PME conversion MEB upgrade

[bloxy]

I am planning on a CCU upgrade (from old Wylex rewirable and replacement of ELCB on TT system) this will be done by a competent person and self certified.

Prior to getting this done I was considering upgrading from TT to PME as the current rod is buried under brick paving and is not accessible. Is conversion to PME a good idea? EON (Central Networks) will do this for £154.

I will need to upgrade the MEB. I was planing on using 16mm and running this through kitchen wall then around outside of building at damp proof course level to the meter box and install a MET there. Is this external routing acceptable with cable clipped to wall or should I run in plastic conduit or use capping?

 

[RF Lighting]

Do you really mean MEB?

It sounds from your description that you are installing a new main earth to your CU.

Is there currently MEB in place, and if so, what size is it?

 

[bloxy]

Yes I mean MEB there is currently a continuous 6mm from the incoming water main copper, which is then follows the rising main and is also looped to clamps on the bathroom h&c pipes and the radiator, this then goes to the earth connection in the CCU.

The main bond and supplementary bonds were combined into a single continuous run. I confess I did this back in the early eighties after moving in, when I found there was no bonding at all. (duff info from an old Collins DIY manual, in pre internet days). I now know better!

I intended to run a new MEB from the incoming water main where it changes to copper (no gas) and remove the existing connection at the CCU and water main (I'll cut it off after last clamps in the bathroom) so leaving the old earth cable as just supplementary bonding.

 

[RF Lighting]

Do you have an oil supply to your property, or is water the only service pipe which enters your house?

Are you going to install a new main earth from the head to your CU ready for the PME upgrade?

 

[bloxy]

I have oil coming into a boiler house, connection from the short length of copper to boiler is by rubber hose. I think there is an argument for not bonding this but I was going to extend the bonding (unbroken) through to the oil.

I was going to add a MET I assumed the DNO would do a link to the MET from the head. I recently had a meter change and the new tails are 25mm so I believe the main earth will need to be 16mm.

 

[RF Lighting]

Yep you need a 16.0mm² main earth from your new MET to your consumer unit.

You also need to leave a tail of 16.0mm² from the MET ready for the DNO to connect into the head. They will not normally provide this link.

Your MEBs can then be connected to your MET or to your CU, which ever is most convenient for you.

The MEBs only need to be 10.0mm²

Without actually seeing your oil setup, it is hard to say whether it would be considered an extraneous conductive part, and require bonding. Any chance of a couple of photos?

 

[bloxy]

I can get a photo of the oil set-up tomorrow. Boiler house is separate to main house but attached to garage which is attached to house. Oil line in copper comes through external wall 300mm then valve then rubber pressure hose to boiler.

I know the requirement is min 10mm but I was going to use 16mm due to outside routing, (mechanical strength etc)

 

[RF Lighting]

Presumably you have some electric in the boiler house to power the boiler? If so then you need to include the boiler room as part of your equipotential zone, which means bonding all extraneous metal work.

I don't really think you would gain any real increase in mechanical strength by using 16.0mm² instead of 10.0mm², but there is nothing to stop you using a cable which is bigger than required if you so wish.

Personally I'd be inclined to put the bonding into PVC conduit outside, just for neatness. I've yet to see a bonding cable clipped on the outside of a property which doesn't look like a dogs dinner.

 

[securespark]

Me neither.

I think black conduit looks the dog's in comparison. It's just a bit fiddly to thread up.

 

[bloxy]

I thought conduit would look neat but at DPC level around the back and side of the house and behind some foliage you won't see much of it (and the Sky cable is already clipped around). May go with the conduit though.

I will bond the oil.

I need the bonding whatever but is the switch to PME a good move?

 

[1john]

imo TNC-S / PME is a more reliable earth although in these days when all circuits generally end up being RCD protected it isn't the end of the world having TT, only problem i can have with it is that the NIC like the reading to be below 100ohms for some reason and in the summer in certain areas arounf here it is hard to get that reading. I would prefere PME so as i could have the option to not have certain things RCD protected like say a smoke alarm circuit or an alarm panel feed fridge freezer ect.