and indeed main bonding) conductors under broken PEN conditions is a tricky one - as often they'll carry return currents from other installations - so there's no guarantee that even 25mm² would be sufficient. The required size has gradually increased over the years as problems have been found, so hopefully we're at the point where the number of actual problems is acceptably low.

  - Andy.

The concept of a MET could be implemented as anything - often the Earth bar in a CU in a simple installation for example. I take it you're referring specifically to the twin-screw exposed terminal block (e.g. https://www.tlc-direct.co.uk/Main_Index/Earthing_Index/Earth_Blocks/index.html ) that are sometimes used.

I don't see a particular shock issue (even under broken PEN conditions) - all the bonded pipework and exposed-conductive-parts (including the CU case now they're steel) will be at the same voltage anyway, so insulating 1% of the problem doesn't seem like much of an improvement.

IP protection - e.g. from damp - may have some benefits - but Henley style blocks aren't particularly damp proof - so you'd probably want something a bit better if going down that route. In practice, the (plated) brass blocks and copper conductors seem to stand up reasonably well even when corroded to some extent.

Size of Earthing (and indeed main bonding) conductors under broken PEN conditions is a tricky one - as often they'll carry return currents from other installations - so there's no guarantee that even 25mm² would be sufficient. The required size has gradually increased over the years as problems have been found, so hopefully we're at the point where the number of actual problems is acceptably low.

  - Andy.

Hi Andy

When I said MET I was absolutely refering too to the style in the link you provided.  I have attached a screen grab for other readers of this thread.

It just seem to me that this can be constituted as an exposed part.  After all a lot of Earth is directly connected to the Neutral in the main cut out. 

With regards to the IP (international Protection) I was more thinking along the line of IP2X.  Think of the MET in a meter cupboard or under the stairs of an old install and apparently some new builds have decided to put them there also.  I think there is a possibility for someone to touch it quite easy, especially children.  Also I have seen many a meter cupbord door missing or hanging off when I go and do the school run. 

Domestic Tails I think would be better as 35mm squared CSA with an Earth of 25mm squared CSA min.  As some domestic installs are getting near the 80 to 100amp usage range with EV, PV and battery storage. 

AJJewsbury said:

I don't see a particular shock issue (even under broken PEN conditions) - all the bonded pipework and exposed-conductive-parts (including the CU case now they're steel) will be at the same voltage anyway, so insulating 1% of the problem doesn't seem like much of an improvement.

And maybe I'm missing something here but...if this is at a dangerous touch potential doesn't that mean that all metal cased appliances also are - so (simplistically) negating the whole point of the earth connection?

Not to say that there aren't houses where this is an issue - in my first house it was not a good idea to touch the kitchen sink with one hand while touching the washing machine with the other (don't ask...) but it wasn't touching the earth block that I was worried about in that case!

But I'm an electronics rather than an electrical engineer so I may well be missing the point here. 

Thanks,

Andy