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

I won't repeat comments about exposed-conductive-parts and extraneous-conductive-parts being at substantially the same potential as the MET.

However, a plastic cover might help keep dirt and dust out ... and possibly make it less prone to "tampering" ... those would be benefits for me.

Sergio Fernandez said:

Green coloured Henley style connector block?

If it were green-and-yellow, OK, but not the single colour green, that would have to be otherwise marked I'm afraid.

It also won't take away the need for a warning notice to meet the requirements of 514.13.1.

The size of bonding conductors where PME conditions apply according to 544.1.1 could be up to 50mm2, depending on the CSA of the distributor's PEN conductor. Whereas the size of the earthing conductor is related to the CSA  of the line conductor (meter tails) OR by using the adiabatic equation (543.1) So we could quite easily have a situation where the bonding sizes are bigger than the main earth.

Where would you get the information about the sizes of the supply PEN conductor(s)? If there were a ring main in the supply, i suppose the CSA should be doubled?

Are there tables where you can measure the OD of the cable sheath to get the required figure?

You can't just open up the head and take a peek

So we could quite easily have a situation where the bonding sizes are bigger than the main earth.

Not for PME though - they've thought of that - Earthing conductors, where PME conditions apply (542.3.1), have to meet the requirements of 544.1.1 as well.

   - Andy.

Are there tables where you can measure the OD of the cable sheath to get the required figure?

Most cable manufacturers have data available - e.g. https://www.elandcables.com/media/39042/aluminium-concentric-bs-7870-pvc-cable.pdf (there might be a small variation between manufacturers or even batches, but they should be of the same magnitudes). Some might be copper rather than aluminium centre core though, so still some guesswork to be done perhaps.

If there were a ring main in the supply, i suppose the CSA should be doubled?

Rings are common for HV, but not sure they're used much at LV in DNO land. Some large urban areas apparently mesh LV, but I've still never heard of a single LV supply having more than one incoming cable (some are looped, one in and one out to next door, but that's not a ring either). Even if it was a ring in the street, it's only the c.s.a. of the 'spur' cable into the property you need worry about for table 54.8.

   - Andy.

I would ask, which is the MET?

House supply has a traditional MET with some very weedy looking copper from the PILC, but it is 4 x 4 mm². I don't quite see how it could be avoided. A length of 16 mm² cable takes it to a switch-fuse in an adjacent cabinet. More importantly, loads of bonding comes together in another terminal in the house. I think of it as a "building earth terminal", but there seem to be other names too.

My garage supply has no bonding so the earthing conductor goes from the (PME) intake to the main board. I suggest that there is no MET.

There is very little point in insulating the earth marshalling blocks from a shock point of view, as the whole point is that is the voltage that is supposed to be safe to touch. If it isn't then we should insulate all the 'earthed' metal work as well, and then we can retire not just blocks, but the whole idea of earthing, as we have re-invented double insulation...

I can see a merit in earthing being being visible - rather like the earth terminals on the outside of the old "PARcan" theatrical lights in the days when earthing was a new thing and not always done very well (*) - "earthed and seen to be earthed", any failure like missing screws or a wire dropped out was/is immediately visible. For much the same reason I prefer clamps on pipes to be in a place they are immediately on show, as opposed to behind a panel or whatever.  I appreciate that this does not match the aesthetic aspirations of some.

Mike

(*) I know that it was actually the 1939 regs that brought compulsory earthing to power sockets in the UK regs, but for a long time after that it was not seen as that serious a thing, and 2 pin sockets were certainly in use well into the 1970s in older buildings, and theatre installations have historically always been a bit ' keep if going for just one more night'.

PPS A lot of kit for use at very high voltages indeed is NOT insulated, as the insulation would only give a false sense of security and may even be prone to ignition from discharge currents that on a bare wire would either have no effect or just corona.

Hi Chris

In my eyes I would say a MET block is one of these.