Stick to TT or go PME?

[FatAlan]

Ok, so you have an old non RCD protected domestic installation on a TT earthing arrangement.
Would it be preferable to upgrade to PME if available by the DNO or stick to TT and protect with up front 100mA RCD if the Ze was ok via the rod?

 

[telectrix]

PME every time if available. i prefer to rely on MCBs for fault protection than some skittish RCD that some snotty-nosed kid has spilt orange juice/coke/superglue into.

 

[Dave OCD]

Alan - does the existing TT installation have an 'up front' voltage trip at least ? If not and unless the Ze is very low it's potentially lethal.

 

[FatAlan]

Cheers Tel. Succinct as ever.

 

[FatAlan]

Alan - does the existing TT installation have an 'up front' voltage trip at least ? If not and unless the Ze is very low it's potentially lethal.

If you mean a VOELD then no

 

[Dave OCD]

Yep that's the one, very odd to find an old TT installation without one [unless already replaced by an RCD] so the Ze must be really low or it's well dodgy.

 

[FatAlan]

Yep that's the one, very odd to find an old TT installation without one [unless already replaced by an RCD] so the Ze must be really low or it's well dodgy.

The later is probably more accurate. Will be checking the Ze.

 

[Carl S]

I had this argument with the NWEB years ago (long before most of you were born). A TT supply the lecky board wanted to make PME, no bloody way! My reasoning, they were wanting to use my “earth nest” for free. It was far better than their puny rod.

UKPN tried the same trick years later on an 11kV system. The earth nest was 100’s of cast iron piles driven eight foot in to the ground. Somewhere in the order of ½ a mile feet squared in area.

 

[l4urence]

When I can, I change TT CU’s to RCBO’s. I don’t really like one central RCD that trips the entire installation. I don’t really get how the overhead TT line could be changed to PME, certainly not in my location.

 

[FatAlan]

If you stayed TT would their be anything wrong with having a 30mA RCBO CU with a 100mA RCD or for that matter split between two 100mA RCDs?

 

[l4urence]

If you stayed TT would their be anything wrong with having a 30mA RCBO CU with a 100mA RCD or for that matter split between two 100mA RCDs?

Why would you use 100mA RCD if you have 30mA RCBO’s in the CU.

 

[FatAlan]

Good point. Getting confused with TT systems with up front 100mA RCDs.

 

[FatAlan]

When I can, I change TT CU’s to RCBO’s. I don’t really like one central RCD that trips the entire installation. I don’t really get how the overhead TT line could be changed to PME, certainly not in my location.

SSE have changed a couple for me both on overhead supplies. All they appear to do is check the available Ze. That’s why I asked the original question really as my DNO SSE appear to be quite happy to swap to PME.

 

[l4urence]

Good point. Getting confused with TT systems with up front 100mA RCDs.

Exactly. RCBO’s are the ways to go right now and not just TT - split load boards (for example) belong in the 80’s.

 

[l4urence]

SSE have changed a couple for me both on overhead supplies. All they appear to do is check the available Ze. That’s why I asked the original question really as my DNO SSE appear to be quite happy to swap to PME.

How very bizarre. Not seen that in my neck of the woods, some changing of PME back to TT for EV charging. The world is changing.